Formability of porous tantalum sheet-metal - IOPscience

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Formability of porous tantalum sheet-metalTo cite this article Paul S Nebosky et al 2009 IOP Conf Ser Mater Sci Eng 4 012018

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Formability of Porous Tantalum Sheet-Metal

Paul S Nebosky1 Steven R Schmid

1 and Timotius Pasang

2

1Dept of Aerospace amp Mechanical Eng University of Notre Dame IN USA 46556

2Dept of Mechanical amp Manufacturing Eng AUT University New Zealand

E-mail schmid2ndedu

Abstract Over the past ten years a novel cellular solid Trabecular Metal

TM has been

developed for use in the orthopaedics industry as an ingrowth scaffold Manufactured using

chemical vapour deposition (CVD) on top of a graphite foam substrate this material has a

regular matrix of interconnecting pores high strength and high porosity Manufacturing

difficulties encourage the application of bending stamping and forming technologies to

increase CVD reactor throughput and reduce material wastes In this study the bending and

forming behaviour of Trabecular MetalTM

was evaluated using a novel camera-based system for

measuring surface strains since the conventional approach of printing or etching gridded

patterns was not feasible A forming limit diagram was obtained using specially fabricated 165

mm thick sheets A springback coefficient was measured and modeled using effective

hexagonal cell arrangements

1 Introduction

Every year orthopaedic surgeons perform around 1000000 total hip replacement (THR) surgeries

worldwide A total hip replacement usually consists of a stem which is inserted into and bonds with

the femur an acetabular cup a head which attaches to the stem and a cup liner for wear resistance

Porous coatings may be included on any orthopaedic implant where tissue ingrowth is desired For

good ingrowth properties scaffolds must have open cells and high through-porosity Arguably the best

scaffold is Trabecular MetalTM

illustrated in Fig 1 which leads to rapid and strong bone ingrowth [1

2] Trabecular MetalTM

has been used as a tissue scaffold and incorporated into acetabular cups knee

spacers and shoulder implants It has a tensile elastic modulus of roughly 7-10 GPa (compared to

subchondral bone with a stiffness of 2 GPa) a yield strength of 45-50 MPa and an ultimate tensile

strength of 59-68 MPa [3]

Currently final part geometries can be obtained in two ways First bulk Trabecular MetalTM

can

be machined to its final shape Conventional machining of Trabecular MetalTM

involves electrical

discharge machining (EDM) or milling As such machining Trabecular MetalTM

is costly not only

because of material loss but also due to expensive manufacturing and cleaning operations The second

approach involves forming before CVD

1 To whom any correspondence should be addressed

Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018

ccopy 2009 IOP Publishing Ltd 1

Figure 1 Porous tantalum foam used for osseointegration in orthopaedic implants (a) SEM image of

Trabecular MetalTM

(b) SEM of trabecular bone (c) total hip replacement with Trabecular MetalTM

bone ingrowth scaffold Source Courtesy of Zimmer Inc

A new approach namely forming Trabecular MetalTM

using bending and forming operations has

several advantages principally cost and reliability To develop sheet forming as a viable method for

shaping Trabecular MetalTM

this research seeks to determine the forming limits bendability and

springback of Trabecular MetalTM

Knowing the forming limits will allow designers to lay out

Trabecular MetalTM

performs and plan such operations

2 Bendability and Springback Some geometries such as interior surfaces of total knee replacements can be produced solely through

bending operations A previous paper investigated the ability of Trabecular MetalTM

to survive

bending without cracking or failure [4] In that paper a manufacturing strategy involving performing

of nestable blanks for CVD followed by bending operations was described to achieve the final part

shape However this presents an additional concern in that control and prediction of elastic springback

is needed to achieve final shapes with tight tolerances Springback results from elastic recovery after

the removal of loads used to plastically deform the workpiece [5]

21 Experimental Investigation of Springback

According to the springback model derived by Gardiner [11] and later expanded by Johnson and Yu

[13] springback depends on the die radius sheet thickness yield strength and Youngs modulus For

this study only die radius R was a factor that could be reasonably varied

Additional factors were necessary to reflect the differences between Trabecular MetalTM

and sheet

metal bending Early forming tests indicated that strain rates affected cracking in Trabecular MetalTM

[4] leading to the inclusion of punch speed clearance C and stroke length

As received the net-formed specimens had dimensions of 254 mm x 1016 mm x 15 mm and

were then prepared using the appropriate embossing Experiments were performed using a wiping die

installed on an Instron 8800 hydraulic test machine (see details in [4])

