Die castings inevitably generate stress during the molding process. Uneven stress release can lead to deformation, a key factor affecting the dimensional accuracy of die castings. Stress deformation not only reduces product assembly performance but, in severe cases, can even render the casting scrapped. Therefore, in-depth analysis of the mechanisms and influencing factors of stress deformation is crucial for improving die casting quality. Stress in die castings is primarily categorized as thermal, mechanical, and phase change stress. Thermal and mechanical stresses are the most common during the die casting process, interacting with each other to influence the degree of deformation in the casting.
Thermal stress is one of the main causes of deformation in die-cast parts, stemming from differential cooling rates across the casting. During the die-casting process, molten metal rapidly cools and solidifies after filling the mold cavity. Due to the complex structure of the casting, varying wall thicknesses, and heat dissipation conditions, cooling rates vary significantly across different parts. Thick-walled areas cool slowly, while thin-walled areas cool quickly. During solidification shrinkage, the thicker-walled areas are constrained by the thinner-walled areas, generating tensile stresses, while the thinner-walled areas generate compressive stresses. When these internal stresses exceed the yield strength of the material, the casting deforms. For example, an aluminum alloy housing with ribs may have thinner walls at the ribs and cool faster, while the main body of the housing has thicker walls and cools slower. This can eventually cause the housing to bend toward the ribs. Simulation software analysis has shown that thermal stress-induced deformation increases significantly when cooling rate differences exceed 20°C/s. Therefore, a suitable cooling system should be incorporated into the mold design to minimize cooling rate differences.
Mechanical stress primarily arises from external forces during the die-casting process, including clamping force, injection force, and ejection force. Excessive clamping force exerts significant restraint on the casting, preventing it from contracting freely during solidification and generating internal stress. Excessive injection force generates significant hydrostatic pressure within the mold cavity, leading to residual stress after solidification. During ejection, if the draft angle is insufficient or the mold surface is rough, the ejection force can cause elastic deformation in the casting. This elastic recovery after ejection can result in permanent deformation. For example, a zinc alloy die-casting with a draft angle of only 0.3° experienced significant friction during ejection, causing bending and deformation of the casting’s sidewalls. Increasing the draft angle to 1° reduced this deformation by 70%. Furthermore, improper external forces during handling and storage of castings after ejection can also generate mechanical stress and cause deformation. Therefore, specialized storage tooling is required to prevent stacking of castings under pressure.
The impact of casting structure on stress deformation cannot be ignored. Improper structural design can exacerbate stress concentration and increase the risk of deformation. Asymmetrical castings experience uneven stress distribution during cooling, making them prone to distortion. For example, in a casting with a boss on one end and a flat surface on the other, shrinkage at the boss can cause the entire casting to tilt toward the boss. Narrow structures and deep cavities in castings are also susceptible to deformation. Narrow structures have poor rigidity and are prone to bending under stress. Deep cavities have different cooling rates on the inner and outer walls, which can cause concave or convex deformation. A company produced a 500mm long aluminum alloy die-cast rod. Due to the lack of reinforcing ribs, it exhibited 2mm of bending deformation after cooling. After adding two ribs in the middle, the deformation was reduced to less than 0.5mm. Therefore, symmetrical structures should be adopted as much as possible during design, and necessary reinforcing ribs should be added to improve the overall rigidity of the casting and reduce deformation.
Controlling stress and deformation in die-cast parts requires both process optimization and structural improvements. Process-wise, stress can be reduced by adjusting die-casting parameters, such as reducing the shot pressure (while ensuring filling), optimizing the cooling system for uniform cooling of the casting, and controlling the mold temperature (e.g., maintaining a mold temperature of 200-250°C for aluminum alloys). Existing stress can be eliminated through aging treatment: heating the casting to 120-180°C and holding it for 2-4 hours to gradually release the stress. For example, residual stress in aluminum alloy die-cast parts can be reduced by 30-50% after aging. Structural improvements include implementing symmetrical structures and reinforcing ribs, adding process allowances in areas prone to deformation, and subsequently correcting the deformation through machining. For complex castings, split die-casting followed by assembly can be used to minimize overall deformation. An automotive parts manufacturer reduced the deformation of a gearbox housing from 1.5mm to within 0.3mm through process optimization (reducing the shot pressure by 10% and optimizing the cooling water system) and structural improvements (adding anti-deformation ribs), meeting assembly requirements.
