Research On The Microstructure And Properties Of Microcrystal/Nanocrystal Co-based Alloys By Laser Additive Manufacturing

Co-base alloy has been widely used for the excellent wear resistance,high temperature resistance and corrosion resistance.A CoCrW alloy was fabricated on H13 steel successfully by Selective Laser Melting?SLM?.The effects of Fe content on the phase composition,microstructure,element distribution,microhardness,wear resistance and corrosion resistance of CoCrW alloy were studied in the optimum process parameters.The effects of solution temperature on the microstructure and microhardness of the multi-layer CoCrW alloy prepared by SLM were revealed.With microhardness and dilution rate of single-layer single pass CoCrW alloy as the evaluation criteria,SLM process was optimized by using the method of orthogonal experiment,and the optimal process parameters are obtained as current 380A,pulse frequency 4Hz,pulse width 8ms and scanning speed 100 mm·min^-1.The influence of matrix elements on the preparation of CoCrW alloy by SLM was mainly in the first and second layers,while the influence of other layers was negligible.The phase composition,microstructure and element distribution of different layers of CoCrW alloy were systematically analyzed.The results showed that the crystal structure of CoCrW alloy were mainly composed of face centered cubic of?-CO and carbides such as cocx and M₂₃C₆.Within three layers of CoCrW alloy,?-?Fe,Ni?solid solution was formed.The seventh layer of CoCrW alloy produced Ni-Cr-Co-Mo.Along the Z-axis direction,the distribution characteristics of microstructure of CoCrW alloy with different layers from bottom to top were in the order of plane crystal?cellular crystal?dendrite?equiaxed crystal,and the grain size showed a decreasing trend.Under the influence of H13 steel matrix,in the Z-axis direction,the element distribution of near surface,middle part and fusion line part of CoCrW alloy was different.The interlaminar structure was coarse,and the dendrite growth direction of the latter layer continues the crystallographic direction of the previous layer.In this paper,the microhardness,wear resistance and corrosion resistance of CoCrW alloy prepared by SLM were tested.The maximum microhardness of the first layer,the second layer,the third layer,and the seventh layer were 596HV_0.2,630HV_0.2,668HV_0.2,and 699HV_0.2,respectively.The wear patterns of the first layer,the second layer,the third layer and the seventh layer at room temperature were abrasive wear+oxidation wear,adhesive wear+oxidation wear+slight abrasive wear,adhesive wear+oxidation wear+slight abrasive wear and adhesive wear+oxidation wear,respectively.The wear behavior of different alloy surfaces at high temperatures was oxidation wear and abrasive wear,and the seventh layer had the least wear degree.The corrosion potential in the 3.5wt.%Na Cl solution was H13<1 layer<2 layers<3 layers<7 layers,and corrosion current was H13>1 layer>2layers>3 layers>7 layers.The impedance spectroscopy was 7 layers>3 layers>2 layers>1layer>H13 steel,so the corrosion resistance was 7 layers>3 layers>2 layers>1 layer>H13steel.After solution treatment at 900?,1100?and 1200?for one hour,CoCrW alloy was mainly composed of face centered cubic?-Co,Cr₂₃C₆ and Cr₇C₃.With the increase of solution temperature,the interlaminar tissue gradually disappeared.The tissue growth was anisotropic,and the near surface tissue was oxidized.After solution treatment at 900?,the microstructure grew up and the element composition did not change significantly.After solution treatment at 1100?,the microstructure grew obviously and the content of Cr-rich carbide increased.After solution treatment at 1200?,the Cr-rich carbides were distributed in a discontinuous granular form.With the increase of solution temperature,the microhardness of CoCrW alloy in the Z-axis gradually decreases.