0102030405
[Practical Tips] Die Casting Defects Series - Cracks
2025-11-13
The cracks in the casting are visible to the naked eye due to external mechanical forces. Analysis of the casting's structural shape and defect appearance determines that the cracks are not caused by casting stress.
I. Defect Characteristics
When the alloy matrix in a die-cast part is damaged or broken, forming fine, thread-like gaps, it is called a crack. These cracks penetrate deep into the casting and can be either penetrating or non-penetrating and tend to grow and expand.
There are macroscopic cracks visible to the naked eye, microcracks that require a low-magnification magnifying glass to observe, and intergranular microcracks that can only be seen with a microscope. Cracks not only appear on the surface of the casting, but also inside the casting.
Cracks can be divided into two types: cold cracks and hot cracks. Their main difference is:
Cold cracks are cracks that occur in castings due to stress or external forces. In cold-cracked castings, the metal at the crack opening is not oxidized; the crack still retains the color of the base metal, which is bright and can penetrate the grains. The crack shape is relatively straight and long.
Hot cracks are caused by the thermal brittleness of metal solidification shrinkage and shrinkage stress. The metal at the crack opening of the casting is oxidized, and the color of the crack becomes darker and duller. Shrinkage cracks usually run along the grain interface and are long and thin, formed by many curved short lines spliced together.
When castings are corroded in an alkaline solution, the cracked areas on the surface of the casting turn dark gray, indicating that the metal matrix is destroyed. The cracks are either straight (cold cracks) or wavy (hot cracks, connected by short, thin, curved lines). The cracks are small and elongated, and tend to develop under external force. Cracks aren't allowed to exist on die castings.
Internal cracks can appear in thicker wall areas and hot spots of castings. Cracks can appear in the appearance, macrostructure, and microstructure of die castings, and these cracks can become sources of fracture. For thin-walled parts, especially magnesium alloy die castings, cracking is one of the most common problems. Several causes of crack formation include: cracks caused by cooling shrinkage; excessive shrinkage due to excessive wall thickness; melting and cracking of the chilled layer; cracking at the fusion line; undercuts or excessively small draft angles; and tearing during mold opening.
II. Causes
Cracks mainly occur when the casting is pulled apart in the mold, due to mechanical stress during mold opening, or when the casting is ejected or when cleaning burrs.
(1) Shrinkage cracks: The appearance of the crack shows obvious dendritic structure, which is a hot crack. The alloy itself has large shrinkage, a wide liquid-solid phase temperature range, or low eutectic content, or poor strength and toughness within the liquid-solid phase temperature range. When the alloy is subjected to shrinkage stress transmitted from other parts due to rapid cooling at high temperature, it will fracture and produce cracks.
(2) Improper selection of alloy or excessive harmful impurities reduce the plasticity of the alloy.
(3) Deviations in the chemical composition of aluminum alloys: ① Excessive silicon segregation in aluminum-silicon alloys, the silicon content should be less than 17%; ② Excessive iron and sodium content and excessively low silicon content in aluminum alloys; ③ Excessive zinc or copper content in aluminum-silicon and aluminum-silicon-copper alloys; ④ Excessive magnesium content in aluminum-magnesium alloys, or when it is between 3.5% and 5.5%; ⑤ Excessively low silicon content in aluminum-copper and aluminum-magnesium alloys; ⑥ Low-melting-point alloys mixed into the alloy.
(4) Excessive melting and casting temperatures of the alloy cause segregation. Excessive holding time results in coarse grains and excessive oxide inclusions.
(5) The temperature of the mold as a whole or in parts, especially the core, is too low (causing cold cracking) or too high (causing hot cracking).
(6) Improperly positioned inlet gates may cause the cavity wall or core to be washed away, resulting in localized overheating of the mold or hindering the shrinkage of the molten alloy.
(7) The shape and structure of the casting are unreasonable, such as the casting wall thickness changes drastically, the shrinkage is large, the shrinkage is hindered, and cracks appear at the hot spot; the casting round corner is too small, and stress concentration occurs at the sharp corner, resulting in cracks; the straight reinforcing ribs in the circular or frame structure cause cracks due to shrinkage, etc.
(8) The casting structure has a large hot spot and the alloy liquid shrinks a lot.
(9) The mold opening and core pulling time is too long, and the shrinkage is hindered and the stress increases.
