Microstructure And Properties Of Ni/WC Cladding Layer On H13 Steel By Laser Remelting

H13 steel（4Cr5Mo Si V1）is widely used as a hot work die steel in various die manufacturing.However,surface problems such as thermal fatigue cracks and wear often occur due to the effect of cold and hot alternating loads on the working environment and the contact friction between the formed parts and the cavity surface,which reduces the service life of the die.Therefore,more and more scholars have been studying the process of WC nickel-based coating on the H13 steel surface.The study found that the defects,such as pores and cracks,appear on the surface and inside of the cladding layer.For the above problems,in this paper,a composite process between laser cladding and laser remelting is used to prepare Ni/WC remelting layer on the H13 steel surface.At the same time,the characterization of its microstructure and properties provides technical support for the application of mold surface repair and reconstruction.In order to study the distribution law in the preparation process,the temperature field of the composite process was simulated and analyzed by finite element software.It was found that the temperature in the molten pool increased rapidly in a short time during laser cladding and laser remelting,reaching 1538.3℃ and 1502.6℃,respectively.At the same time,the temperature differences between the simulation and experimental results are 69℃ and 52℃,respectively.The difference of penetration depth and width of 0.2 mm is within a reasonable range,which proves the accuracy of the simulation result.In addition,the effects of different process parameters on the temperature field during remelting were studied.The results show that the laser current,defocusing,and scanning speed significantly affect the temperature field.Therefore,the process parameters need to be optimized.The laser remelting process of Ni/WC cladding layer on the H13 steel surface was studied.The effect of WC addition on the forming quality of laser remelting pre-coating was explored.The results show that with the increase of WC addition,the grain size of the pre-coating is smaller,and the fluctuation of crack sensitivity and microhardness value is more significant.The pre-coating prepared by adding30% WC shows the best performance.The number of pre-coating cracks and the fluctuation of microhardness value are both small.On this basis,to study the influence of different process parameters on the microstructure and properties of remelting layer,laser remelting of the pre-coating was studied in detail using the control variable method.The experimental results show that when the input heat of laser remelting is small,remelting process only occurs in a small area,which is not enough to discharge pores and heal cracks.When the input heat of laser remelting is significant,large thermal stress is generated in the molten pool,and solid shrinkage is caused during solidification,resulting in secondary cracks and pores in the remelting layer.The grain size in the remelting layer is reduced with the decrease of laser current,increased defocusing,or increased scanning speed.To sum up,the optimized parameters are as follows: the conditions of laser current 110 A,defocusing amount 7 mm and scanning speed 90 mm/min.Under such conditions,there are no apparent pores and cracks in the remelting layer.The elements are evenly distributed.The microhardness value is stable.And the average microhardness reaches 735.5 HV.To study the mechanism of laser remelting on the forming quality of laser cladding,the microstructure and properties of the coating before and after laser remelting were compared and analyzed.The results show that laser remelting play the role of secondary slag removal,exhaust and crack healing.After remelting,the surface of the coating is smooth,and the fish scale is more regular.There are no obvious pores and cracks in the remelting layer.The microstructure of the remelting coating is more acceptable and denser to achieve the purpose of fine-grain strengthening.At the same time,laser remelting makes the molten pool melt again,which can improve the uneven distribution of elements.The average microhardness of the coating before and after remelting is 638.8 HV and 735.5 HV,respectively,which is increased by about 15%.After remelting,the self-corrosion potential of the layer moves forward,the self-corrosion current density is decreased,the capacitive arc diameter is more significant,and the corrosion resistance is better.