High Integrity Magnesium Automotive Castings (HI-MAC)

DOE/USAMP Funded

Project Period: April 2006 to March 2010

Yuan-Dong Li, Jay Keist, and Diran Apelian

Project Update

December 3rd, 2008

HI-MAC Project

Goal

Reduce vehicle weight to meet future CAFE requirements by use of light weight Mg alloys in structural components

Objective

Develop cost effective casting technologies

(existing and new) for high integrity casting of

Mg alloys for structural automotive components

Key Technical Issues Address by HI-MAC

• Lower manufacturing costs

• Improve casting quality

• Develop Infrastructure

Major Tasks

Task 1: Squeeze casting process development

Task 2: Low pressure casting process development

Task 3: Thermal treatment of Mg castings

Task 4: Microstructural control during casting

Task 5: Computer modeling and properties

Task 6: Controlled molten metal transfer and filling

Task 7: Emerging casting technologies

Task 8: Technology / commercial transfer throughout the automotive value change

Sub-Tasks: Thermal Treatment

• Evaluate fluidized bed (FB) versus conventional heat treatment of Mg alloys

• Study strengthening kinetics of Mg alloys

• Investigate step heat treatment for enhancement of mechanical performance

Project Deliverables

• Optimum heat treatment process for Mg alloysReduced cycle timeCost effectiveApplicable for high volumes

• Fundamental understanding of the strengthening kinetics for Mg alloys

• A microstructure-property correlation of Mg alloys

Status of Investigation

• Fluidized bed heat treating of Mg alloysAM60MRI-206SAZ91D

• Analysis Influence of solution time

Tensile properties Microstructural behavior

Influence of aging time and temperature Tensile properties Precipitation behavior

AM60

• Mg-Al-Mn alloy

• Exhibits excellent ductility in the as cast condition

• Typically not heat treated

Fluidized bed heat treating trialsSolution at 424°C for 2 to 10 hrsAging at 232°C for 4 hrs

AM60Microstructural Evolution

Mg17Al12 rapidly dissolves into the Mg matrix

2 hr Solution

As-Cast

10 hr Solution100 µm

AM60Aging Precipitates Mg17Al12

5 µm

50 µm

Solution424°C - 2hr

Age232°C – 3hr

AM60 Results

• Solid solution and precipitation occurred but resulted in no strengthening after agingDiscontinuous precipitation at the grain

boundariesLarge, plate like precipitates

• No improvement in mechanical properties were obtained after heat treatment

MRI-206S

• Dead Sea Magnesium Alloy• Mg-1.5%Nd-0.2%Y-0.2%Zn-0.4%Zr• Sand or PM alloy designed for high temperature

applications

Fluidized bed heat treating trialsSolution at 540°C for 30 min to 4 hrsAging at 250°C for 1 to 8 hrs

MRI-206S – Microstructural Evolution

1hr Solution

As-Cast

4hr Solution

Mg41Nd5 dissolves rapidly into the Mg matrix

50 µm

MRI-206SAging Precipitates MgxNdx

1 µm 20 µm

Scales

Solution540°C - 2hr

Age250°C – 3hr

MRI-206S Properties (T6) vs. Solution Time

• Solution

540°C• Quenching

FB – 20°C• Aging

250°C – 3.5 h

Tensile Properties Vs. Aging Time

• Solution

540°C – 2 hrs

• Quenching

FB – 20°C• Aging

250°C

Tensile Properties for Various Aging Treatments

T6a (Under)230°C – 1 hr

T6b (Peak)230°C – 4 hrs

T6c (Peak)250°C – 4hrs

T6d (Over)270°C – 4hrs

MRI-206S Results

• Mg41Nd5 dissolves rapidly at 540°C

Most secondary phases dissolved within 1 hrSome secondary phases remain and grow (MgxNdx)

• Size and shape of aging precipitates remained constant despite varying solution and aging treatments

