JIANG Bin, ZHANG Ang(), SONG Jiangfeng, LI Tian, YOU Guoqiang, ZHENG Jiang, PAN Fusheng
National Key Laboratory of Advanced Casting Technologies, National Engineering Research Center for Magnesium Alloys, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
Cite this article:
JIANG Bin, ZHANG Ang, SONG Jiangfeng, LI Tian, YOU Guoqiang, ZHENG Jiang, PAN Fusheng. Defect Control of Magnesium Alloy Gigacastings. Acta Metall Sin, 2025, 61(3): 383-396.
The demand for lightweighting is rapidly increasing to meet the carbon peak and neutrality goals. Gigacasting integrates stamping and welding processes into a single high-pressure die-casting operation, streamlining production workflows and considerably enhancing production efficiency, thereby accelerating advancements in automotive lightweighting. Magnesium alloys, which are the lightest metallic structural materials at present, are superior choices for lightweighting because of their low density, high strength, and excellent casting performance. Magnesium alloy gigacasting has enormous potential for automotive applications, enabling the production of lightweight automotive components with superior mechanical properties. However, this process faces challenges because magnesium alloys' active chemical properties and high susceptibility to hot tearing, combined with their large size, thin wall thickness, and complex geometries, make defects like porosity and hot tearing prevalent. These defects greatly impair the performance of gigacast components. Preventing and mitigating casting defects is critical for improving the yield and quality stability of magnesium alloy gigacastings, thereby facilitating their widespread application in industries like automotive and aerospace. To address these issues, the causes and control measures for three common defects (porosities, defect bands, and hot tearing) are briefly explored in this study. Progress and challenges in defect control, focusing on melt treatment, alloy development, process optimization, and structural design, are also outlined. This review aims to provide valuable insights into defect control strategies for developing high-performance magnesium alloy gigacastings.
Fund: National Key Research and Development Program of China(2021YFB3701000);National Natural Science Foundation of China(52471118);National Natural Science Foundation of China(U21A2048);Young Elite Scientists Sponsorship Program by CAST(2022QNRC001)
Gas trapped during die casting process and gas generated by the decomposition of mold release agent
Equipping vacuum system, reasonable selection of process parameters and coating, and control of the amount of spraying
Adhesion
Strip-shaped scratches along the mold-opening direction on the casting surface
Damaged or rough mold surface, high casting temperature or mold temperature, and bad release agent effect
Repair the damaged part of the mold surface, adjust the balance of the ejector rods, use the release agent with good release effect, and adjust the pouring and mold temperatures
Cold shut
Irregular sunken linear lines on the casting surface
Poor fluidity of the alloy melt, low filling rate, low pouring temperature, and low mold temperature
Increase the pouring temperature, shorten the filling time, enhance the fluidity of the liquid metal, and improve the injection rate
Defect
band
A band of pores with solute segregation, distributed near the casting surface and some also in the core area
The shear force in the solid-liquid two-phase region causes the fragmentation, remelting, and coalescence of externally solidified crystals. It is difficult for liquid metal to fill at late solidification, resulting in a large number of pores
Increase the shear force, by increasing the injection rate and adjusting the vacuum time, to break externally solidified crystals
Undercasting
Insufficient filling parts or incomplete casting contour
Poor fluidity, low pouring temperature, low mold temperature, too much involved gas, and poor operation
Optimize the alloy composition and improve pouring and mold temperatures
Flow mark
Clearly visible, non-directional stripes that
differ in color from
the metal matrix
Low mold temperature, splashing due to too small cross-section area and inappropriate position of the inner gate, insufficient pressure on the metal, and too much coating
Raise the mold temperature, adjust the cross-sectional area and position of the inner gate, adjust the injection rate and pressure, and use appropriate amount of coating
Deformation
Overall or partial deformation of the
casting
Poor structural design, insufficient casting rigidity due to premature mold opening, uneven force during ejection caused by unreasonable setting of ejector rods
Improve casting structure, adjust mold opening time, reasonably set the number and position of ejector rods, and eliminate mold pulling problem
Burr
Metal flakes appear on the edge of the parting surface
High injection rate, insufficient locking force, high pouring temperature, and worn and deformed hinge of die casting machine
Check the locking force, correct the mold, clean the cavity and parting surface, and reduce poring temperature and the
injection rate
Slag inclusion
Irregular impurities on the casting surface and inside the casting
Unclean furnace materials, insufficient alloy purification, unclean casting mold, and slag and oxides brought into melt
Ensure the cleanliness of furnace materials, refine melt, and promptly clean the mold
Crack
Network-like protrusions or indentations resembling hairs on the casting surface
Cracks on the surface of the mold cavity, high pouring temperature, rough surface of the mold cavity
Select high-quality mold materials, avoid too high pouring temperature, and sufficient and uniform mold preheating
Drawing
die
Difficult to smoothly demold
Insufficient mold surface roughness, carbon deposits and oxidation on the mold surface, improper or insufficient use of lubricants, unreasonable design of mold structure
Spray special coatings, perform surface treatment on the mold surface, select appropriate lubricants, and minimize mold deformation and wear
Table 1 Common defects in die casting
Fig.1 Defect morphologies of porosities (including gas pore, gas-shrinkage pore, and island-shrinkage)[29] (a), defect band[32] (b), and hot tearing[33] (c)
Fig.2 Simulations of bubble dynamics and dendrite-bubble interaction (a) changes of dimensionless shape parameter Cb with hw / d0 of the obstacle[35] (Cb—revised Blaschke coefficient, hw—dimensionless horizontal interval width, d0—dimensionless bubble diameter, vw—dimensionless vertical obstacle width; inset shows the rising bubble in the bilateral arc obstacle cases)(b) changes of the length (L, mesh in unit) of the closed liquid phase region (CLPR) with the dimensionless undercooling (ΔT / ΔTf)[39] (ΔTf—undercooling, K; ΔTf—the freezing temperature range, K; g—given gravita-tional acceleration, m/s2; g0—gravitational acceleration benchmark, m/s2; inset shows the definition of the length CLPR)
Fig.3 Schematic of continuous non-flux refining system for magnesium melt[76]
Fig.4 “Partial”-“whole” collaborative design based on typical structural features
Fig.6 Simulations and predictions of die-casting defects of magnesium alloy dashboard bracket (a) air entrainment (b) misrun sensitivity (c) total shrinkage porosity (d) hot tearing indicator
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