Al 8 Mn 5 in High-Pressure Die Cast AZ91: Twinning, Morphology and Size Distributions
- PDF / 4,214,347 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 60 Downloads / 175 Views
ODUCTION
AUTOMOTIVE magnesium components are often Mg-Al-based alloys produced by high pressure die casting (HPDC). When conducted with an optimized die, process parameters and vacuum system,[1,2] HPDC can mass produce large, thin-walled, complex shapes containing microstructures with fine a-Mg grains (5 to 20 lm),[3,4] and a fine-scaled percolating eutectic Mg17Al12 network.[5,6] While a large body of research has investigated microstructure formation in Mg HPDC, including the formation of a-Mg grains,[3,4,7] the surface ‘skin’,[4,8] the eutectic Mg17Al12,[5,9,10] and casting defects,[11–17] less work has explored the formation of Al-Mn-(Fe) intermetallic particles.[18–21] These particles play an important role in determining micro-galvanic corrosion in HPDC Mg parts[22,23] and can initiate cracks under tensile loading.[24,25] Most Mg-Al-based HPDC alloys (e.g. AM50A, AM60B, AZ91D[26]) contain sufficient Mn and Al that
G. ZENG is with the School of Materials Science and Engineering, Central South University, Changsha 410083, China and also with the Department of Materials, Imperial College London, London, SW7 2AZ, UK. Contact e-mail: [email protected]. S.S. SHUAI is with the State Key Laboratory of Advanced Special Steel, School of Materials Science & Engineering, Shanghai University, 200444, Shanghai, China. X.Z. ZHU and S.X. JI are with the BCAST, Institute of Materials & Manufacturing, Brunel University London, Uxbridge, UB8 3PH, UK. J.W. XIAN and C. M. GOURLAY are with the Department of Materials, Imperial College London. Contact e-mail: [email protected]. Manuscript submitted October 8, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Al8Mn5 begins to form before a-Mg during solidification. For example, Figure 1 shows the sequence of phase formation assuming Scheil solidification of AZ91D with the composition in Table I, calculated with the Thermo-Calc TCMG magnesium database version 4.[27] It can be seen that Al8Mn5 is the first solid phase to form, and becomes stable ~ 44 K above the a-Mg liquidus temperature for this composition. It has been confirmed by in situ X-ray imaging that Al8Mn5 forms at higher temperature (i.e. earlier on cooling) than a-Mg in a similar alloy.[28,29] A consequence of this in HPDC is that Al8Mn5 can form and settle in the holding pot,[29,30] for example, during temperature drops when charging the furnace with new ingots, leading to die casting sludge.[30] Furthermore, in cold chamber HPDC, heat loss in the shot chamber can cause Al8Mn5 formation prior to injection as Al8Mn5 externally solidified crystals (ESCs)[20] in addition to the a-Mg ESCs that are widespread in HPDC Mg components.[3,14,31] This occurs because a feature of Mg HPDC is partial solidification in the shot chamber that leads to large a-Mg ESCs being injected into the cavity.[3,32] The volume fraction of a-Mg ESCs has been shown to depend on the melt superheat, the fill fraction and the temperature of the sleeve walls and plunger tip, and is typically 10 to 30 vol pct[3,14,31,33]; similar factors might be expected to
Data Loading...
