Study On Multi-stage Thixoforging Of Rolled SKD11 Cold Work Die Steel

The inhomogeneous quality of products manufactured by semisolid forming is mainly attributed from the liquid phase segregation of semisolid metal billets during forming.Although phase segregation can be inhibited by lower forming temperature,higher strain rate,and lower stroke,the realization of net-shape forming with above parameters is quite difficult.In this study,a multi-stage thixoforging strategy including a partial melting,a fast first compression,a partial solidification,and a secondary compression was proposed to weaken phase segregation of thixoformed samples.A physical simulation of multi-stage thixoforging was conducted on commercial rolled SKD11 cold work die steel,using a multistage hot compression test machine.The microstructural evolution and thixoforming behaviors of the rolled SKD11 cold work die steel in different stages of multi-stage thixoforging with various forming parameters were investigated experimentally.Out flow of liquid phase does not occur throughout the thixoforming,and can be inhibited effectively by partial solidification and subsequent secondary compression.The results showed that during the remelting process,as the remelting temperature increases,microstructure evolutions such as austenization,austenite grain growth,the dissolution of two different morphology carbides and partial melting took place in the rolled SKD11 cold work die steel;Grain sizes of austenite firstly increased rapidly and then decreased slightly,while the liquid fraction and shape factors increased continuously.The results showed that in the fast first compression,the flow stress-true strain curve of rolled SKD11 cold work die steel could be divided into three stages by two critical true strain valuesε_c1 andε_c2.When the deformation temperature increased and the strain rate decreased,the flow stress and liquid phase segregation decreased and increased respectively.Compared with the first and third stage,liquid segregation increased more rapidly in the second stage.In the partial solidification stage,the shape factors of the solid grains in the center and edge region decreased with decreasing cooling temperature and cooling rate.Due to the difference in the liquid fractions of the center and edge region,on the one hand,the microstructure evolutions of the center and edge region after partial solidification is significantly different;On the other hand,the solid grains grew in different types during solidification.When the strokes of double-passes semisolid compression are 2mm and 4mm respectively,the liquid phase segregation increased in the secondary compression stage;when the double-passes semisolid compression strokes are 4mm and 2mm respectively,the liquid phase segregation remaind basically unchanged in the secondary compression stage,and strain rate only slightly affected liquid segregation under these parameters.SKD11 cold work die steel samples with more homogenous microstructure and smoother surface was manufactured by multi-stage thixoforging including a fast first compression with a stroke of 4 mm and a strain rate of 2.0/s at 1280°C,a partial solidification to 1240°C with a cooling rate of 2°C/s,and a secondary compression with a stroke of 2 mm and a strain rate of 0.1/s at 1240°C.Finally,the feasibility of multi-stage thixoforging strategy was verified.