Corrosion and Mechanical Properties of Friction Stir

Corrosion and Mechanical Properties of Friction Stir Formed Aluminum and Steel Joints

A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE

UNIVERSITY OF HAWAI‘I AT MĀNOA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

IN

MECHANICAL ENGINEERING

AUGUST 2014

By

Jonathan Earl C. Jaberina

Thesis Committee:

Scott F. Miller, Chairperson

Lloyd Hihara

Blair E. Carlson

ii

© Copyright 2014

By

Jonathan Earl C. Jaberina

iii

Acknowledgments

I would like to thank the following:

My family, for supporting my decision to continue graduate school in a different state

and for their continuous support throughout this endeavor. I would also like to thank my

friends from all over the world for keeping in touch and cheering me along the way.

Dr. Lloyd Hihara for being part of my committee and for invaluable consultation periods

with giving advice and input with the research. I’d also like to thank members of the

Hawaii Corrosion Laboratory: Ryan Sugamoto, Raghu Srinivasan, Shruti Tiwari, Daniel

Hong, Jeffrey Nelson, Jan Kealoha and Zhitong Chen for assistance and help with the lab

access, sample preparation, equipment loans, trips to the corrosion test sites and their

time out of the day.

Dr. Blair Carlson for taking time from his schedule to be part of my committee. Thank

you for sharing this project to become a research avenue for us.

Our research group of Sladjan Lazarevic, Kaimiao Liu and Kenneth Ogata. Thanks for

being a good bunch of colleagues to be working with. Special thanks to Sladjan for all his

help with the longer nights doing research in the lab.

And finally, Dr. Scott Miller. for giving me the opportunity to become a research

assistant for this project over the past two years. Thank you for being a great adviser and

mentor. My deepest gratitude goes out to you.

Funding for the research was from the Air Force Office of Scientific Research (AFOSR),

award number FA7000-10-2-0010 and MOD P00003. This is under the Correlation of

Field and Laboratory Studies on the Corrosion of Various Alloys in a Multitude of

Hawaii Micro-Climates project.

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Abstract

In the automotive industry, there is a big push to improve performance, fuel

efficiency and other tailor-made engineering properties of vehicles. Manufacturers are

now looking at other better, lightweight materials like magnesium and aluminum to use

alongside steel. Traditional joining methods like welding are not suitable for these

dissimilar materials. The friction stir forming process was developed and was able to

create a single-pin fusion joint between aluminum and steel.

This research now focuses on the issues that stemmed from the newly developed

friction stir forming process. The first part studies the corrosion that occurred due to the

usage of dissimilar metals. Friction stir formed samples were deployed to three different

test sites, each with different atmospheric conditions for an 8-month period. The

corrosion products were characterized and the effects of these were related to the

mechanical properties of the joints. Zinc was discovered to be the first metal to be

attacked, followed by aluminum, and formed corrosion products in the joint. Higher

chloride concentrations proved to be a more corrosive environment. Corrosion caused

significant decrease in the strength of the joints.

The second part of the research aimed to change the structure and makeup of the

previous single-pin fusion joint. By developing a new anvil and the usage of a different

tool, a new joint called the clinch joint was produced. The optimal parameters for the

friction stir clinching process were determined. The clinch joint is a mechanical interlock

and its mechanical properties were compared to the previous single-pin fusion joint. The

clinch joint had slightly lower shear strength but had more toughness compared to the

single-pin fusion joint.

v

Table of Contents

Acknowledgments iii

Abstract iv

Table of Contents v

List of Tables vii

List of Figures viii

1. Introduction 1

1.1Background 1

1.2 Motivation 1

1.3 Literature Review 2

1.4 Research Issues 3

1.4.1 Effect of corrosion and corrosion products on the properties of the joint 3

1.4.2 Effect of different pin structures and potential improvement of mechanical properties 4

1.4.3 Effects of corrosion and joint structure on microstructure 4

2. Corrosion Investigation of FSF 6

2.1 Introduction 6

2.2 Fabrication of FSF Samples 6

2.3 Corrosion Exposure Experiments 7

2.3.1 Corrosion Exposure Test Sites 8

2.3.1.1 HECO Waipahu 9

2.3.1.2 Lyon Arboretum 9

2.3.1.3 Marine Corps Base Hawaii (MCBH) 10

2.3.2 Corrosion Area Fraction Determination 10

2.3.3 Immersion Experiments 12

2.3.4 Lap Joint Shear Strength Testing 13

2.4 Results and Analysis 14

2.4.1 Chloride Candle Data 14

2.4.2 Area Fraction Results 15

2.4.3 Immersion Experiment Results 17

2.4.4 Lap Joint Shear Strength Results 19

2.4.4.1 Waipahu Test Site Results 20

2.4.4.2 Manoa Test Site Results 22

2.4.4.3 MCBH Test Site Results 24

2.4.4.4 8-Month Shear Strength Cross-site Comparison 26

2.5 Conclusions 27

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3. Friction Stir Experiments to Produce Clinch Joints 28

3.1 Introduction 28

3.2 Joint Design 29

3.3 Anvil Design 30

3.4 Fabrication of FS Clinch Joint Samples 31


3.5.1 Structure of the Clinch Joint 33

3.5.2 Failure Mechanism 34

3.5.3 Brazing Layer 35

3.5.4 Shear Strength Test Results 36

3.5.5 Comparison to Cold Worked Clinch Joints 38

3.6 Conclusions 39

4. Metallographic Analysis for Corrosion and Clinching 40

4.1 Introduction 40

4.2 Sample Preparation for Electron Microscopy 40


4.3.1 Metallographic Analysis of FS Clinch Joints 42

4.3.1.1 CL-07 42

4.3.1.2 CL-10 43

4.3.1.3 FS Clinching Joint Conclusions 44

4.3.2 Metallographic Analysis of Corroded FSF Joints 45

4.3.2.1 Standard Cross-Section Sample (ST) 45

4.3.2.2 Waipahu Cross-Section Samples (W1 and W5) 46

4.3.2.3 Manoa Cross-Section Samples (L1 and L5) 49

4.3.2.4 MCBH Cross-Section Samples (B1 and B5) 52

4.3.2.5 MCBH Surface Samples (B6) 55

4.4 Corrosion Mechanisms 59

4.5 Conclusions 62

5. Conclusions 63

6. References 65

vii

List of Tables

Table 2.1 Fixed FSF Parameters for Aluminum on Steel Forming 7

Table 2.2 Chemical Composition of Al-5182 7

Table 2.3 Summary of Electrodes and Immersion Solutions used for Potential Determination 13

Table 2.4 Area Fraction Corrosion Percentages for Steel at MCBH 15

Table 3.1 Optimal Parameters for Friction Stir Clinching Process 32

viii

List of Figures

Fig. 2.1: Setup of CNC for fabrication of FSF samples 6

Fig. 2.2 FSF Test Samples at Lyon Arboretum 8

Fig. 2.3 Wet Candle Chloride Apparatus 8

Fig. 2.4 Waipahu Test Site 9

Fig. 2.5 Manoa Test Site 10

Fig. 2.6 MCBH Test Site 10

Fig. 2.7 Area Fraction Scan of MCBH 4-Month Samples 11

Fig. 2.8 Immersion experiments to determine potentials of submerged coupons in solutions 12

Fig. 2.9 Tensile Testing Machine used for Lap Shear Joint Strength Determination 13

Fig. 2.10 Chloride Deposition Rates 14

Fig. 2.11 Close up of B2 and B6 steel coupons 16

Fig. 2.12 Potentials in Sodium Chloride Solution over time 17

Fig. 2.13 Potentials in Sodium Sulfate Solution over time 18

Fig. 2.14 Waipahu Samples Shear Strength Results 20

Fig. 2.15 Individual Waipahu Coupons after shear strength testing 21

Fig. 2.16 Manoa Samples Shear Strength Results 22

Fig. 2.17 Individual Manoa coupons after shear strength testing 23

Fig. 2.18 MCBH Samples Shear Strength Results 24

Fig. 2.19 Individual MCBH coupons after shear strength testing 25

Fig. 2.20 8-Month Shear Strength Results 26

Fig. 3.1 Cross Section of Single Pin FSF Joint 28

Fig. 3.2 (a) Schematic of the Clinching Process and (b) Cross-Section of a Clinch Joint 29

Fig. 3.3 Original Single Pin Anvil 30

Fig. 3.4 Comparison of the Schematics of FSF Single Pin vs FSF Clinch Joint 30

Fig. 3.5 FSF Clinch Anvil 31

Fig. 3.6 Clinch Anvil Setup on the CNC 31

Fig. 3.7 (a) straight clinch joint and (b) T-clinch joint 32

Fig. 3.8 (a) CLT-10 cross-section and (b) CL-09 cross-section with tool and anvil 33

Fig. 3.9 (a) CL-11 top of joint and (b) CL-11 back of joint 34

Fig. 3.10 Views of the aluminum and steel coupons of CL-11 after the shear strength test 34

Fig. 3.11 Brazing Layers on FS Clinch Joints 35

Fig. 3.12 FS Straight Clinch Joint Shear Strength 36

Fig. 3.13 FS T-Clinch Joint Shear Strength 37

Fig. 3.14 (a) Cold-Worked Clinch Strengths and (b) Friction Stir Clinch Strengths 38

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Fig. 4.1 FSF specimens before carbon coating process 41

