Analysis of shrinkage defects in biomedical CoCrMo alloy castings and its process improvement (Tech Center of Shanghai HY Industry Co., Ltd)

Biomedical materials are artificial non-living materials that are combined with living bodies for medical purposes, i.e. “natural or artificial materials used to replace and repair living tissues”. Its definition has been changing with the rapid development of medical materials.Nowadays, biomedical materials mainly refer to new high-tech materials used to diagnose, treat, repair or replace diseased tissues and organs of living organisms or enhance their functions. Metal biomedical materials have a long history of application, and in recent years biomedical cobalt-based alloys have been widely used in clinical medical and scientific research. Cobalt-based alloy castings have the advantages of high strength, good toughness, high bending fatigue strength and excellent processing performance, as well as many other excellent properties that cannot be replaced by medical materials. However, the physiological environment in which biomedical cobalt-based alloy castings are exposed to is corrosive, which can cause the diffusion of metal ions into the surrounding tissues and the degradation of the implant material itself, the former may lead to toxic side effects, and the latter may lead to the failure of the implant material. Therefore, the research and development of new cobalt-based alloy biomedical materials with better performance and biocompatibility is the constant goal of medical materials researchers.

Researchers such as Feng Yuting, Hu Xiaoli and Li Qiang from Tianjin Sino-German University of Applied Sciences and Hebei Guangde Precision Machinery Co., Ltd. focused on the problem of fluctuation of mechanical properties due to shrinkage in CoCrMo alloy femoral stem castings of surgical implants – bone joint prosthesis (see Figure 1), analyzed the production process of its shell and casting casting process, and explored the reasons for elongation failure and measures to reduce the shrinkage rate in order to improve product quality and production efficiency.

1 Test

1.1 Test material

• CoCrMo alloy is melted by vacuum and cast by investment casting process, using group tree process, each group of 8 moulds to form 8 femoral shanks

1.2 Test method

• According to GB/T 228.1-2010 “Tensile test of metal materials part 1: room temperature test method”, the three states of CoCrMo alloy femoral shank castings without heat treatment, solution treatment and solution + aging treatment were processed into tensile specimens of size ϕ8mm×10mm (the scale distance is 5 times the diameter), and two specimens were set for each state. After the test, a cylindrical specimen with a height of 10 mm was intercepted near the fracture of the drawn specimen and observed by ZEISS scanning electron microscope (SEM).

2 Test results and analysis

2.1 Tensile test

• Tensile strength of the specimens after solution treatment increased to different degrees, the tensile strength of the solid solution treated specimens reached 1150MPa, and the tensile strength of the solid solution + aging treated specimens reached 1110MPa, the standard requirement of tensile strength of not less than 600MPa; the elongation after fracture of the specimens of different heat treatment processes were mostly less than 5%, and did not reach the standard requirement of more than 5%. Therefore, it is necessary to further study the reasons for the failure of elongation after fracture through fracture analysis.

2.2 Fracture analysis

• 0-1 specimen fracture belongs to the brittle fracture, the fracture can be seen intact dendritic morphology, indicating that there are original defects between the dendrites, these defects have become the source of cracks, under the action of tensile force to gather growth, resulting in crack expansion fracture; 1-1 specimen is also brittle fracture, can be seen in the cast dendritic morphology, indicating that the solid solution treatment failed to eliminate defects; 2-1 specimen fracture along the crystal fracture characteristics, the grain is obviously grown, crack Along the grain boundary expansion, and fracture local smooth. A large number of shrinkage holes can be seen from the edge to the heart of the specimen, with more shrinkage holes in the heart.

• In the process of casting and specimen tensile test, the shrinkage in the material will become a source of cracking, which will seriously affect the continuity of the material, and the material will fracture and fail quickly under the action of external force. Combined with the mold making process, casting casting pouring process, analysis that the main reasons for shrinkage defects in CoCrMo alloy castings during investment casting are the following.

1. casting structure: the wall thickness of the casting is not uniform, in the wall thickness of the larger parts of the slower heat dissipation, thermal joints increase, wall thickness increases resulting in smaller diameter of the cast hole, cast hole paraffin heat greatly reduces the solidification rate of the metal on the surface of the cast hole, resulting in shrinkage and loosening of the hole wall.

2. Melting: unclean furnace charge, rust and moisture absorption causes the metal liquid gas content to rise, when the furnace temperature and pouring temperature is not stable, the metal liquid fluidity becomes poor, the gas does not completely escape during the cooling process of the casting, preventing the neighboring metal liquid to flow to the shrinkage, resulting in shrinkage or sparse.

3. pouring: unstable pouring temperature, the temperature is too high, resulting in an increase in the shrinkage of the metal liquid, the temperature is too low will reduce the shrinkage capacity of the riser, the phenomenon is more obvious when using the bottom injection pouring system; the riser is not full, will reduce the shrinkage capacity of the riser, resulting in shrinkage or loose castings; pouring speed is low, pouring time is long, will lead to a greater reduction in the temperature of the metal liquid, the viscosity becomes large, the bubble is difficult to float, and then increase the number of porosity defects.

3 Process improvement

• Comprehensive analysis of the above, combined with the characteristics of the existing casting mold and the existing problems, taking into account the difficulty of filling and shrinkage of the specimen, on the basis of strict control of the melting and pouring process, the existing casting mold head is modified to add an internal sprue, and the mold head is also changed from two teeth to three teeth, which greatly enhances the shrinkage capacity of the riser and reduces the probability of shrinkage holes.

• After the improvement of the mold making process and pouring process of CoCrMo alloy castings, the failure rate of castings was reduced from 9.8% to 3.1%, and the qualified rate of castings was greatly improved and the production cost was significantly reduced.

4 Conclusion

• CoCrMo alloy castings after break elongation failure is mainly due to the presence of a large number of shrinkage defects in the castings.

• By increasing the inner gate of the alloy castings and setting up additional three-tooth die, the shrinkage capacity of the riser is greatly enhanced, and the failure rate of the castings is reduced from 9.8% to 3.1%.

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