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High Integrity Magnesium Automotive Castings (HI-MAC)

This project aims to reduce vehicle weight to meet CAFE requirements by using lightweight Mg alloys in structural components. The objective is to develop cost-effective casting technologies for high-integrity casting of Mg alloys for automotive structural components.

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High Integrity Magnesium Automotive Castings (HI-MAC)

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  1. 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

  2. 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

  3. Key Technical Issues Address by HI-MAC • Lower manufacturing costs • Improve casting quality • Develop Infrastructure

  4. 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

  5. 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

  6. Project Deliverables • Optimum heat treatment process for Mg alloys • Reduced cycle time • Cost effective • Applicable for high volumes • Fundamental understanding of the strengthening kinetics for Mg alloys • A microstructure-property correlation of Mg alloys

  7. Status of Investigation • Fluidized bed heat treating of Mg alloys • AM60 • MRI-206S • AZ91D • Analysis • Influence of solution time • Tensile properties • Microstructural behavior • Influence of aging time and temperature • Tensile properties • Precipitation behavior

  8. AM60 • Mg-Al-Mn alloy • Exhibits excellent ductility in the as cast condition • Typically not heat treated • Fluidized bed heat treating trials • Solution at 424°C for 2 to 10 hrs • Aging at 232°C for 4 hrs

  9. AM60 Microstructural Evolution Mg17Al12 rapidly dissolves into the Mg matrix As-Cast 2 hr Solution 10 hr Solution 100 µm

  10. AM60Aging Precipitates Mg17Al12 5 µm Solution 424°C - 2hr Age 232°C – 3hr 50 µm

  11. AM60 Results • Solid solution and precipitation occurred but resulted in no strengthening after aging • Discontinuous precipitation at the grain boundaries • Large, plate like precipitates • No improvement in mechanical properties were obtained after heat treatment

  12. 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 trials • Solution at 540°C for 30 min to 4 hrs • Aging at 250°C for 1 to 8 hrs

  13. MRI-206S – Microstructural Evolution Mg41Nd5 dissolves rapidly into the Mg matrix As-Cast 1hr Solution 4hr Solution 50 µm

  14. MRI-206S Aging Precipitates MgxNdx Solution 540°C - 2hr Age 250°C – 3hr Scales 1 µm 20 µm

  15. MRI-206S Properties (T6) vs. Solution Time • Solution • 540°C • Quenching • FB – 20°C • Aging • 250°C – 3.5 h

  16. Tensile Properties Vs. Aging Time • Solution • 540°C – 2 hrs • Quenching • FB – 20°C • Aging • 250°C

  17. 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

  18. MRI-206S Results • Mg41Nd5 dissolves rapidly at 540°C • Most secondary phases dissolved within 1 hr • Some secondary phases remain and grow (MgxNdx) • Size and shape of aging precipitates remained constant despite varying solution and aging treatments • Fractography • Cleavage fracture with facets and steps • Peak aged (T6) → Some shearing along preferred plans • Optimum FB HT → 2 hrs at 540°C, 3-4 hrs at 230°C

  19. AZ91D • Recommended heat treatment • Solution • 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 Mg • 12.6 wt% at 437°C • 1 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

  20. AZ91 Literature Review Aging 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 morphology • Secondary phase morphology

  21. AZ91 Literature Review Heat 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

  22. 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

  23. 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

  24. E1: Influence of Solution Temperature on T4 Properties Notes Solution Time 2 hours HT Ramp < 3 min Quench FB at 21°C

  25. 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

  26. 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

  27. 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

  28. Optimum Treatments E1 2 hrs @ 370°C E2 2 hrs @ 390°C 8 hrs @ 200°C E3 2 hrs @ 415°C 8 hrs @ 200°C E4 2 hr Ramp, 6 hrs @ 415°C 8 hrs @ 200°C

  29. AZ91D Results • Properties improved slightly for samples undergoing 2 hours at 370°C • Properties were not reduced by • A rapid heating rate to solution temperature (< 3 min) • Decreased solution times of 2 hrs compared to 6 hrs • Optimum treatment Solution at 390°C for 2 hrs Aging at 200°C for 8 hrs

  30. Finishing Up • Complete investigation on strengthening kinetics • AZ91D • Conduct fluidized bed heat treating on actual parts (automotive control arm) • Cost analysis of fluidized bed heating of magnesium parts

  31. Fluidized Bed Heating of the Automotive Control Arm Work holder and Control Arms Batch fluidized bed HT system

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