(10) Poor filling and lack of fusion of the metal matrix result in insufficient strength after solidification, especially in areas far from the gate where cold shuts are more likely to occur.
(11) Cracks in the casting are caused by mechanical forces such as operation, mold opening, and casting ejection. Uneven setting of the core pulling and ejection positions and quantities leads to uneven force and skewing. The lengths of the ejector bars ejected by hydraulic ejection are inconsistent.
(12) The core is bent, the forming surface of the mold has depressions and machining marks along the demolding direction, the forming parts are inherently skewed, and there is a backslope that hinders the casting from being ejected, and the casting tears when it is demolded.
(13) Cracks in die castings caused by the mold and core retracting when the metal is pumped into the mold cavity.
(14) Cracks are caused by misalignment of the moving and fixed molds. When the mold opens and separates, the moving mold moves downwards, and the fixed mold core leaves traces of pulling or scratching on the upper half of the casting hole. Similarly, the guide pillars on the mold parting surface will also lift the moving mold upwards when the mold closes. In this case, the mold usually needs to be reinstalled.
(15) After injection filling, at the moment of pressurization, due to insufficient clamping force, poor clamping mechanism, poor mold slider fit, insufficient mold strength, etc., the slider moves under pressure and cracks are generated.
(16) Cracks are caused when using a cutting die to cut edges and deburr.
III. Countermeasures
Cracks often appear at the root of long cylinders, the root of cantilever sections, the edge of large flat parts where shrinkage is hindered, and the parts where the thermal temperature is higher than the target temperature.
(1) The main cracks are located near the ingate. Improve water cooling at the ingate of the mold; reduce the thickness of the ingate, increase the trimming residue of the ingate, repair the trimming mold and clean the cutting edge.
(2) Select or modify alloy types with low shrinkage, narrow liquid-solid phase temperature range, high content of eutectic during crystallization, or high high-temperature strength.
(3) Strictly control harmful impurities and correctly control the alloy composition to ensure it reaches the specified range: ① The iron content in aluminum alloys should not be too high, and the silicon content should not be too low; ② Reduce the zinc, copper, and iron content in aluminum-silicon and aluminum-copper alloys; ③ Add aluminum-silicon alloy to aluminum alloys with low silicon content to increase the silicon content and reduce the shrinkage of the alloy; ④ Cracks in magnesium alloy castings: If the aluminum, silicon, and iron content in magnesium alloys is too high, add pure magnesium to the alloy to reduce the aluminum (cold cracking) or too low (hot cracking); control the iron content in brass alloys. ⑤ Strictly control the content of low-melting-point metals.
(4) Reducing the solidification range of the alloy, increasing the isothermal solidification time, refining the grains, and ensuring uniform solidification of the entire casting can all reduce hot cracking. The pouring temperature shouldn’t be too high to reduce the shrinkage of the alloy.
(5) Reducing the mold dwell time can reduce the clamping force of the casting on the mold.
(6) Lower the surface temperature of the mold to eliminate thermal cracking stress; conversely, increase the temperature of the mold and core to reduce shrinkage and shrinkage stress, slow down the cooling rate of the alloy liquid, and eliminate cold cracking stress.
(7) Heat the areas of the mold around the crack where the temperature is low, especially the areas with a large surface area.
(8) The process measures of increasing pressure during solidification to improve shrinkage cavities in areas where shrinkage causes cracks are also applicable to cracks.
(9) Cracks inside the casting are caused by high temperature shrinkage. They can be eliminated by using lower pouring temperature, lower mold temperature, higher filling speed and greater casting pressure, timely pressure supplementation or local extrusion.
(10) Improve the position and direction of the ingate to avoid the alloy liquid from scouring the cavity wall and core, which would hinder the shrinkage of the casting and cause cracks.
(11) Improve the casting structure and make the wall thickness uniform; maximize the casting radius.
(12) Due to cracks caused by ejection, there may be adhesion and sticking of the mold or core. The mold structure needs to be modified. Observe the smoothness of ejection, adjust the core pulling mechanism and push rod to ensure uniform ejection force, and ensure that the casting moves forward in parallel without tilting and is ejected evenly. The push rod must have sufficient strength to prevent elastic bending. The ejection time of the casting must be appropriate, and the ejection should be slow and smooth.
(13) There should be no gap between the movable core and the movable core. When pulling the core, the movable core should move parallel to the axial direction without tilting.