Fig. 4.2 Sample CL-07 inside the EPMA chamber 42

Fig. 4.3 Backscatter image at 40x magnification of right side of CL-07 clinch joint 42

Fig. 4.4 Sample CL-10 inside the EPMA chamber 43

Fig. 4.5 Backscatter images of CL-10 joint 43

Fig. 4.6 Sample ST inside the EPMA chamber 45

Fig. 4.7 Backscatter images of ST joint 45

Fig. 4.8 (a) Sample W1 and (b) sample W5 inside the EPMA chamber 46

Fig. 4.9 SE and backscatter images of W1 joint 47

Fig. 4.10 EDS analysis of point 4 from the 250x magnification shown in Figure 4.7b 47

Fig. 4.11 Backscatter images of W5 joint 48


Fig. 4.13 (a) Sample L1 and (b) sample L5 inside the EPMA chamber 49

Fig. 4.14 Backscatter images of L1 joint 50

Fig. 4.15 EDS analysis of point 1 from the 190x magnification image shown in Figure 4.14b 50

Fig. 4.16 Backscatter images of L5 joint 51

Fig. 4.17 (a) Sample B1 and (b) sample B5 inside the EPMA chamber 52

Fig. 4.18 Backscatter images of B1 joint 52


Fig. 4.20 Backscatter images of B5 joint 53-54


Fig. 4.22 Surface of the brazing layers on (a) B6 aluminum and (b) B6 steel 55

Fig. 4.23 Backscatter images of B6 aluminum surface 56

Fig. 4.24 EDS analysis of points 1 and 2 from the 40x magnification shown in Figure 4.23a 56

Fig. 4.25 Backscatter images of B6 steel surface 57

Fig. 4.26 EDS analysis of points 1 and 2 from the 5000x magnification shown in Figure 4.25b 58

Fig. 4.27 Simonkolleite on galvanized steel surface 58

Fig. 4.28 Crevice corrosion schematic 59

Fig. 4.29 Stability diagram of Zn2+

60

Fig. 4.30 Pourbaix diagram of Aluminum 61

1

1. Introduction

1.1 Background

Friction stir forming (FSF) is a process designed to produce a high quality joint

between dissimilar materials, without using additional material like rivets, bolts, fluids, or

welding wires. FSF between aluminum and steel is feasible and has a higher joint

strength compared to the currently used method for joining called self-piercing riveting

(SPR) [Lazarevic et al., 2013]. Research is now aimed on how to further increase the

strength and toughness of the FSF joint as well as potential issues such as corrosion.

1.2 Motivation

As there is more push for use of better materials for automobiles to improve

performance and fuel efficiency or other tailor-made engineering properties,

manufacturers are now looking at other lightweight materials like aluminum and

magnesium, alongside the use of steel. Traditional welding techniques (like arc or gas

welding) are not suitable for these dissimilar materials because of difference in physical

characteristics (e.g., melting temperature and chemical incompatibility). The friction stir

forming process has shown potential in joining these dissimilar materials.

One of the major concerns with this process is the possibility of corrosion due to the

galvanic coupling of two different metals, aluminum and steel. Dissimilar metals, when

in electrical contact with each other, form galvanic couples which lead to corrosion of

one or both of the materials. With the aluminum-steel couple, aluminum will be expected

to corrode and steel will be protected. Even if the joining is feasible and the strength of

the joint is stronger than currently used methods, corrosion may decrease the strength

significantly as time goes on. Crevice corrosion is the potential mechanism seen to occur

due to the arrangement of the lap joint. To analyze this, the joints were studied in an

accelerated corrosive environment.

2

The other main concern with this process is that the mechanical properties are lacking

in certain areas. The current FSF joint is a 3 mm diameter single-pin and that holds the

two materials together. A different type of pin structure is needed to increase properties

of the joint such as strength and toughness. A clinch joint is a possible pin structure

which can provide those increases in properties. The clinch joint is a mechanical interlock

between two sheet metals and it will be a joint much larger than the current single-pin

joint. To address this, a clinch joint that forms with the friction stir process was

developed.

1.3 Literature Review

Self-piercing rivets are an effective way of combining high strength steel and

aluminum alloy sheets in automobile parts. Mori et al. [2006] found that joinability using

self-piercing rivets for an upper sheet of steel to a lower sheet of aluminum is higher than

the reverse configuration. This is due to the softer material being at the bottom, the

spread of the rivet leg is larger [Abe et al., 2006]. Dynamic strength and fatigue behaviors

for self-piercing rivets (SPR) and resistance spot welds (RSW) for joining several

aluminum alloys and steels were studied and compared [Sun et al., 2007]. Fatigue

strength for a SPR joint is greater than its RSW counterpart. SPR is the current method

being utilized for dissimilar metal joining; unfortunately, this method adds additional

mass.

Corrosion studies on friction stir welding (FSW) have been done for different

aluminum alloys. The welded samples of Al-2024-T3 were found to be more susceptible

to pitting corrosion than the original base alloy [Abbass et al., 2005]. Corrosion behavior

of Al-6061 joints using both friction stir welding and gas tungsten arc welding methods

were compared. FSW joints had better corrosion resistance than GTAW (gas-tungsten arc

welding) joints; however, both joints still showed poorer corrosion resistance than the

base metal [Fahimpour et al., 2012].

3

Clinching is a fastening method to form a mechanical interlock between two sheet

metals. It is used to replace spot welding, which is an issue when using aluminum as

aluminum is not spot welded easily. Clinching of aluminum and steel has been done;

although the joining range of aluminum alloys and high-strength steel is small due to low

ductility of the steel [Ahn et al., 2012]. This will be addressed by the prefabricated hole

in the steel work piece and introduction of the heat in the FSF process. Lee et al., [2010]

joined high strength steel and Al 6061 with a predrilled hole in the steel piece in

comparison with FE simulation of the joint. Lap shear strength for the joint was found to

be 2.56kN. Control of metal flow for the mechanical clinching process was done by

optimizing the shape of the die [Abe et al., 2012].

1.4 Research Issues

The goal of this research was to determine the effect of the galvanic coupling of

aluminum and steel as well as the effect of different pin structures on the mechanical

properties of the friction stir formed joint. The major research issues towards this

research were: (1) the effect of corrosion and corrosion products on the properties of the

joint; (2) the effect of different pin structures and potential improvement of the

mechanical properties; and (3) the effects of corrosion and joint structure on the grain

microstructure.

1.4.1 Effect of corrosion and corrosion products on the properties of the joint

The effect of corrosion on the strength of the joint needed to be quantified. When

dissimilar metals are in contact with each other, galvanic corrosion would occur due to

the difference in their potentials. With the aluminum-steel FSF joint, it was expected that

aluminum would cathodically protect the steel. The corrosion of each individual metal

would not be of much concern here; it would be the corrosion of the area where the joint

is formed. Since the GMW2 steel was hot-dip galvanized, zinc from the layer was also

expected to play a role in the corrosion product formation. The mechanical properties of

the corroded joint would be expected to decrease. Additionally, previous studies of

4

friction stir welding indicate that the welds have lower corrosion resistance than the base

metals. The rate of corrosion at the accelerated environment can be determined. This rate

of corrosion can then be extrapolated in a normal environment, as an estimation. In

Chapter 2 the effect of corrosion on the joint strength properties is discussed. An

experiment was setup to test the strength of the joint at different stages of corrosion in a

comparative analysis.

1.4.2 Effect of different pin structures and potential improvement of mechanical

properties

The current single pin structure is too small to generate sufficient joint strength. It is

thought that a joint geometry that is similar to a clinched joint with larger cross section

could be stronger and tougher. FSF single-pin structure of aluminum to steel offered

higher strength to self-piercing rivets [Lazarevic et al., 2013], but an increase in the

toughness is also warranted. The previous study also created a joint with a multi-pin

structure. The idea was to increase the overall neck area and increase the strength.

Generally, a larger joint would be stronger and tougher; however, the previous four-pin

structure was not sufficient. There were four different holes needed and the mechanical

interlocking wasn’t pronounced. The structure of a clinch joint, using only one hole but

of much larger diameter than the four-pins is the new design used to address the

interlocking aspect. This method currently employs a cold working process. Introduction

of the FSF process aims to not only form a material bond but also a mechanical interlock

as well. In Chapter 3, the effect of the new joint structure was discussed. The geometry

needed to be experimentally developed for joint structure and strength.

1.4.3 Effects of corrosion and joint structure on microstructure

To gain more understanding of the effects of both corrosion and clinching joints to

the mechanical properties, metallographic techniques were needed to analyze both. The

expected corrosion products of aluminum and steel are their respective oxides, aluminum

oxide (Al2O3) and iron oxide/rust (Fe2O3). Al2O3 can act as a passive layer which protects

5

the aluminum from corrosion while Fe2O3 cannot. Zinc is also present on the steel

surface, as the steel underwent hot-dip galvanization. Zinc can also form several

corrosion products like zinc oxide (ZnO) and zinc carbonate (Zn5(CO3)2(OH)6). The

corrosion products that can form at the joint where aluminum, zinc and steel bonded

together are of interest, and will be inspected using metallography. Clinching joints will

also be inspected using metallography to understand the composition of the much larger

joint as well as inspect how the flow of material to form the clinch joint is generated.

Chapter 4 presents the metallurgical analysis of the results from Chapters 2 and 3.

6

2. Corrosion Investigation of FSF

2.1 Introduction

This chapter focuses on the effects of corrosion on the mechanical properties of

friction stir formed single pin joints. The main goal of this investigation is achieved by

exposing the FSF samples to different types of corrosive environments over time.

