Study And Optimization Of Interface Of "Skin Layer"-"Bone Layer" And Surface Crack Of "Fist-Like" Die

In order to solve the problem of microcracks on the surface layer of the “sandwich layer” forging die due to the continuous high temperature and high pressure of forming difficult deformation material,a new method of “fist-type” forging die design and manufacturing was put forward.In this method,the material with high strength and high hardness can play a supporting role is used as the “bone layer” of the forging die,and then the material with good high temperature mechanical properties is added as the “skin layer”.In order to get the practical application of the “fist-like” forging die,this paper adopted the method of combining experiment and simulation,starting from the arc additive manufacturing process,mainly carried out the research on the performance of the “skin layer”-“bone layer” dissimilar metal interface and controlling the transverse propagation crack on the forging die surface.The numerical simulation model of “skin layer” single pass multi-layer additive manufacturing was established by using Marc finite element software.The effects of different welding heat source power(4000-6000W)and welding speed(8-15mm/s)on the distribution of temperature field and stress field were studied.The results showed that when the welding speed was constant,with the increase of the welding heat source power(or when the welding heat source power was constant,with the decrease of the welding speed),the weld temperature increased and the weld pool became larger,which can promote the formation of reliable metallurgical bonding between the “skin layer” and“bone layer” to a certain extent.But the high temperature residence time increased and the cooling rate decreased of the metal near the joint face,which easily leads to coarse grains.At the same time,the longitudinal residual stress of the joint face increased,while the transverse residual stress decreased.Therefore,the welding heat source P=5000W and the welding speed V=10-12mm/s can be selected to improve the comprehensive mechanical properties of the joint interface.The grey based Taguchi method was used to optimize the welding voltage,wire feeding speed and welding speed by taking the high temperature ultimate tensile strength and hardness of the “skin layer”-“bone layer” joint interface as multi-response parameters.The results showed that when the welding voltage is 26 V,the wire feeding speed is9000mm/min,and the welding speed is 500mm/min,there was a greater grey correlation value.Compared with the initial optimal group,the grey correlation degree of specimen optimization was increased by 42%,in which the tensile strength was increased by 5MPa,and the hardness was increased by 32 HV.Therefore,the comprehensive mechanical properties of the joint face were improved to a certain extent.Meanwhile,the effect of welding speed on the microstructure and mechanical properties of the joint face was studied.The results showed that there was more retained austenite in iron-base alloy at lower welding speed and finer columnar structure in nickel-based alloy at higher welding speed.At lower welding speed,the microhardness of iron-base alloy was slightly lower,but that of nickel-based alloy was higher,and the ultimate tensile strengthen was higher.The numerical simulation models of “skin layer” and “bone layer” multi-pass and multi-layer additive were established by using Marc finite element analysis software,and the effects of “single-path-multi-layer by single-path-multi-layer in the same direction /staggered” and “layer by layer in the same direction / staggered” on the welding stress field and residual deformation were studied.The results showed that the “staggered layer by layer” additive path was more conducive to reduce the residual stress near the interface and residual deformation.In addition,the welding stress field and deformation of “flat plate type” and “concave convex type I / II” joint face structure models were compared.The results showed that the welding residual stress and deformation near the joint face can be reduced to a certain extent by placing part of the joint face under the concaveconvex connection mode,and the forging die with better forming quality and smaller residual stress can be obtained.In order to explore the formation of transverse propagation cracks on the surface of forging die,the parallel path specimen of JX07 iron-based material were prepared and the high temperature tensile test was carried out.The results showed that the tensile strength along the weld direction was better than that of the vertical weld.Based on this,the cross path between vertical and transverse weld was designed and prepared.The results showed that the tensile strength of the vertical junction specimen was between the weld direction and the vertical weld specimen,and the elongation of cross path specimen was higher than that of parallel path specimen.Therefore,the cross additive path can not only using the whole vertical weld with better mechanical properties to hinder the further propagation of cracks at the junction of transverse weld in the layer,but also improving the overall mechanical property,so as to reduce the length and number of transverse propagation cracks on the surface of forging die to a certain extent and slow down the failure rate of forging die.