Study On The Process And Mechanical Properties Of Fe-8Ni Alloy Prepared By Metal Injection Molding

Metal injection molding（MIM）technology combines the advantages of powder metallurgy and plastic forming processes,which is particularly suitable for the nearest shaped forming of small and medium-sized metal parts with complex shapes.Fe-Ni alloys have high hardness and strength,but the poor processing properties limit their application as parts of high precisions.In this paper,Fe-8Ni alloy samples were prepared by MIM technology.The effects of key parameters of injection molding,degreasing and sintering processes on the apparent quality,microstructure and mechanical properties of Fe-8Ni alloy were studied.Moreover,the microstructure and mechanical properties of Fe-8Ni alloy were compared with those of Fe-2Ni alloy basing on the optimized processing parameters.The main results are as follows:The optimum loading of alloy powder is calculated to be 60%in the feed.A uniform feed consisted of powder and binder is achieved after the mixing at 185℃for 30 min.The effects of injection temperature,pressure and speed on the apparent quality of injected green bodies were studied by orthogonal testing method.Results show that the three parameters of high values favor the filling of mixed feed in the mould,whereas higher or lower values lead to many defects formed in the injected green bodies.When the feed is injected with a speed of 50 mm/s,the injected green body exhibits high formability and few defects are found at the temperature of 190℃and pressure of 50 MPa.The relationship between the degreasing temperature and time vs the degreasing rate of injected green bodies was studied under the catalytic atmosphere of HNO₃vapor.Meanwhile,the effect of sintering temperature on the apparent quality of Fe-8Ni alloy was analyzed.The degreasing rate of green bodies at 120℃is higher than that at 110℃.The degreasing rate increases with the extending time,while its rate tends to slow and then to be stable after 150 min.The injected green bodies of high degreasing rate of 91.8 wt%and good apparent quality can be obtained when the temperature is at 120℃holding for 300 min.As the sintering temperature elevates from 1210℃to 1290℃,the shrinkage ratio and density of Fe-8Ni alloy increase firstly before a decrease.When the degreased bodies are sintered at 1270℃for 180min,the density of Fe-8Ni alloy samples exhibits a maximum of 7.68 g/cm³ and defect-free.Relatively low or high sintering temperatures and high heating rate contribute to the increase of internal pores and surface defects.The variations of microstructure and mechanical properties of Fe-8Ni alloy versus the sintering temperature were investigated.Results show that the pearlite structure and mechanical properties tend to increase as the temperature elevates from1210℃to 1270℃.The hardness and tensile strength have maxima of 226.32 HV and 773.21 MPa respectively,with the temperature at 1270℃.When the temperature is further elevated to 1290℃,the extreme growth of crystal grains and the number increase of pores cause a decline of the mechanical properties.Fe-8Ni alloy sintered at different temperatures exhibits a ductile fracture under tensile loading,whose toughness can be improved at high temperatures.Compared to Fe-2Ni alloy,the density of Fe-8Ni alloy after sintering at 1270℃is evidently increased since high content of Ni element can promote the sintering densification of iron-based alloys.Low content of Ni element leads to the increase of ferrite structure and decrease of solid solution strengthening effect,which are responsible for the low mechanical properties of Fe-2Ni alloy.The corresponding hardness and tensile strength are 222.30HV and 680.89 MPa.Moreover,the tensile failure of Fe-2Ni alloy exhibits a quasi-cleavage manner with a toughness decrease.