Documentation of their performance was done afterwards. Supporting experiments in the

laboratory were also performed in order to further explain the events ongoing in the

outside environment.

2.2 Fabrication of FSF Samples

The FSF joints were prepared at the Engineering Machine Shop at Holmes Hall,

University of Hawaii at Manoa. The CNC machine used was a Bridgeport Discovery 308,

capable of up to 6000 rpm. Figure 2.1 shows the setup for the CNC machine.

(a) (b)

Fig. 2.1: Setup of CNC for fabrication of FSF samples (a) CNC machine with measurement equipment and

(b) close up of FSF fixture and tool

There were several parameters that could be varied with the machine for producing a

FSF joint: spindle speed, plunge speed, plunge depth, tool diameter and bottom sheet hole

diameter. From the previous study of Lazarevic et. al., [2013] the parameters were

7

determined and fixed for these experiments. Table 2.1 shows these fixed parameters for

the fabrication of all the specimens.

Table 2.1 Fixed FSF Parameters for Aluminum on Steel Forming

Spindle Speed 2250 rpm

Plunge Speed 2.54 mm/min

Plunge Depth 0.9 mm

Cavity Depth 0.4 mm

Tool Diameter 14 mm

Bottom Sheet

Hole Diameter

3 mm

The materials used for the samples for the corrosion experiments were 0.7 mm thick

GMW2 steel (bottom sheet) and 1.0 mm thick 5182 Aluminum (top sheet). Table 2.2

shows the composition of the aluminum alloy 5182.

Table 2.2 Chemical Composition of Al-5182 (ASM Metals Handbook Vol. 2)

Al Si Fe Cu Mn Mg Cr Zn Ti Other

Al-5182 Rem. 0.20 0.35 0.15 0.20-0.50 4.0-5.0 0.10 0.25 0.10 0.20

2.3 Corrosion Exposure Experiments

Several samples were produced to be deployed at different test sites for the corrosion

exposure experiments. Figure 2.2 shows an example of a mounted set for one of the test

sites. There will be four samples per mount. These mounts, as well as the test site racks,

are designed and maintained by the Hawaii Corrosion Laboratory. The exposure

timeframe for the samples are split into two phases: 4-month exposure time and 8-month

exposure time. Samples will be monitored once a month (once every two months for

Waipahu) and pictures would be taken for documentation by the HCL Team. After each

exposure time phase, the samples were removed and further tests were performed with

them. The chloride candle data, area fraction of corrosion and electrode potentials of the

metal coupons were analyzed to study the effect of corrosion.

8

Fig. 2.2 FSF Test Samples at Lyon Arboretum

2.3.1 Corrosion Exposure Test Sites

Three different exposure test sites were selected for their differing amount of

corrosion levels: they are HECO (Hawaiian Electric Company) Waipahu, Lyon

Arboretum, and Marine Corps Base Hawaii. For each test site, a chloride candle is setup

which measures the amount of chlorides suspended in the atmosphere. This candle has a

wet wick of known diameter and surface area and particles of salt or spray are trapped by

the wet wick and retained there. The wick is kept wet by the reservoir of water below.

The candles are collected and replaced on set intervals and quantitative determination of

the cations and anions is performed. Figure 2.3 shows the schematic of a chloride candle.

Fig. 2.3 Wet Candle Chloride Apparatus

9

2.3.1.1 HECO Waipahu

The Waipahu test site is found in a mildly industrial, relatively dry area in Waipahu,

approximately 14 miles from downtown Honolulu. This property is owned by Hawaiian

Electric Co. (HECO). Minimal rainfall, dry climate and different types of corrosive salts

are key factors in this area. This site is expected to have the least amount of corrosion

effect on the samples. Fig 2.4 shows the Waipahu test site setup.

Fig. 2.4 Waipahu Test Site

2.3.1.2 Lyon Arboretum

The Lyon Arboretum test site is deep in the rainforests up in the Manoa area,

located 6 miles from downtown Honolulu, and represents a tropical rainforest

microclimate. This area has the most amount of rainfall (an average of 165 inches/year),

has a humid climate and very low amount of corrosive salts. Figure 2.5 shows the Lyon

Arboretum test site.

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Fig. 2.5 Manoa Test Site

2.3.1.3 Marine Corps Base Hawaii (MCBH)

The MCBH test site is situated at Kaneohe Bay, on the north side of Oahu. Varying

temperatures, saltwater mist spraying onto the samples and a very high concentration of

corrosive salts from the seawater are the major factors here. This is a very severe marine

environment. This site was expected to have the highest amount of corrosion effect on the

samples. Figure 2.6 shows the MCBH test site.

Fig. 2.6 MCBH Test Site

2.3.2 Corrosion Area Fraction Determination

After the samples were taken back from the sites, individual samples were scanned

for high resolution close-ups pictures of the visual corrosion. The area fraction of

11

corrosion for each steel plate was determined by a proprietary program used in the

Hawaii Corrosion Lab. Area fraction of corrosion would be reported as percentages of a

selected area in the picture. The program has set thresholds to what is considered

“corroded” e.g., red areas in the picture. Fig 2.7 shows the scanned pictures for the test.

The samples from the Waipahu and Manoa sites were not scanned due to them not

showing any visible corrosion on the steel coupons.

(a) from top to bottom B4, B3, B2, B1 (b) from top to bottom B4, B3, B2, B1

(c) from top to bottom B5, B6, B7, B8 (d) from top to bottom B5, B6, B7, B8

Fig. 2.7 Area Fraction Scan of MCBH 4-Month Samples (a) front view and (b) back view and 8-Month

Samples (c) front view and (b) back view

12

2.3.3 Immersion Experiments

Fig. 2.8 Immersion experiments to determine potentials of submerged coupons in solutions

Additional immersion experiments were performed in the laboratory to determine the

galvanic potentials of the materials used. Fig 2.8 shows one of the tests being performed.

Four coupons, two each of GMW2 steel and Al-5182 were to be tested. Each coupon was

converted into an electrode by attaching copper wires to them using silver epoxy. The

immersion solutions were 3.15 wt. % sodium chloride (NaCl) and 0.5 M sodium sulfate

(Na2SO4). Each coupon was then immersed for a couple of weeks and potential

measurements were taken at roughly the same time each day. The potentials were

measured against a standard calomel reference electrode.

After the first set of runs of the immersion experiments, inspection of the solutions

showed that the zinc coating on the galvanized steel was corroding too fast in the sodium

sulfate solutions. A second set of runs were done using a lower concentration of sodium

sulfate (0.05 M instead of 0.5 M). Another two coupons each of steel and aluminum were

turned into electrodes for this run. The individual coupons were tested in the solutions

against the standard electrode like the first run; they were also paired to each other and

tested for the galvanic potentials and current between the two metals. Table 2.3

summarizes the electrodes and their immersion solutions.

13

Table 2.3 Summary of Electrodes and Immersion Solutions used for Potential Determination

First Run Second Run

3.15% NaCl solution 0.5 M Na2SO4 solution 3.15% NaCl solution 0.05 M Na2SO4 solution

Al – Calomel Al – Calomel Al – Calomel Al – Calomel

Steel – Calomel Steel – Calomel Steel – Calomel Steel – Calomel

Al - Steel Al- Steel

These immersion solutions correspond to the outside corrosion test sites; the NaCl

solution corresponds to the MCBH site (for its presence of chlorides in saltwater) while

the sulfate solution is for the other two sites (since there would be a lesser concentration

of chlorides there). The potential and current data gathered from this experiment would

be used to correlate the behavior of the samples in the external test sites.

2.3.4 Lap Joint Shear Strength Testing

The joint strength of three samples per site per exposure time phase were determined.

The test was carried out by using an INSTRON tension-testing machine (Model # 4206-

004). This is shown in Figure 2.9. The samples were placed in the grips of the machine

and were pulled apart at a rate of 3 mm/min. Shims were used on both grips to minimize

any potential bending during the test due to the orientation of a lap joint specimen. The

maximum force was recorded by the machine and shear strength was reported in

Newtons (N). Extension was also recorded and measured in millimeters (mm).

Fig. 2.9 Tensile Testing Machine used for Lap Shear Joint Strength Determination

14

2.4 Results and Analysis

2.4.1 Chloride Candle Data

Fig. 2.10 Chloride Deposition Rates

Figure 2.10 shows the chloride deposition rates (in mg/m2/day) at specific sites for the

time period starting from September 2013 to February 2014 (a period of 6 months). The

samples for this research were exposed from September 2013 to May 2014 (4-month and

8-month groups) so data for two months (March and April) are missing. This is a

limitation that cannot be addressed since the chloride data samples are only measured

every six months. Looking at the data, the Manoa site has the least amount of chlorides

(maximum at 1.59 mg/m2/day) with Waipahu having approximately 4 times that amount.

As expected, since the MCBH site is near the ocean, it has the largest amount of chlorides

present, three orders of magnitude more than the other two sites. The approximate area of

15

one of the specimens is 150 cm2. If we take into account just the minimum amount of

chloride deposition from MCBH for the month of October (929.7 mg/m2/day), the total

amount of chlorides deposited on one of samples is about 13.9 mg. The amount of sulfate

ions were measured as well, since it is also a hygroscopic salt. In atmospheric corrosion,

these salts are a factor in the amount of water adsorbed on the surface of the materials

being tested. The effect of these salts would be determined later in the immersion

experiments.