22 Results

Figure 2 breaks the data into the four significant categories and plots the springback factor as a

function of RT The curve indicates the springback factor of Trabecular MetalTM

as calculated using

the approach of Gardiner [10] The results show several important trends First springback factor

increases with decreasing die radius which is consistent with previous studies Similarly springback

factor decreases with increasing clearance In addition clearance appears to have a greater effect on

springback at larger die radii

Figure 3 shows the results of the numerical simulations plotted on top of confidence regions

determined experimentally for the four combinations of R and C In general the springback factor

increases with decreasing aspect ratio


2

Figure 2 Springback factor as a function of

RT The only two significant factors were die

radius and clearance


aspect ratio

23 Discussion

From lot to lot the springback properties of Trabecular MetalTM

have varied greatly While some lots

of specimens such as those net-formed in a low nitrogen environment proved very ductile and

bendable others such as those shaped using EDM with nitrogen in the CVD chamber have been

more brittle Nitrogen in the CVD chamber can react with tantalum to cause nitrogen embrittlement of

the specimen Likewise the EDM process may reduce bendability due to the presence of a recast layer

and the formation of tantalum oxide on the surface of the specimen Under net-formed conditions the

surface chemistry would not be altered

The samples used for these experiments were created under the standard CVD atmosphere

subjecting them to nitrogen embrittlement However net-forming the samples prevented recast and

the creation of tantalum oxide Thus they showed reasonable bendability

During bending it can be assumed that all deformation occurred at the bend radius Outside this

region the workpiece was assumed to remain unbent Experimentally this assumption proved

accurate as curvature outside the region of the bend was undetectable at full punch extension (Fig

4a) However simulations run at the larger clearance showed significant curvature outside the area of

the bend (Fig 4b) Deformation in this region was found to be elastic as releasing the punch

straightened the workpiece away from the bend radius

Figure 4 The observed workpiece behaviour

during wiping die bending (a) The

experimentally observed workpiece behaviour

for the larger clearance at full punch extension

(b) The deformation behaviour as predicted by

the numerical models

3 Determination of the Forming Limit Diagram

31 Experimental Set Up

Forming-limit diagrams (FLDs) have been widely applied since the 1960s for evaluating the ability of

sheet metal to be plastically deformed to desired geometries The general approach to FLDs is beyond

the scope of this paper but the general theory is reviewed by [5 15] and the approach for determining

a FLD has been standardized by ASTM [16] It should be noted however that determining an FLD

for a particular sheet metal involves inscribing or printing a (usually circular) grid pattern on the sheet

deforming the sheet and measuring the resulting surface strains The FLD is a plot of major versus

minor strain at incipient failure resulting from these experiments

Since the bone ingrowth sections of orthopaedic implants can consist of thin sheets of Trabecular

MetalTM

the extension of forming-limit diagrams to this material is useful for implant designers


3

Because of the unique characteristics of the material several departures were necessary from the

ASTM standardized test method

The first testing change involved specimen size Traditional sheet-metal is readily available in

large blanks and can be easily evaluated using relatively large tooling However bulk Trabecular

MetalTM

is never as large as typical sheet-metal parts as found in automobiles for example In most

cases the scaffold sections of implants are relatively small For example a proximal ingrowth pad on

a hip stem may only be 50 mm long and ingrowth layers covering entire stems are not much longer

perhaps only 150 mm in length Thus reducing specimen size provided a more realistic simulation of

the forming conditions associated with implant geometries and greatly simplified sample preparation

Manufacturability and porosity also contributed to a change in specimen size By decreasing

specimen size the effect of changing porosity within a given specimen and from sample to sample on

the forming-limit curve was reduced For the tests performed here the sheet porosities ranged from

752 to 814 For these reasons a specimen length of 100 mm was selected Specimen widths in

punch tests ranged from 10 mm to 100 mm in increments of 10 mm while tensile tests were

performed on specimens 25 mm wide In most cases four samples of each width were tested with

three samples taken at widths of 50 and 80 mm and six analyzed at a width of 40 mm A visual

examination of samples was performed and samples of poor quality were not tested

For orthopaedic implants the desired thickness of the ingrowth layer usually ranges from 15-2

mm However limitations of the CVD process restrict the smallest sheet thickness to roughly 3 mm

With thinner layers localized tantalum deposition occurs and causes warping resulting in a wavy

sheet To avoid this problem sheets 100 x 100 x 33 mm were manufactured in the CVD reactor

These were then cut in half using wire EDM to obtain a target thickness of 165 mm As received and

prior to testing sheet thickness ranged from 152 mm to 20 mm with an average of 18 mm