• FractographyCleavage fracture with facets and stepsPeak aged (T6) → Some shearing along preferred

plans• Optimum FB HT → 2 hrs at 540°C, 3-4 hrs at 230°C

AZ91D

• Recommended heat treatmentSolution

415°C for 16 to 24 hr 2 hr controlled ramp from 260 to

415°C → Avoid fusion voids

Aging 168°C for 16 hr Alternate: 216°C for 6 hr

• Eutectic phase → Mg17Al12

• Al solubility in Mg12.6 wt% at 437°C1 wt% at RT

Chemistry

● 8.3-9.7% Al● 0.15% Mn min.● 0.35-1.0% Zn● < 0.10% Si● < 0.005% Fe● < 0.030% Cu● < 0.002% Ni

AZ91 Literature ReviewAging Behavior

• β-Mg17Al12 precipitates out without the appearance of G.P. zones

• Forms both continuous and discontinuous precipitates (similar to AM60)

• Low strengthening response from aging• Mechanical properties influenced by

Solid solution (Al, Zn)Aging precipitate morphologySecondary phase morphology

AZ91 Literature ReviewHeat Treatment Observations

• Cerri and Barbagallo, Mat Letters 56 (2002) 716-720 Solution at 395°C yielded similar mechanical properties as

obtained at 415°C1

Aging at 220°C for a shorter time yielded better hardness than aging at 170°C

Solution time can be significantly reduced (2 hrs)

• Celotto, Acta Mater 48 (2000) 1775-1787 Optimum aging → 10 to 20 hours at 200°C

• Wang et al. Scripta Mater 54 (2006) 903-908 Aging precipitates had minimal influence on strength properties Suggested single step heat treatment at 370°C, No Aging

Heat Treating Analysis

• Break into 4 Experiments E1: Analyze T4 properties at 370, 390, and 415°C E2: Analyze influence of aging time on properties for samples

solutioned at 390°C E3: Influence of the heating ramp to 415°C

< 3 min 2 hr (260 to 415°C)

E4: Influence of solution time at 415°C 2 hr 6 hr

• All aging conducted at 200°C• Analyze mechanical properties

Controlled Heating Ramp

• Fluidized Bed set at 260°C

• Insert samples• Increase bed

temp by 40°C every 30 min

• Final temp of 415°C

E1: Influence of Solution Temperature on T4 Properties

Notes

Solution Time

2 hours

HT Ramp

< 3 min

Quench

FB at 21°C

E2: Influence of Aging Time on T6 Properties (390°C)

Notes

Solution

2 hr @ 390°C

Ramp

< 3 min

Quench

FB at 21°C

Age Temp

200°C

E3: Influence of Ramping Time on T6 Properties (415°C)

Notes

Solution

2 hr @ 415°C

Quench

FB at 21°C

Age

8 hr @ 200°C

E4: Influence of Solution Time on T6 Properties (415°C)

Notes

Solution

415°C

Ramp

2 hr

Quench

FB at 21°C

Age

8hr @ 200°C

Optimum Treatments

E12 hrs @ 370°C

E22 hrs @ 390°C8 hrs @ 200°C

E32 hrs @ 415°C8 hrs @ 200°C

E42 hr Ramp, 6 hrs @

415°C8 hrs @ 200°C

AZ91D Results

• Properties improved slightly for samples undergoing 2 hours at 370°C

• Properties were not reduced byA rapid heating rate to solution temperature (< 3 min) Decreased solution times of 2 hrs compared to 6 hrs

• Optimum treatmentSolution at 390°C for 2 hrs

Aging at 200°C for 8 hrs

Finishing Up

• Complete investigation on strengthening kineticsAZ91D

• Conduct fluidized bed heat treating on actual parts (automotive control arm)

• Cost analysis of fluidized bed heating of magnesium parts

Fluidized Bed Heating of the Automotive Control Arm

Work holder and Control Arms

Batch fluidized bed HT system