2.4.2 Area Fraction Results

In the previous section, Figure 2.6 shows the scans for the samples exposed at

MCBH. The scanned pictures were then run through a proprietary program by the Hawaii

Corrosion Laboratory. The program lets you select a certain area of a picture and the

percentage of corrosion is calculated over that area. There is a threshold set to determine

red coloration of iron oxide (Fe2O3) over the picture and that is considered the corroded

area. Table 2.4 shows the area fraction percentages for steel at MCBH. The area fraction

percentages for the Manoa and Waipahu sites were not determined as there were no

visible signs of rust formation.

Table 2.4 Area Fraction Corrosion Percentages for Steel at MCBH

Sample Exposure Duration Front Back

B1 4 months 6.8 0.9

B2 4 months 21.1 0.5

B3 4 months 9.3 0.0

B4 4 months 9.9 0.0

B5 8 months 100 15.2

B6 8 months 100 45.9

B7 8 months 93.1 11.6

B8 8 months 89.6 26.4

16

The samples with the largest area fraction percentages, for the exposure durations of

4-months and 8-months, were B2 and B6 respectively. Figure 2.11 shows the close up

views of the area where these percentages were determined.

(a) B2 steel front side (b) B2 steel back side

(c) B6 steel front side (d) B6 steel back side

Fig. 2.11 Close up of B2 and B6 steel coupons (orientated top to bottom as shown)

These diagrams show most of the corroded area to be on the lower edge, which is due

to the fact that the coupons were oriented top to bottom, as shown in the previous figure,

on the mounts. The flow of the saltwater spray and the salts move downward and settle at

the lower end. For the front side pictures, the aluminum is covering part of the steel so

the area of interest is just the uncovered area on the right. The area situated in between

the two coupons would be studied later after the samples have been tested for their shear

strengths.

The main thing to note with this test is that it only shows the window in the middle of

the whole corrosion process of galvanized steel. Galvanized steel is composed of two

metals, zinc and steel, and both of them underwent corrosion simultaneously. For an area

corrosion percentage of 0%, as in the case of the Waipahu and Manoa samples, it does

not mean that corrosion has not occurred: the zinc layer may have been already corroding

but there was still enough zinc to protect the steel and no rust was visible. Similarly, for

samples with an area corrosion percentage of 100%, corrosion was still possible. It means

that the area of interest was composed of iron oxide / rust (thus the 100% red result) and

that the entire protective zinc layer was gone. The steel could continue to corrode until

17

the remaining iron mass disintegrates. The definite conclusion drawn is that sometime

between the 4-month and 8-month exposure at MCBH, the protective zinc layer corroded

away for samples B5 and B6 and was almost gone for samples B7 and B8.

One of the shortcomings of this program is the sensitivity of the threshold. For the B6

front sample, based on the picture, there are still some white spots visible to the naked

eye (which could be an oxide of zinc) but the program determined it was 100% corroded.

The only way to get specific corrosion rates from this test is knowledge of the mass of the

original steel specimen and the mass of the applied zinc layer. A mass loss method could

then be utilized.

2.4.3 Immersion Experiment Results

Figures 2.12 and 2.13 below show the potential measurements over time of the

aluminum and steel coupons in sodium chloride and sodium sulfate solutions,

respectively. The reason for testing in two different solutions was to see the behavior of

the metals independently to chloride and sulfate ions, which were together in the

atmospheric environments.

Fig. 2.12 Potentials in Sodium Chloride Solution over time

18

Fig. 2.13 Potentials in Sodium Sulfate Solution over time

In the sodium chloride solution, the values range from -1020 to -1115 mV for Al, -

1051 to -1076 mV for steel(Zn) and -1053 to -1082 mV for Al-steel(Zn) coupled

together. Except for that one instance of the potential dipping to -1082 mV for the Al-

steel(Zn) couple, the potentials for the couple and steel alone have little difference

between them. The same trend can be seen in the sodium sulfate solution, where the

potential of Al-steel(Zn) couple has also little difference to the potential of the steel

coupon alone as well. Since aluminum is the more active metal when coupled to steel, the

assumption was that aluminum would protect the steel from corrosion. This would show

if the coupled potential values move towards the more negative potential of aluminum.

Due to this result, it could be concluded that aluminum was not acting as an efficient

sacrificial anode to steel. Since zinc was still present due to the galvanization process

done on the steel, it was protecting the steel as the anode (which was the original purpose

of galvanization). Also, since the behavior of the metals in both solutions are the same,

the effect of sulfates can now be neglected since majority of the ions present in the

outdoor test sites are chlorides.

19

2.4.4 Lap Joint Shear Strength Results

There were three different test sites, each with two different exposure periods (4

months and 8 months) for a total of six groups of samples. Each group had three samples

for shear strength testing and one for metallography. Based on the general environmental

conditions for the three different test sites, it was predicted that Waipahu would have the

least corrosion damage and MCBH would have the most. It was also expected that the 4-

month samples would be stronger than the 8-month samples regardless of test site.

20

2.4.4.1 Waipahu Test Site Results

Fig. 2.14 Waipahu Samples Shear Strength Results; green is the baseline strength of 4500 N for a non-

corroded sample, blue for 4-month samples (dark blue as average) and red for 8-month samples (dark red

as average)

Figure 2.14 shows the shear strength values of samples W2 to W8 as compared to a

standard non-corroded sample benchmark. Immediately noticeable are the strengths of

the first two samples, W2 and W3, which are lower than the strengths of any of the 8-

month samples. Had the two samples strengths been similar to W4’s, then there would

have been a proper trend of slow strength depreciation of an average of 4000 N at 4

months to about 3784 N at 8 months. All individual samples were photographed and

tested for their shear strengths within one week of recovery from the sites and no damage

occurred to the samples prior to testing. Figure 2.15 shows the state of these samples after

shear testing. Visual inspection of the samples after also showed no signs of damage.

There does appear to be some white deposition around the brazing joint area, where the

aluminum and steel were in contact.

21

(a) W2 aluminum

(c) W3 aluminum

(e) W4 aluminum

(g) W6 aluminum

(i) W7 aluminum

(k) W8 aluminum

(b) W2 steel

(d) W3 steel

(f) W4 steel

(h) W6 steel

(j) W7 steel

(l) W8 steel

Fig. 2.15 Individual Waipahu Coupons after shear strength testing. Samples do not have any major cracks

or other signs that would indicate major damage that would have caused the 4-month low strength results.

22

2.4.4.2 Manoa Test Site Results

Fig. 2.16 Manoa Samples Shear Strength Results; green is the baseline strength of 4500 N for a non-


as average)

Figure 2.16 shows the shear strength values of samples L2 to L8 also compared to a

standard non-corroded sample benchmark. For this group, the 4-month samples’ strengths

are also way lower than the 8-month samples. This is actually a more severe case than the

Waipahu group because there is no 4-month sample even close to the strengths of the 8-

month samples. Figure 2.17 shows the state of these samples after shear testing. Similar

to the Waipahu samples, there also appears to be some form of white deposition around

the brazing joint area. The rest of the samples do not have visible signs of damage. The

braze joint of L3 does appear to have some damage as there are ragged edges around the

area, which could have contributed to the low strength.

23

(a) L2 aluminum

(c) L3 aluminum

(e) L4 aluminum

(g) L6 aluminum

(i) L7 aluminum

(k) L8 aluminum

(b) L2 steel

(d) L3 steel

(f) L4 steel

(h) L6 steel

(j) L7 steel

(l) L8 steel

Fig. 2.17 Individual Manoa coupons after shear strength testing. Most of the samples do not have any major

cracks or other signs that would indicate major damage that would have caused the 4-month low strength

results. The braze joint of L3 on aluminum (2.17c) has some additional jagged material around the area and

is of particular interest.

24

2.4.4.3 MCBH Test Site Results

Fig. 2.18 MCBH Samples Shear Strength Results; green is the baseline strength of 4500 N for a non-


as average)

Figure 2.18 shows the shear strength values of samples B2 to B8 also compared to a

standard non-corroded sample benchmark. For this group, the predicted trend of 8-month

samples being much less weaker than the 4-month samples was observed. B6 and B7

both have strengths of 0 N because even before they were removed from their mounts,

the coupons already broke apart. Due to this catastrophic failure, 8-month exposure at

this site is considered the maximum threshold for corrosion testing of friction stir formed

joints. Figure 2.19 shows the state of samples of the coupons after separation. The white

deposition around the brazing joint is much more pronounced in this site’s samples. It is

also worth noting that where the steel area is covered and in contact with aluminum, rust

has not completely formed for B2, B3 and B4. For B6, B7 and B8 steel coupons, the rust

has started to form and creep toward the joint area. For B6 and B7 coupons, the rust has

moved very close to the brazing joint area which indicated that corrosion has reached that

level. This may explain why these particular samples already broke even before they

25

were tested for strength. As such, B6 was selected for additional metallographic analysis

because of its possible insight to the catastrophic failure.

(a) B2 aluminum

(c) B3 aluminum

(e) B4 aluminum

(g) B6 aluminum

(i) B7 aluminum

(k) B8 aluminum

(b) B2 steel

(d) B3 steel

(f) B4 steel

(h) B6 steel

(j) B7 steel

(l) B8 steel

Fig. 2.19 Individual MCBH coupons after shear strength testing. Most of the samples have significant rust

formation as well as white deposition around the brazing joint area. B6 and B7 in particular have major

corrosion creeping into the joint area which could explain the immediate failure of the joints prior to

testing.