An ARAMIS camera system was used to monitor the strain evolution of the surface during

forming The ARAMIS system consists of two computer integrated CCD cameras Typically the

angle γ between the cameras ranges from 10-40deg with the optimal angle ranging from 25-30deg During

testing the cameras record images at a specified time interval For testing images were captured once

each second Once testing is complete the computer software converts this two-dimensional image

data to three-dimensional object data

The experimental setup for the punch tests is given in [4] Two pull-action latch clamps with a

maximum combined hold-down capacity of 6200 N fixed the specimens in place For the tensile tests

hydraulic grips were used All tests were performed on Instron 8800 test machines Deformation rates

were 635 mmmin for the punch tests and 0635 mmmin for the tensile tests Cross-head

displacement continued until the formation of visible cracks

32 Results

While most specimens showed some formability others failed after little or no plastic deformation

Of the 44 specimens tested 37 were formable while the remaining 7 performed poorly Upon closer

examination each of these brittle specimens had surface defects that led to quick failure These were

in the form of a serrated surface (Figure 5a) or an undiscovered crater Specimens with smooth non-

serrated surfaces without defects generally performed well (Figure 5b) To compensate for the effect

of brittle samples two forming-limit curves were constructed (Figure 6) The lower curve shows the

forming limits for the brittle samples while the upper curve represents the forming limits of the other

more ductile specimens These curves were determined as follows First each deformed specimen

was classified as brittle or ductile The failed strain states those within the immediate vicinity of the

crack were recorded and plotted on the forming-limit diagram Critical strain states those where

failure did not occur but which directly surrounded areas of failure were also recorded Finally the

forming limits were derived empirically by drawing the curves below the failed strain states but above

the safe and critical ones

For a plane-strain condition the limiting major strain for formable Trabecular MetalTM

was found

to be roughly 5 Likewise minor strains generated during testing ranged from -8 to 14 By


4

comparison low-carbon steel can be formed with major strains of 40 in the plane-strain condition

Minor strains generated during testing can range from -50 to 60 Clearly Trabecular MetalTM

has

much less formability than conventional sheet metals most likely attributable to its porosity and

material brittleness

Figure 5 Trabecular Metal

TM specimens

showing samples with (a) little formability -

note the visibly striated surface and (b)

good formability

Figure 6 Forming-limit curves for

Trabecular MetalTM

Brittle specimens displayed little formability To the naked eye all deformation appeared to be elastic

due to springback after punch removal However the cameras were able to capture slight plastic

strains For the seven brittle specimens the limiting major strain was less than 1 in the plane-strain

condition Minor strains measured during testing varied from about -1 to 4

33 Discussion

Not surprisingly Trabecular MetalTM

had much less formability than conventional sheet metals This

result was anticipated from prior testing and its nature as a cellular solid In essence the pores in the

material act as stress concentrations reducing formability By this reasoning formability should

increase with decreasing porosity However less porous foams would provide smaller volumes for

bone ingrowth making them less desirable for orthopaedic applications

In general formability increases with the uniformity of the strain field developed during forming

Lubrication improves formability by fostering a more uniform strain distribution One way to

incorporate lubrication into Trabecular MetalTM

forming would be to use sheets of solid lubricant such

as polyethylene between the punch and workpiece Such sheets provide lubrication while eliminating

costly cleaning procedures that would be needed to remove the remnants of lubricating oils or fluids

The result would be a more uniform strain distribution an ability to develop strain states with larger

positive minor strains and ultimately the ability to produce more complicated parts

Based on the empirical experiences with this material it is clear that commercial application will

require some acceptable failure threshold Although the sheets used for forming appear to be free of

surface defects a porous material contains a large number of stress concentrations While many

successful parts may be made using sheet forming there still will be some that fracture prematurely


5

4 Conclusions The manufacturing burden associated with Trabecular Metal

TM can be significantly reduced by

forming performs This paper examined the behavior of Trabecular MetalTM

in bending and forming

operations Based on the results presented herein it can be concluded

1 Statistical analysis of the experimental results showed that die radius R and clearance C were

the only factors that influenced springback Although material properties such as the yield

strength σo and Youngs modulus E do influence springback they were assumed to be

constant for a given sample lot

2 Trabecular MetalTM

was shown to have non-negligible formability which introduces new

manufacturing and design strategies for orthopaedic implants Simple geometries that do not

require high forming strains can be manufactured using forming operations

3 Before commercial application sheet forming of Trabecular MetalTM

must overcome several

remaining hurdles First the sheets used for forming must be free of surface defects or

blemishes These include surface striations along with any craters or pits Thickness

variations throughout the sheet must also be avoided Finally it must be determined if

forming operations are actually cheaper than machining

5 Acknowledgments

The authors would like to thank the financial support of the Trabecular Metal Division of Zimmer

