Cracking characteristics of mold parts are shown in the oval in Figure 2. An aluminum alloy die-casting mold part made of 8407 mold steel developed a large through crack after serving 2000 molds, which could not be repaired and led to its scrapping. Crack cross section is smooth and neat, as shown in Figure 3. Color of cross section changes from surface gray to light gray. Gray is mainly concentrated in the area 30mm from surface, with a clear dividing line from light gray. White aluminum alloy flakes can also be seen in gray area (see circle in Figure 3). Judging from crack cross-section characterization analysis, crack evolves from surface to bottom in depth.

 

 

During gradual evolution process, oxygen penetrates into crack tip, forming an oxide layer on crack section, crack at the tip of oxide layer continues to spread to depth. During die-casting process, liquid alloy aluminum continuously penetrates into cracks and adheres to crack cross-section. Finally, under action of combined force, crack expanded larger and larger, breaking through yield strength of steel at a position 30mm away from surface, and mold part collapsed as a whole, forming a light gray cross-section area.

Carry out sampling and testing at location shown in Figure 4, analyze whether material is qualified in terms of material composition and metallographic structure. Material composition testing results are shown in Table 1, which meets standards of 8407 materials. Metallographic structure examination is shown in Figure 5. Matrix is lath martensite + flake martensite + retained austenite structure, there is possibility of cracking caused by local band-like structural defects.

 

Table 1 Material composition test results (quality score)

 

 

Mold parts are quenched according to heat treatment process shown in Figure 6, and design hardness is 46~48HRC. Measured hardness on the surface of mold parts is 45~48HRC, and measured hardness inside is 47~50HRC. Internal hardness is on average 2HRC higher than external hardness. Test results show that hardness of mold parts is uneven, material may have cracks caused by uneven internal and external structures.

 

CAE software is used to simulate and analyze force, temperature distribution, thermal stress distribution, deformation, etc. of mold parts. It is possible to predict working conditions of mold parts in advance, predict location of problem defects and evolution trend of problems, provide a reference for guiding mold design and manufacturing processes.

Stress deformation trend is shown in Figure 7. Mold parts are subject to periodic cyclic changes in pressure, which causes elastic deformation. After mold molds a certain number of parts, fatigue occurs, which eventually develops into plastic deformation, causing mold parts to yield and crack. Maximum limit deformation value is shown in Figure 8. Under condition that aluminum liquid temperature is 680℃, mold temperature is 200℃, and casting pressure is set to a peak of 100MPa, CAE software analysis shows that maximum peak deformation of mold parts is 0.4374mm, and mold parts may have fatigue cracking after a period of service. Maximum deformation peak appears in cavity wall area of mold part, which is the area where cracking first occurs. However, actual cracking of mold parts occurs in runner area and cavity wall area close to runner area, which is inconsistent with results of CAE software analysis. It can be judged that fatigue fracture factors caused by stress and deformation are not cause of actual cracking of mold parts.

 

 

During die-casting process, mold cavity temperature changes periodically (200~471℃). As shown in Figure 9, runner area maintains a relatively high temperature. Temperature factor causes dent deformation trend in mold parts. CAE software analysis results are shown in Figure 10. Maximum deformation peak is 0.17mm. Thermal stress distribution caused by temperature factors is shown in Figure 11. From analysis results in Figure 11, it can be seen that there are stress concentration areas caused by local heat accumulation in mold parts, resulting in uneven stress. Stress is concentrated in corners and sharp corners, and stress distribution is consistent with actual cracking position of mold part. Stress in runner area is large and risk of cracking is high. Stress in cavity wall area of mold part is small, and risk of cracking is low. Maximum stress value is about 1139MPa, which is close to yield strength of material 8407 δ0.2=1250~1600MPa, indicating that plastic deformation of mold parts caused by temperature factors is more likely to cause cracking.

 

 

 

According to CAE software analysis results, mold parts have deformation trends and uneven stress distribution, which may cause cracking. For further verification, comparisons and verifications were made from mold part materials and mold structure design changes.

Same mold structure design was carried out using P20 material and domestic H13 material, production and service conditions were compared and verified. After mold parts made of domestic H13 material have been in service for 1,500 molds, cracks have begun to appear at the center push rod of runner area. Location of crack occurrence is same as crack location of original mold part, as shown in Figure 12. It is inferred that initial crack of original mold part also appears at this location.

 

Yield strength of mold parts made of P20 material is δ0.2=900~1100MPa, which is lower than domestic H13 material and original mold parts 8407 material. After serving 800 mold times, large-area cracks will occur. As shown in Figure 13, crack occurrence area is consistent with thermal stress distribution area of mold flow analysis, cracks have appeared on corners and sharp corner features of mold parts. It can be seen that uneven distribution of thermal stress caused by temperature is one of main factors that cause cracks in mold parts, yield strength of surface of mold parts is one of factors that affects cracks. Yield strength of surface of mold parts is low, and thermal fatigue resistance is low, which can easily cause cracks.

 

Design of central push rod in runner area is cancelled. As shown in Figure 14, mold parts are also made of domestic H13. After serving for 3,000 molds, fine cracks were found at corners of mold parts and sharp corners of other push rod openings, but there were no cracks at canceled push rod positions. It can be seen that eliminating push rod in this position also avoids sharp corners of its mouth and avoids cracks caused by sharp corners of mouth.