26

2.4.4.4 8-Month Shear Strength Cross-site Comparison

Fig. 2.20 8-Month Shear Strength Results; green is the baseline strength of 4500 N for a non-corroded

sample, red for 8-month samples (dark red as average)

Figure 2.20 shows the comparison of the 8-month shear strength results between the

three sites. From the chloride candle data, the sites are ranked from least amount to most

amount of chloride concentrations: Manoa, Waipahu and MCBH. Manoa has least

amount of chlorides and have the largest average strength. Conversely, MCBH has the

largest amount of chlorides and have the lowest average strength of joints. It can be

correlated that the higher the chloride concentrations, the more corrosive the environment

is and is more detrimental to the mechanical properties of the friction stir formed joints.

The last step now involves determining what the corrosion products are using

metallography. Identifying those corrosion products will then allow the determination of

the corrosion mechanisms that occur on these joints. This will be discussed in chapter 4.

27

2.5 Conclusions

Several different simultaneous tests were performed and correlated to determine the

effect of different atmospheric environments to the mechanical properties of friction stir

formed joints.

Area fraction percentages of rust formation on steel pieces are a good estimation of

how much corrosion has occurred on the samples. It was determined, for the MCBH site,

that most of the zinc has corroded from the galvanized steel because most of the samples

are already 100% covered in rust. However, the zinc coating mass on the galvanized steel

was unknown and the corrosion rate could not be determined.

Immersion experiments determine how active a metal is and its relative activity to a

galvanically coupled metal. Al-5182 was found to be more active than galvanized steel.

However, the Al-galvanized steel couple’s potential was almost the same as the

galvanized steel potential alone, meaning aluminum wasn’t protecting galvanized steel at

all. The reason for this is the existence of the zinc coating on the steel. In the friction stir

formed joint then, zinc is an effective sacrificial anode and the Al-5182 is a poor

sacrificial anode.

The determination of the amount of salts was done at each of the specific test sites

using chloride candle data. The presence or absence of specific salts would determine the

corrosion products that would form on these samples. The site with the highest amount of

chlorides was found (and predicted) to be at MCBH and the samples exposed there had

the lowest shear strengths of all the samples. As for the 4-month samples which

performed worse than the 8-month samples (Waipahu and Manoa), there is currently not

enough evidence to show the failure was due to corrosion or during some other issue

during the fabrication process.

At the outset, the mass of the zinc coating on the galvanized steel needs to be

determined to be able to determine specific corrosion rates for a test site. Immersion

experiments could be performed on friction stir formed samples and not just single

coupons to establish the galvanic effect of the coupling better.

28

3. Friction Stir Experiments to Produce Clinch Joints

3.1 Introduction

The previous research done regarding the friction stir forming process between

aluminum and steel by Lazarevic et al. [2013] has produced a friction stir formed joint

with a single pin design. The design proved to be a strong joint, having lap shear

strengths ranging from 4 to 5 kN. Much of the strength is from the formed brazed layer in

between the aluminum and steel pieces and is composed mostly of zinc, which is present

on the surface of the galvanized steel. The design of this new friction stir clinch joint is

believed to increase the toughness properties as an effect of the mechanical interlock.

Figure 3.1 shows the schematic for the cross-section of the single pin FSF joint from

Lazarevic et al. [2013]. The major points of interest that were changed and studied for the

clinch joint are the brazed layer (G), the neck (D) and the head (E). Ahn et al. [2012]

changed their approach to the clinching process by making a prefabricated hole in the

steel coupon as well, similar to the friction stir forming process. Since it employed

similar configurations with the prefabrication of the hole in the lower coupon, their

results will be used as a comparison for the friction stir clinching process.

Fig. 3.1 Cross Section of Single Pin FSF Joint: (A) steel coupon, (B) aluminum coupon, (C) shoulder, (D)

neck, (E) head, (F) flash, (G) brazed joint and (H) core

29

3.2 Joint Design

A clinch joint is produced by the conventional clinching process, which is a cold

working process. The two metal sheets are placed over a die and a tool will punch the two

sheets downward into the die and form a mechanical interlock between them. The process

does not involve tool rotation. Figure 3.2(a) shows the schematic for the clinching

process and Figure 3.2 (b) shows an example of a clinch joint. St is the residual base

thickness and D is the diameter.

(a) (b)

Fig. 3.2: (a) Schematic of the Clinching Process and (b) Cross-Section of a Clinch Joint: St – residual base

thickness, D - diameter

This is comparable to the friction stir forming process because there is no additional

material added to form the joint (like self-piercing rivets or traditional welding); it is only

composed of the two metal sheets. A major drawback with this cold working process is

that due to the nature of the two dissimilar materials being joined, the joining range for

aluminum and steel is limited. This is because of the low ductility of the steel which

limits the amount of deformation that can occur to form the joint [Ahn et al. 2012]. With

the friction stir clinching process, the heat generated will “melt” the aluminum and make

that form the mechanical interlock; the steel will not be deformed.

30

3.3 Anvil Design

To create this new FSF clinch joint, a different kind of anvil had to be designed. The

original anvil was just a circular cavity with 0.6 mm depth with a diameter of just slightly

larger than the 3.0 mm pre-punched hole in the steel work pieces. Figure 3.3 shows the

original anvil.

Fig. 3.3 Original Single Pin Anvil

A different tool also needed to be accommodated with the new anvil. The new tool

was made of silicon nitride (Si3N4). Figure 3.4 shows the schematic for the clinch anvil

design compared to the previous single-pin design.

Fig. 3.4 Comparison of the Schematics of FSF Single Pin (left) vs FSF Clinch Joint (right)

The design involves a pin protruding at the top of the anvil and a circular cavity

around this pin. The height from the bottom of the cavity to the top of the pin is 1.3 mm

and from the bottom of the cavity to the anvil surface is 0.6 mm. The pin is protruding

0.7 mm because the thickness of the bottom steel sheet used in these experiments is 0.7

mm as well. It was decided that the diameter of that hole in the steel coupon should be

31

close to the size of the nitride tool to allow the largest amount of material flow. This then

dictated the dimensions of the canal around the pin like the diameter and depth. A lathe

was used to create the cavity for this anvil. Figure 3.5 shows the final clinch anvil

produced, which was used to fabricate the clinch joint samples.

Fig. 3.5 FSF Clinch Anvil

3.4 Fabrication of FS Clinch Joint Samples

The FSF clinch joints were prepared using the same CNC machine used for the FSF

corrosion samples. Figure 3.6 shows the CNC fixture with the clinch anvil.

Fig. 3.6 Clinch Anvil Setup on the CNC

The materials used for this process were the same: 0.7 mm GMW2 steel and Al-5182,

but of a different thickness of 1.4 mm. Since the cavity in the anvil increased in volume

and additional material was needed to fill in that volume, a thicker aluminum sheet was

utilized. To allow the flow of the top material into the cavity, a prefabricated hole was

introduced into the lower steel sheet. Preliminary experiments were done to determine the

32

optimal machining parameters for the clinching process like the spindle speed, plunge

speed, plunge depth, etc. The parameter that was tested the most was the plunge depth,

which had the following values during the experimentation: 1.0, 1.25, 1.5, 1.7, 1.9, and

2.1 mm. This was due to the fact that the flow of the material was unknown in the new

anvil. Upon testing the strengths of the preliminary joints, it was found that the best

plunge depth was at 1.9 mm. Table 3.1 shows the optimal parameters used for all the

succeeding clinch joints.

Table 3.1 Optimal Parameters for Friction Stir Clinching Process

Spindle Speed 3000 rpm

Plunge Speed 5.0 mm/min

Plunge Depth 1.9 mm

Tool Diameter 11.1 mm

Bottom Sheet

Hole Diameter

11.1125 mm (7/16 in.)

Two types of clinch joints were produced: straight clinch and T-clinch. The straight

clinch utilizes a steel piece of about 38 mm wide while the T-clinch has additional

flanges made of the same steel material on either side, which effectively doubled its

width. This increase in width was to determine if the joints’ strength was affected at all

by the deformation of the steel. Figure 3.7 shows the two types of clinch joints produced.

(a) (b)

Fig. 3.7 (a) straight clinch joint and (b) T-clinch joint

33

Five joints of each type were produced; three of them were tested for shear strength

and the other two were for metallographic analysis, randomly selected. The straight

clinch joints were labeled CL-09 to CL-13 while the T-clinch joints were labeled CLT-04

to CLT-08.


To determine how the joint performed, three specific areas of particular interest were

noted: the brazing layer, the head and the neck. The performance of these joints would be

explained by those three specific parts of the joint.

3.5.1 Structure of the Clinch Joint

Prior to metallographic analysis, one of each type of clinch samples was cross-

sectioned and reassembled in between the tool and anvil. Figure 3.8 shows the

reassembly of these joints. Notice that centering has large influence on how the

aluminum flows into the cavity of the anvil.

(a) (b)

Fig. 3.8 (a) CLT-10 cross-section and (b) CL-09 cross-section with tool and anvil

Upon observation of these cross-sections, there is indeed formation of the head of the

joint, where the aluminum is curling around the edge of the steel and under it. Due to the

34

large volume of aluminum flowing into the cavity, the neck part of the joint is just a very

thin layer and part of it adhered to the steel anvil. This is more pronounced in the

following close-up pictures of CL-11 below. The neck has adhered to the anvil and

peeled off thus leaving a substantial hole in the joint. The formation of the head is still

intact and can be seen clearly in the picture of the back of the joint. The aluminum has

flowed all around and curled under the steel plate to form the interlock. As for the third

point of interest, the brazing layer can only be observed after the samples have been

tested for their shear strengths, when the two metal sheets separate.