Holdings LLC The authors would also like to thank the personal support of Dr Robert Pogge Mr

Robert Cohen and Dr Michael Hawkins for this research In addition the efforts of Dr David Coe of

Trilion Quality Systems to assist with experimental setup and camera operation are gratefully

acknowledged

6 References [1] Bobyn J Stackpool G Hacking S Tanzer M and Krygier J 1999 J Bone amp Joint Surg 81-B

907-913

[2] Bobyn JD Toh K-K Hacking A Tanzer M and Krygier JJ 1999 J Arthroplasty 14 347-353

[3] Zardiackas LD Parsell LD Dillon DW Mitchel LA Nunnery R and Poggie R 2001 J

Biomedical Materials Research Part B Applied Material 58 180-187

[4] Nebosky PS and Schmid SR 2007 Trans NAMRI 35 57-64

[5] Kalpakjian S and Schmid SR 2006 Manufacturing Processes for Engineering Materials 5th ed

(Prentice-Hall)

[6] Sturm R and Fletcher B 1941 Product Engineering 12 526-528


[8] Chapman F Hazlett T and Schroeder W 1942 Product Engineering 13 382-383

[9] Schroeder W 1943 Transactions of the ASME 65 817-827

[10] Sachs G 1951 Principles and Methods of Sheet-metal Fabricating New York (Reinhold

Publishing Corporation)

[11] Gardiner F 1957 Transactions of the ASME 79 1-9

[12] Queener C and DeAngelis R 1968 Transactions of the ASM 61 757-768

[13] Sidebottom O and Gebhardt C 1979 Experimental Mechanics 19 371-377

[14] Johnson W and Yu T 1981International Journal of Mechanical Sciences 23 619-630

[15] Hecker SS 1975 Sheet Metal Industries 13 42-48

[16] ASTM E 2218-02 2006 (American Society for Testing and Materials)

[17] Kwon JW Lee DN and Kim I 1994 Scripta Metallurgica et Materialia 35 613-618


6

Formability of Porous Tantalum Sheet-Metal

Paul S Nebosky1 Steven R Schmid

1 and Timotius Pasang

2

1Dept of Aerospace amp Mechanical Eng University of Notre Dame IN USA 46556

2Dept of Mechanical amp Manufacturing Eng AUT University New Zealand

E-mail schmid2ndedu

Abstract Over the past ten years a novel cellular solid Trabecular Metal

TM has been

developed for use in the orthopaedics industry as an ingrowth scaffold Manufactured using

chemical vapour deposition (CVD) on top of a graphite foam substrate this material has a

regular matrix of interconnecting pores high strength and high porosity Manufacturing

difficulties encourage the application of bending stamping and forming technologies to

increase CVD reactor throughput and reduce material wastes In this study the bending and

forming behaviour of Trabecular MetalTM

was evaluated using a novel camera-based system for

measuring surface strains since the conventional approach of printing or etching gridded

patterns was not feasible A forming limit diagram was obtained using specially fabricated 165

mm thick sheets A springback coefficient was measured and modeled using effective

hexagonal cell arrangements

1 Introduction

Every year orthopaedic surgeons perform around 1000000 total hip replacement (THR) surgeries

worldwide A total hip replacement usually consists of a stem which is inserted into and bonds with

the femur an acetabular cup a head which attaches to the stem and a cup liner for wear resistance

Porous coatings may be included on any orthopaedic implant where tissue ingrowth is desired For

good ingrowth properties scaffolds must have open cells and high through-porosity Arguably the best

scaffold is Trabecular MetalTM

illustrated in Fig 1 which leads to rapid and strong bone ingrowth [1

2] Trabecular MetalTM

has been used as a tissue scaffold and incorporated into acetabular cups knee

spacers and shoulder implants It has a tensile elastic modulus of roughly 7-10 GPa (compared to

subchondral bone with a stiffness of 2 GPa) a yield strength of 45-50 MPa and an ultimate tensile

strength of 59-68 MPa [3]

Currently final part geometries can be obtained in two ways First bulk Trabecular MetalTM

can

be machined to its final shape Conventional machining of Trabecular MetalTM

involves electrical

discharge machining (EDM) or milling As such machining Trabecular MetalTM

is costly not only

because of material loss but also due to expensive manufacturing and cleaning operations The second

approach involves forming before CVD

1 To whom any correspondence should be addressed


ccopy 2009 IOP Publishing Ltd 1


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6

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Formability of porous tantalum sheet-metal - IOPscience