Study On Microstructure And Friction Behavior Of Bulk Al_0.5CoCrFeNi High Entropy Alloy Prepared By SLM

High entropy alloy（HEA）tends to form a simple solid solution structure,which has excellent properties such as high strength,high hardness,good thermal stability,fatigue resistance,corrosion resistance and oxidation resistance.It has important application potential in aerospace,petroleum industry,national defense and military industry,biomedicine and other fields.At present,the preparation process of high-entropy alloys mainly focuses on traditional preparation methods such as vacuum arc melting,casting and mechanical alloying.Most of the prepared high-entropy alloy parts have problems such as coarse grains,composition segregation and limited macroscopic forming size.Selective Laser Melting（SLM）has the advantages of ultra-fast laser heating,rapid cooling and multiple thermal cycles,as well as the characteristics of near-net forming,high material utilization,and the ability to form parts of any complex shape.It is a potential technology for forming bulk high-entropy alloys with uniform structure and excellent performance.In this paper,AlxCoCrFeNi（x=0.5）high-entropy alloy gas atomized powder was used as raw material.Bulk Al_0.5CoCrFeNi high-entropy alloy with 10 mm×10 mm×5 mm was successfully prepared by optimizing SLM forming process.The phase composition,microstructure and wear properties of SLM formed Al_0.5CoCrFeNi high-entropy alloy were analyzed.The influence of SLM forming process on the microstructure and friction behavior of high-entropy alloy was systematically studied,which laid a theoretical and technical foundation for SLM forming complex high-entropy alloy parts.The process optimization of SLM forming Al_0.5CoCrFeNi bulk high-entropy alloy was carried out.When the thickness of the powder layer t=25μm,the laser scanning power P=100W,the scanning speed v=1500 mm/s,the scanning spacing h=0.06 mm,and the interlayer rotation angleα=90°,the SLM sample surface has the least defects such as pores,cracks and ravines,and the sample forming quality is the best.The main phase composition of SLM formed Al_0.5CoCrFeNi bulk high-entropy alloy is face-centered cubic phase.With the increase of volume energy density（VED）,the face-centered cubic phase gradually decreases and the body-centered cubic phase gradually increases.The microstructure of SLM formed bulk high-entropy alloy is affected by different forming parameters.The cross section parallel to the printing direction is in the shape of‘fish scale’and‘horizontal line’.The interior is dominated by columnar crystals.A single grain passes through the multi-layer deposition layer,showing epitaxial growth characteristics,and the maximum length of columnar crystals is about 90μm.The crystal texture of the bulk high-entropy alloy shows that compared withα=0°andα=90°,whenα=67°,the grain orientation is the most‘messy’,and the texture uniformity density is the lowest.The microhardness and friction and wear properties of different forming surfaces of SLM formed Al_0.5CoCrFeNi high-entropy alloy show anisotropy.The surface microhardness and friction and wear properties parallel to the forming direction are better than those perpendicular to the forming direction.Based on the analysis of microhardness and friction and wear properties,it can be seen that with the increase of VED from 118.5 J/mm³ to 148.1 J/mm³,the average friction coefficient of SLM formed Al_0.5CoCrFeNi high entropy alloy is 0.64,0.68 and0.61,respectively.The wear volumes are 0.088 mm³,0.094 mm³ and 0.073 mm³,respectively.The wear resistance decreases first and then increases,which is consistent with the influence of VED on the microstructure of high entropy alloy,such as grain size,crystal texture and dislocation density.In addition,in the analysis of the interlayer rotation angle on the wear resistance of the sample,it was found that the wear volume and average friction coefficient of theα=90°sample with high texture strength were 0.046 mm³ and 0.52,respectively.Compared with theα=67°sample with low texture strength,the wear volume and average friction coefficient were 0.073 mm³ and 0.61,respectively,and the wear resistance was better.