(a) (b)

Fig. 3.9 (a) CL-11 top of joint and (b) CL-11 back of joint

3.5.2 Failure Mechanism

The following figures below show the condition of the CL-11 joint and both the

aluminum and steel coupons after the shear strength test.

(a) CL-11 aluminum coupon (b) CL-11 joint on aluminum coupon

(c) CL-11 steel coupon (d) CL-11 hole on steel coupon

Fig. 3.10 Views of the aluminum and steel coupons of CL-11 after the shear strength test (2079 N)

35

Inspection of the aluminum coupon shows some degree of formation of the brazing

layer, albeit not as pronounced as single pin joints. Looking at the steel coupon, the

original hole has enlarged and the edge part of the coupon has bent outward. This type of

failure is typically seen in bolted connections and is called bearing failure of the plate.

The clinch joint was stronger than the steel coupon thus the steel deformed and the hole

elongated. Whereas the single pin joint has a very small hole diameter compared to the

tool and most of the strength of the joint comes from the brazing layer, the clinch joint

has a hole with the same diameter as the tool itself. All the other formed clinch joints are

also that way. Based on the failure mechanism, the strength of the joint is more

dependent on the formation of the mechanical interlock. The brazing layer has a lesser

effect on the strength of the clinch joint compared to the single pin one. The brazing layer

might be more pronounced in the area where the curling of the aluminum occurs around

the steel piece. To better understand the formation of the clinch joint, metallography was

performed on one of the straight clinch joints and one of the T-clinch joints.

3.5.3 Brazing Layer

(a) (b) (c) (d) (e) (f)

Fig. 3.11 Brazing Layers on FS Clinch Joints (a) CLT-05 (b) CLT-06 (c) CLT-08 (d) CL-11 (e) CL-12 and

(f) CL-13

Figure 3.11 shows the brazing layers formed between the aluminum and the steel

coupons. The amount of zinc on the surface of the steel may have an effect on the amount

of brazing seen in each of the different joints. For CLT-06, the brazing layer is much

larger but the clinch did not form too well (seen at the lower left side). This also seems

like the similar case for CL-12 and CL-13. For the case of CLT-08, there was very little

brazing except underneath the clinch joint itself. Centering of the tool with the hole may

36

be a large factor as the steel hole diameter and tool diameter are very close in dimension.

Brazing will not occur if proper centering of the tool is achieved.

3.5.4 Shear Strength Test Results

The shear strength test results of the straight clinch joints and the T-clinch joints are

shown in Figures 3.12 and 3.13 respectively. A standard single pin sample, ST-18, was

also plotted alongside in the force vs extension curves for comparison. The maximum

strength for ST-18 was 3065 N at 1.315 mm extension. Also, toughness was calculated to

be 5553 N/mm.

Fig. 3.12 FS Straight Clinch Joint Shear Strength

For the case of the straight clinch joints, the maximum strengths achieved were:

CL-11 was 2079 N at 0.908 mm extension; 7313 N/mm (toughness)



37

Fig. 3.13 FS T-Clinch Joint Shear Strength

For the case of the T-clinch joints, the maximum strengths achieved were:

CLT-05 was 2424 N at 1.683 mm extension; 6053 N/mm (toughness)



In spite of the large spike at the beginning for the standard single pin joint, the area

under the load-displacement curves for the clinch joints is slightly larger, thus showing an

increase in the toughness of the joint. It is also much more pronounced with the T-clinch

joints. There is also a slight increase for the maximum strength in the T-clinch joints

compared to the straight ones. The reason for this is that the additional width of the

flanges of the steel allows the steel to distribute the stresses more and thus increase the

strength and toughness. This should have little effect when larger sheets of aluminum and

steel are joined together and not just smaller strips of coupons. The general behavior of

the clinch joints as seen in the graphs, while undergoing shear forces, is that the load hits

a maximum value and then the strength slowly tapers off with a small slope. This is

unlike the single-pin samples in which after it hits the maximum value, it then drops

38

straight down significantly before undergoing the elongation process. This spike for the

single-pin samples indicate the strength of the brazing layer, followed by an immediate

drop. The rest of the graph shows the strength of the small mechanical interlock formed

with the head/neck formation (approximately 1400 N). Comparing this to the strengths of

the mechanical interlock formed with the clinch joint, this is lower. There is also much

more elongation for the clinch joints showing that this type of joint maximizes the use of

the ductility of the lower plate. This shows that the joint is stronger than the two base

materials used, which is desirable.

3.5.5 Comparison to Cold Worked Clinch Joints

The research by Ahn et al. [2012] showed a cold working process of producing clinch

joints, with a similar goal of forcing the aluminum to form a clinch joint under the steel.

The materials used were SPFC 440 (steel), 1.6 mm in thickness, and Al6061, 2 mm in

thickness. There was also a prefabricated hole in the steel coupons they used. Figure 3.14

shows the load-displacement curves for the joints from (a) the cold worked clinch process

and (b) the friction stir clinch process.

(a) (b)

Fig. 3.14 (a) Cold-Worked Clinch Strengths (from Ahn et al., 2012) and (b) Friction Stir Clinch Strengths

The maximum shear strengths of the cold worked and friction stir clinch joints are

comparable ranging from about 2.0 kN to 2.75 kN. Given that the coupons used for the

friction stir clinch process were thinner, the joint strengths may even be larger if same

39

thicknesses are used. The immediate apparent difference is noticeable in toughness.

Toughness for the best cold worked joint (specimen #5) was approximated to be 2.66

kN/mm; the comparable joint from the friction stir clinch joints was CLT-05with a

calculated toughness of 6.05 kN/mm The maximum elongation achieved by the cold

worked joints is 1.4 mm; for the friction stir joints, it was nearly 12 mm. This may be due

to the difference in ductility between SPFC 440 and GMW2 steel. Additionally, the types

of aluminum used were different. A better comparison could be made if the friction stir

clinching process is applied to the same type of materials used in the cold working clinch

process.

3.6 Conclusions

A friction stir clinching joint was developed by creating a different anvil which

allows the aluminum to flow under the steel coupon and form a mechanical interlock.

This new method of joining aluminum and steel has shown potential in creating a much

tougher joint than the single pin joints. The strength of the joint is mostly from the

interlock but the brazing layer may still have some slight effect on the material

properties. The optimal parameters have been determined and are reproducible. The

failure mechanism is similar to those of bolted connections, where bearing failure of the

plate (in this case coupon) is prevalent. The strength of the friction stir clinch joints is

comparable to the cold worked clinch joints but have much higher toughness. The

toughness of the friction stir clinch joints is also much higher than for the single pin

friction stir formed joints.

The effect of proper centering of the hole with the tool may need to be taken into

account for future fabrication of FS clinch joints. The random orientation of the brazing

layer seen on the pieces may be due to the improper centering during fabrication. For a

better comparison of friction stir clinch joints to cold worked clinch joints, the same type

of aluminum and steel coupons must be used with the same dimensions as well.

Additionally, a more accurate way to create the anvil could be utilized, using better

automated equipment.

40

4. Metallographic Analysis for Corrosion and Clinching

4.1 Introduction

To better understand the mechanism of corrosion and the formation of the clinching

joint, metallographic techniques were used on the FSF specimens. Metallography would

be able to provide information about different phases, chemical composition, grain

boundaries and sizes, and deformations present on the sample joints.

4.2 Sample Preparation for Electron Microscopy

A total of 11 samples were analyzed for this research: 2 for clinching and 9 for

corrosion. The clinch samples were both cross-sections of samples to study the formation

of the clinch joint. For the corrosion samples, 7 were cross-sections and 2 were surface

samples. Six of the cross-sections each represented a test site and an exposure time; the

last one was a standard sample left in the laboratory. The surface samples were from B6

to try to explain the catastrophic failure even before any shear strength testing could be

performed.

The samples were cut into the appropriate sizes using a diamond saw. Then they were

mounted in epoxy resin and left to cure overnight. After curing, the samples were then

grinded and polished using silicon-carbide papers and emulsions of alumina with

different particle sizes. After all the polishing was done, the samples were run under an

ultrasonic cleaner with ethanol to completely clean the surface of any possible particles

left over from the polishing step. A final drying of the samples overnight in an oven was

done before the application of a carbon coating. Carbon coating is used to improve

imaging of samples and inhibits charging on the specimens. The coated samples were

then analyzed with the electron probe micro analyzer (EPMA) at POST 621. Figure 4.1

shows the samples just before the carbon coating process.

41

Fig. 4.1 FSF specimens before carbon coating process


Each sample was analyzed with the EPMA. A couple of different detectors for

imaging used were the backscattered electrons detector (BSE) and secondary electrons

detector (SE). BSE was used mostly for compositional contrast between the three metals

(zinc, aluminum and steel) in the joints. Energy dispersive x-ray spectroscopy (EDS) was

used with the corrosion samples to determine the chemical composition of the various

phases found.

42

4.3.1 Metallographic Analysis of FS Clinch Joints

4.3.1.1 CL-07

Fig. 4.2 Sample CL-07 inside the EPMA chamber

Figure 4.2 shows CL-07 as mounted inside the EPMA chamber. The lower right part

of the clinch joint is the spot of interest in this analysis (marked in the figure by the white

square). Aluminum is apparently seen to form the mechanical interlock at that end.

