Study On The Structure And Properties Of Cold Mold Surface Repaired By CO₂ Laser Cladding

According to the statistics,75 percent roughing and 50 percent finishing mechanical parts will be completed by the mold to finished within the next five years. The level of manufacturing mold technology has become the important indicator to measure the manufacturing level of a country. Most of our molds are produced for own used, Among only about 1/3 mold are commercial mold, and the domestic mold is still in short supply, particularly the sophisticated, large-scale, complex and long-life mold had to rely on imports. Cold work die mold output always stay at the first of manufacturing, the common feature of working conditions is: operating temperature is not high, normal working temperature is not exceed 200~300℃,bear high pressure or impact on work, and continuous with the processed material surface for friction,make the work surface to wear, excessive wear will lead to failure and scrap.During the manufacturing process of cold work mold, not only require the mold surface or parts have high hardness and wear resistance,but also have good toughness inside, while this comprehensive requirements is restricting the practical application of cold-working die steel bottleneck.And cold work mold will appear overload failure, wear failure and fatigue failure during the using process, only wear failure can repaired to use again.For the above, this paper use the laser cladding technology to prepare a certain thickness of nickel-based alloy cladding on the surface of typical cold-working die steel Cr12,to improve the surface hardness and wear resistance,but the matrix still has a good strength and toughness.Study the laws of different laser power, scanning speed, powder feed rate and carrier gas and other process parameters influence on the macro-morphology and size of cladding layer,optimizate the laser cladding technology parameters;Comprehensive use the modern means of physical testing(include X-ray diffraction, scanning electron microscopy and energy spectrum analyzer)to analyse the different processing parameters laser cladding of microstructure, phase composition, micro-ingredients, reveal the laser parameters affect on the microstructure of cladding layer;Testing the laser cladding layer microhardness of the surface and cross section, reveal the relationship between microhardness and Laser power as well as scanning speed,further optimizate the process parameters;Testing the laser cladding layer friction and wear properties, reveal the wear mechanism of laser cladding layer and the friction and wear properties with variation of laser parameters.Research results are as follows: 1.Study of the process on laser cladding repairing the cold work moldThe quality of laser cladding repairing the mold is closely related to the process parameters of laser cladding process parameters: With the increase of laser power P ,the cladding layers width W first increased and then decreased,the height H gradually reduced;Increase the scanning speed V make the cladding layers width W and height H are all reduced; With the powder feed rate Rp increased, the cladding layers width W and height H are all increased; With the containing gas Rg increased ,the cladding layers width W and height H are equally decreased,but change not significant, the reason is containing gas Rg only affect the powder feed speed,does not affect the powder feed rate;The single-channel single track cladding layer convex was mainly caused by surface tension gradient lead to the convection,horizontal temperature gradient caused the pool convection and the wettability of pool and substrate interaction;Appropriate lap rate can ensure the multi-track cladding layer smooth;Optimization of single-layer laser cladding process parameters are:laser power P is between 1800W to 2000W, scanning speed V is between 240mm/min to 300 mm / min, powder feed rate Rp is between 5g/min to 7.2g/min, containing gas Rg is 200L / h and overlap rate LR is 30%.Using Z-axis incrementΔZ is between 0.4mm to 0.6mm and smaller than the first cladding layer thickness to prepare single-channel multi-layer cladding layer,Shape consequent is better, this process can restorate the surface nick of mold also can be used for forming thin-walled parts. When multi-channel multi-cladding, the adjacent two cladding layer should be adopted vertical stacking, not only can prevent the defects inherited and amplificated,but also can smooth the surface of cladding layer, this process can be used for repairing a large area surface of the mold and prepare the block materials.2.Study of the laser cladding layer microstructure on the die steel surfaceLaser cladding of die steel samples and the general laser surface treatment of samples have similar cross-section tissue distribution, the cross section of samples along the depth direction have four regions: cladding layer, bonding zone, heat- affected zone and substrate.EDS line scanning analysis shows: cladding layer is mainly enriched in Ni element, Cr element, Fe element and Si element; XRD analysis shows:cladding layer mainly haveγ-Ni solid solution, CrB, Ni4B3, Ni3Si, Ni2.9Cr0.7Fe0.36 phase; Different laser cladding process parameters will obtained different microstructure of the cladding layer; Choose reasonable parameters of laser cladding, just like the laser power P is between 1800W to 2000W and scanning speed V is between 240mm/min to 300 mm/min,can refine cladding layer tissue and make the cladding layer and the substrate have a better metallurgical bonding.If the laser cladding process controlled improper, cladding layer will formed cracks, porosity, inclusions and other defects; to pretreat the cladding powder and substrate,and use the optimized process parameters of laser cladding, can minimize the presence of defects.3.Study of the laser cladding layer surface properties on the die steelLaser cladding can improve the surface microhardness of the die steel significantly,with the increase of laser power P,the surface microhardness of cladding layer also increased, but the laser power P is not the higher the better,when the laser power P is between 1800W to 2000W,the microhardness of cladding layer is maximum.With the increase of scanning speed V, cladding layer tissue refine, microhardness followed increase;when the scanning speed V is too high,surface microhardness of cladding layer will be appeared greater fluctuation, when the scanning speed V is between 240mm/min to 300 mm/min, the microhardness of cladding layer is maximum. When the die steel by laser cladding treated, the microhardness of cladding layer over the substrate significantly increased, for constant laser power P or scanning speed V, the cross section microhardness of die steel along the depth gradually decreases, and the microhardness of heat-affected zone is also higher than the substrate.Laser cladding can improve the wear resistance of die steel significantly, under the same conditions, the wear resistance of laser cladding treated die steel is better than the original substrate.With the increase of laser power P, wear quality gradually reduced, coefficient of friction became smaller,but excessive laser power will reduce the wear resistance of cladding layer.