Figure 4.3 below shows the backscatter image for this.

Fig. 4.3: Backscatter image at 40x magnification of right side of CL-07 clinch joint; gray areas are

aluminum while lighter areas are steel

43

The microscopic analysis shows that the joint did not completely form in this

situation. The aluminum did indeed flow downwards into the cavity as shown by the

black arrow; however, it did not flow and curl under the steel coupon. This can be

attributed to possible depth issues with the cavity of the anvil. Additionally, a third phase

of zinc is not seen in between the aluminum and steel coupons (indicated by the orange

arrow).

4.3.1.2 CL-10

Fig. 4.4 Sample CL-10 inside the EPMA chamber

Figure 4.4 this time shows CL-10 inside the EPMA chamber. Again, the right side of

the clinch joint was the area of interest due to the seen formation of the mechanical

interlock there (marked in the figure with a white square). Figure 4.5 below shows two

backscatter images of this specific area.

44

(a) (b)

Fig. 4.5: (a) backscatter image at 40x magnification of right side of CL-10 clinch joint and (b) backscatter

image at 75x magnification; gray areas are aluminum while lighter areas are steel

Downward flow of the aluminum material is still apparent and is indicated by the

black arrow. This time, curling of the aluminum around the steel coupon is observed

(shown by the blue arrow). Similar to CL-07, there is no indication of any zinc phase

present in the joint. This means that the brazing layer does not form with a clinch joint

(with proper centering). This is because for this type of joint the hole diameter is larger

than the tool diameter. There is no steel directly underneath the tool, thus, no zinc will be

heated up by the friction process and no brazing layer will form. The brazing layer will

only form if the centering is not correct.

4.3.1.3 FS Clinching Joint Conclusions

Metallographic analysis verifies the formation of a clinch joint that forms a

mechanical interlock between the aluminum and steel coupons. Because of the nature of

the dimensions of the tool and coupon holes, the brazing layer of zinc does not form.

Should a brazing layer be present, it is due to improper centering of the tool with the

hole. The clinch joint shear strength is therefore mainly dependent on the mechanical

interlock. Formation of the brazing layer may actually be detrimental to the strength of

the joint.

45

4.3.2 Metallographic Analysis of Corroded FSF Joints

4.3.2.1 Standard Cross-Section Sample (ST)

Fig. 4.6 Sample ST inside the EPMA chamber

A standard single-pin sample was left in the laboratory as a control sample for

analysis. This was analyzed at the same as all the other corroded samples. Figure 4.6

shows ST inside the EPMA chamber and Figure 4.7 shows backscatter images for the ST

joint.

(a) (b)

Fig. 4.7: (a) backscatter image at 40x magnification left side of ST joint and (b) backscatter image at 40x

magnification of the neck and head area of ST joint; gray areas are aluminum, lighter areas are steel and

areas indicated by the orange arrows are zinc

46

Flow of aluminum is indicated by the black arrows on the figures. The areas

indicated by the orange arrows are zinc, which form the brazing layer in the middle of the

two coupons. The other areas where zinc is prevalent is where the material has been

pushed during the friction stir process. In figure 4.7a, the brazing layer is about 10

microns thick and is typical for this type of single-pin sample.

4.3.2.2 Waipahu Cross-Section Samples (W1 and W5)

Fig. 4.8 (a) Sample W1 and (b) sample W5 inside the EPMA chamber. Notice the visible crack in the

middle of the neck area of W5

Two samples from this site were cross-sectioned for analysis: W1 for the 4-month

exposure and W5 for the 8-month exposure periods. Figure 4.8 shows them in their

mounts inside the EPMA chamber. Immediately noticeable is the crack in the middle of

the neck area in sample W5. Figure 4.9 shows a couple of backscatter images for sample

W1 while Figure 4.10 shows the EDS analysis of the head area of the joint where some

degree of corrosion is suspected.

47

(a) (b)

Fig. 4.9: (a) scanning electron image at 40x magnification left side of W1 joint and (b) backscatter image at

250x magnification of area indicated by the black arrow in (a)

Fig. 4.10 EDS analysis of point 4 from the 250x magnification shown in Figure 4.9b

The area where EDS was performed indicates it to be mostly composed of aluminum

and zinc, which is expected, since that is where the aluminum is extruded through the

steel to form the head. The zinc is pushed in that direction as well, much like the standard

sample. However, a small peak of Cl is observed, which means that the chlorides have

started to attack the samples in this site. The brazing layer in between the aluminum and

steel seems to be intact.

Figure 4.11 this time shows the backscatter images for W5. Figure 4.12 shows the

EDS analysis of the brazing layer area on the right side of the joint.

48

(a) (b)

(c) (d)

Fig. 4.11: (a) backscatter image at 200x magnification left side of W5 joint (b) backscatter image at 40x

magnification of crack at the neck area (c) backscatter image at 100x magnification right side of W5 joint

(d) backscatter image at 1000x magnification of area indicated by the black arrow in (c)

49

Fig. 4.12 EDS analysis of point 4 from the 250x magnification shown in Figure 4.11d

The large crack in the middle of the neck area may or may not have stemmed from

corrosion. It could be due to some type of mechanical failure. The crack could have

initiated at the jagged edge of the steel coupon indicated by the blue arrow. Another

possible explanation for this would be stress corrosion cracking, to which aluminum is

susceptible. The area where EDS was performed, which is believed to be the brazing

layer, indicates that some type of aluminum oxide has formed and that chlorides are also

present now.

4.3.2.3 Manoa Cross-Section Samples (L1 and L5)

Fig. 4.13 (a) Sample L1 and (b) sample L5 inside the EPMA chamber

50

Two samples from this site were cross-sectioned for analysis: L1 for the 4-month

exposure and L5 for the 8-month exposure periods. Figure 4.13 shows them in their

mounts inside the EPMA chamber. Figure 4.14 shows a couple of backscatter images for

sample L1 while Figure 4.15 shows the EDS analysis of the brazing layer on the right

side of the joint with suspected corrosion.

(a) (b)

Fig. 4.14: (a) backscatter image at 40x magnification right side of L1 joint and (b) backscatter image at

190x magnification of area indicated by the black arrow in (a)

Fig. 4.15 EDS analysis of point 1 from the 190x magnification image shown in Figure 4.14b

The area where EDS was performed shows it to be mostly composed of zinc (which

is what the brazing layer is known to be composed of). Peaks of O and Cl indicate that

some salts have started to attack inside the joint.

Figure 4.16 shows the backscatter images for L5. Inspection shows that the brazing

layer and the rest of the joint is still intact. All EDS analysis done indicated similar

results to the L1 analysis. For this particular site, the corrosion attack has not yet occurred

51

to the state where there are obvious physical changes to the joint. This can be tied up to

the data that this site has the least amount of chloride salts in the atmosphere.

(a) (b)

Fig. 4.16: (a) backscatter image at 130x magnification left side of L5 joint (b) backscatter image at 40x at

the neck and head are of L5 joint

52

4.3.2.4 MCBH Cross-Section Samples (B1 and B5)

Fig. 4.17 (a) Sample B1 and (b) sample B5 inside the EPMA chamber. Notice the visible crack in the

middle of the neck area of B1

Two samples from this site were cross-sectioned for analysis: B1 for the 4-month

exposure and B5 for the 8-month exposure periods. Figure 4.17 shows them in their

mounts inside the EPMA chamber. Immediately noticeable is the crack in the middle of

the neck area in sample B1. Figure 4.18 shows a couple of backscatter images for sample

B1 while Figure 4.19 shows the EDS analysis of the area where the brazing layer is on

the right side of the joint.

(a) (b)

Fig. 4.18: (a) backscatter image at 40x magnification of the crack at the neck area of B1 joint and (b)

backscatter image at 60x magnification of the area where the brazing layer is on the right side of the joint

53


The area where EDS was performed indicates it to be mostly composed of

aluminum. Also worth noting is that zinc is just a small part of this area now. This layer,

which used to be just 10 microns in thickness, is now more than 100 microns and is

mostly composed of aluminum, some oxygen and even chlorine and sulfur. This area of

the joint has definitely started to corrode. Similar to the case for W5, the sample may

have undergone stress corrosion cracking due to the edge of the steel piece and aluminum

being susceptible to it.

Figure 4.20 this time shows the backscatter images for B5. Figure 4.21 shows the

EDS analysis of the brazing layer area on the right side of the joint.

(a) (b)

54

(c) (d)

Fig. 4.20: (a) backscatter image at left side of B5 joint (b) backscatter image of brazing area at the left side

of B5 joint (c) backscatter image at right side of B5 joint (d) another backscatter image of the right side of

B5 joint (all images at 40x magnification)


What used to be a 10 micron brazing layer in between aluminum and steel has now

turned into more than 250 microns of aluminum oxides, chlorine and sulfur. The brazing

layer composed of mostly zinc, which was the integral part of the single-pin joint, is no

longer present in between the two coupons. The major effect of the corrosive

environment can be seen here completely in sample B5, where the oxide growth had

compromised the structure of the joint itself. This would explain the very low strength of

the remaining B8 joint and why B6 and B7 have already failed and broken apart before

being tested.

55

4.3.2.5 MCBH Surface Samples (B6)

Fig. 4.22 Surface of the brazing layers on (a) B6 aluminum and (b) B6 steel. The areas studied in the

EPMA are marked by the black squares

Figure 4.22 shows the surface of the two coupons from B6. The circular areas are the

brazing layer and those were cut off and prepared for analysis with the EPMA. Figure

4.23 shows the backscatter images for the B6 aluminum coupon surface brazing layer and

Figure 4.24 shows the EDS analysis on the two different phases seen in the image.

56

(a) (b)

Fig. 4.23: (a) backscatter image at 40x magnification for EDS and (b) backscatter image at 100x

magnification for B6 aluminum surface

Fig. 4.24 EDS analysis of points 1 and 2 from the 40x magnification shown in Figure 4.23a

The area where EDS was performed indicates the lighter phase to be composed of

mostly zinc and the darker phase to be composed more of aluminum. A significant

presence of oxygen suggests that these phases are oxides formed with those metals. Small

peaks of chlorine also indicate inclusion into the phases of the joint. Figure 4.23b shows

the corrosion products on the aluminum surface.

57

Figure 4.25 this time shows the backscatter images for B6 steel coupon surface

brazing layer and the EDS analysis of the white, lamellar structures observed.

(a) (b)

(c) (d)

Fig. 4.25: (a) backscatter image of B6 steel surface at 40x, (b) backscatter image of same area as (a) at

5000x, (c) another backscatter image of B6 steel surface at 300x, (d) backscatter image of same area as (c)

at 5000x, (e)

58

Fig. 4.26 EDS analysis of points 1 and 2 from the 5000x magnification shown in Figure 4.25b

The EDS analysis shows the white lamellar phase to be comprised mostly of zinc,

some oxygen and a low but significant amount of chlorides. This phase is called

simonkolleite (Zn5(OH)8Cl2·H2O). Figure 4.27 shows a similar micrograph from Vera et

al. [2013]. The structure of simonkolleite in the brazing layer of the friction stir samples

show that these corrosion products have formed and that the brazing layer is no longer

just composed of zinc. These corrosion products have chemically altered the original

fusion bond and have decreased the strength of the joint to the point of failure.

Fig. 4.27 Simonkolleite on galvanized steel surface (Vera et al. 2013)

59

4.4 Corrosion Mechanisms

The mechanism believed to have occurred in these joints is through crevice corrosion,

due to the nature of the lap joint. There exists a crevice between the two metal surfaces of

aluminum and steel. Figure 4.28 shows the schematic of crevice corrosion for a typical

steel plate with a “shield” on top.

Fig. 4.28 Crevice corrosion schematic (Substech.com)

The main difference for the friction stir forming case is that the zinc is getting

attacked by the chlorides instead of the steel. This is supported by the immersion

experiment data indicating that zinc is the most active in the Zn-Al-steel system. The

“shield” in this case is the aluminum coupon and the passive film is zincite/zinc oxide

(ZnO). The following equations show the possible formation of simonkolleite in the

crevice:

Zn(s) Zn2+

+ 2 e- (1) (anodic dissolution)

½ O2 + H2O + 2 e- 2 OH- (2) (cathodic reaction)

5 Zn2+

(aq) + H2O + 8OH-(aq) + 2 Cl

-(aq) Zn5(OH)8Cl2·H2O (3) (formation) or

60

4 ZnO(s) + Zn2+

(aq) + 5 H2O + 2Cl-(aq) Zn5(OH)8Cl2

•H2O (4) (formation)

Figure 4.29 shows the stability diagram of zinc ions, simonkolleite and zinc oxide

[Karlsson, 2011]. This explains why there are two equations that can produce

simonkolleite, either from Zn2+

ions or from zincite (ZnO). It is dependent on the pH and

the concentration of the chloride ions.

Fig. 4.29 Stability diagram of Zn2+

As the zinc from the brazing layer keeps getting used up due to these corrosion

reactions, the aluminum component of the joint is also corroding at the same time. The

tendency for aluminum is to form aluminum oxide (Al2O3), although it is dependent on

potential and pH concentrations as well. Figure 4.30 shows the Pourbaix diagram for

aluminum. However, this formation of aluminum oxide is not passivation; but rather the

formation of the oxide between the two metal coupons. This actually causes the joint to

further deteriorate and is not a corrosion protecting aspect at all.

61

Fig. 4.30 Pourbaix diagram of Aluminum

Finally, for the case of samples W5 and B1, where there were major visible cracks

seen through the neck of the joint, the attributed mechanism is stress corrosion cracking.

Stress corrosion cracking needs three aspects to be feasible: type of material (Al-5182),

corrosive environment (exposed to chlorides) and induced or residual stress. The angled

edge of the steel, which can be seen in Figures 4.11b and 4.18a, caused induced stresses

on the aluminum. This angled edge of the steel stemmed from the prefabrication of the

holes.

62

4.5 Conclusions

Metallographic analysis has confirmed how the friction stir clinching process creates

the clinch joint and the composition of the corrosion products that were present for the

single-pin samples.

The clinch joint is a mechanical interlock with no brazing layer formation. Brazing

layers occur when the centering is not proper during the fabrication of the joint. The

design where the prefabricated hole is larger than the tool allows the formation of this

joint and avoids the addition of the brazing layer. A better, more accurately machined

anvil should provide stronger and better clinch joints.

The corrosion products of the single-pin specimens have been identified and fall into

two types: zinc corrosion products (simonkolleite) and aluminum corrosion products

(aluminum oxide). Since both these metals are the backbone of the FSF joint, any

corrosion to either of them is detrimental to the mechanical properties of the joint. A way

to lessen the effect of corrosion on this joint would be to use sealants around the crevice

or other types coatings to protect it like paints. Additional quantitative analysis on the

corrosion products (like x-ray diffraction) could be utilized to determine the exact

composition.

63

5. Conclusions

One of the main concerns with the friction stir forming process is corrosion, due to

the galvanic coupling of two different metals, aluminum and galvanized steel. The effects

of corrosion on the joints and their mechanical properties were studied by the exposure of

the formed joints in three different atmospheric test sites, each with their own

characteristic atmospheric conditions. The joints did not perform well in high chloride

environments, where the shear strength of the joints was the lowest. Electrochemical

experiments of the metal coupons and metallographic analysis of the corroded samples

confirmed and characterized the corrosion products from the joint. They were found to be

simonkolleite and aluminum oxide, derived from the zinc layer of the galvanized steel

and the aluminum coupon, respectively. Due to the large contribution of the brazing layer

to the shear strength of the single-pin joint getting, any significant corrosion damage to

zinc or aluminum will be detrimental to the integrity of the joint.

The development of a different type of friction stir joint was done to address the issue

of the single-pin joints having high shear strength but unsatisfactory toughness. The new

joint is called the friction stir clinch joint, which is a mechanical interlock between the

aluminum and steel. A different anvil was developed to create this clinch geometry. The

brazing layer seen in the single-pin joints was absent here indicating that the joint is a

mechanical joint and not a fusion one. The goal was achieved of getting improved,

tougher joints.

This research led to three specific scientific contributions in the area of friction stir

processes:

1. Corrosion was found in the zinc-aluminum bonded area of the joint and was

found to be detrimental to shear strength of the joint

2. The new clinching joint configuration was found to have slightly lower overall

strength but improved toughness compared to the previous single-pin

configuration

64

3. The microstructure analysis showed a better understanding of the corrosion

mechanism and its products as well as a better understanding of the formation of

the clinch joint

Further characterization and quantitative analysis could be done for better

understanding of the corrosion process of the joints and the effect on the mechanical

properties. Additionally, corrosion tests could also be performed on the newly developed

clinch joint.

65

6. References

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sheets using conventional rivet and die, Journal of Materials Processing Technology,

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Abe, Y. et al., Joining of high strength steel and aluminum alloy sheets by mechanical

clinching with dies for control of metal flow, Journal of Materials Processing

Technology, 2012.

Ahmad, Z., Principles of Corrosion Engineering and Corrosion Control, 2006.

Ahn, N.S. et al., Joining High-Strength Steel and Al6061 Sheet Using Hole Clinching

Process, 2012.

ASM Metals Handbook, Vol. 2 Properties and Selection: Nonferrous Alloys and

Special Purpose Materials, ASM International.

Callister, William D., Materials science and engineering: an introduction. 4th ed. New

York: John Wiley & Sons, 1997. Print.

Clinch Joint Parameters and control, accessed at:

http://www.clinchsystems.com/process/control.html, 2014.

Fahimpour, V. et al., Corrosion behavior of aluminum 6061 alloy joined by friction

stir welding and gas tungsten arc welding methods, Materials and Design, 2012.

Hosking, N.C. et al., Corrosion resistance of zinc-magnesium coated steel, Corrosion

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Kopeliovich, D., Crevice Corrosion, accessed at:

http://www.substech.com/dokuwiki/doku.php?id=crevice_corrosion, 2012.

Lazarevic, S. et al., Experimental analysis of friction stir forming for dissimilar

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Lee, C.J. et al., Characteristic of Mechanical Clinching for Al5052 to High-Strength

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Lee, C.J. et al., Parametric study on mechanical clinching process for joining

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Liu, Y. et al., EIS Investigation and Structural Characterization of Different Hot-

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TECHNOLOGY WHITE PAPERS, 2014.

Mori, K. et al., Plastic Joining of Ultra High Strength Steel and Aluminum Alloy

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Duplex Stainless Steel, 2009.

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Corrosion and Mechanical Properties of Friction Stir