Research On Microstructure,strengthening Mechanism And Performance Of Laser Cladding Nickel-based Alloy And Its Surface Laser Peening

316L stainless steel is widely used in aerospace,petrochemical industry,nuclear power and other fields due to its superior corrosion resistance and mechanical properties.However,lower hardness and poor wear resistance have limited the application of 316L stainless steel in key frictional components.Nickel-based alloys have broad application prospects in engineering applications due to their higher bonding strength,better wear resistance,and excellent corrosion resistance.As a new manufacturing technology integrating laser processing technology,numerical control technology,materials technology,and other disciplines,laser cladding deposition technology,based on the advanced“discrete-superimposition”idea,adopts a point-by-layer forming method,and has attracted wide attention in recent years due to these advantages of fast speed,no mold,and material saving.Based on the advantages,this thesis focuses on the actual application requirements of 316L stainless steel surface strengthening/repair,combined with the poor controllability of mechanical laser cladding deposition technology.It was proposed to apply laser peening technology to nickel-based alloy cladding layers,which can provide the theoretical foundation and engineering experience for practical application of composite strengthening technology.Firstly,316L stainless steel was used as the test substrate,and nickel-based alloy powder was used as the raw material,then single-layer single-pass laser cladding experiments were carried out under different process parameters to determine the best process parameters.Used these optimized process parameters,a single-layer multi-pass nickel-based alloy cladding layer was prepared.With modern analytical testing methods,phase composition and microstructure of the cladding layer were analyzed systematically,and its mechanical properties were studied.Multi-layer multi-pass nickel-based alloy cladding layers were prepared under different laser powers.Through XRD,OM,SEM,EDS,EBSD,TEM,friction and wear performance,electrochemical performance,and other tests,the influence of laser power was discussed on the structure and property of the cladding layer.The study found that the nickel-based cladding layer prepared with optimized process parameters had a good metallurgical bond between the substrate and the cladding layer without obvious defects.The dislocation density is not high but its distribution is uniform in the cladding layer.Increasing the laser power had no significant effect on the phase composition in the multi-layer multi-pass layer.However,the size of dendrite was become big,the?-Ni solid solution phase was shifted left significantly,thicker columnar dendrites were formed in the bonding zone of the cladding layer,more equiaxed dendrites were obtained in the cladding layer,the element distribution among the cladding layers was different,and the microhardness of the cladding layer was decreased.With?-(Fe,Ni)solid solution phase,Cr₅B₃,Ni₁₇Si₃,and Ni₃B strengthening phase being in the cladding layer,the friction and wear performance and corrosion resistance of the substrate were improved significantly.Laser peening technology was used to post-process the nickel-based alloy cladding layer,study the microstructure change law of the cladding layer after laser peening,discuss the evolution mechanism of grain refinement and dislocation configuration,study the distribution of residual stress field,analyze the microhardness change of the cladding layer,analyze the friction coefficient and wear scar morphology under different loads,and discuss the friction and wear mechanism.The results show that the dendrites near the outer surface of the cladding layer had a certain preferred orientation after laser peening,the grain was refined obviously,the proportion of subgrain boundary was reduced,but small angle grain boundary was still dominated,dislocations were piled up and entangled,dislocation substructure was formed,the distribution of residual stress field was produced,and the distribution was relatively uniform,which improves the microhardness of the cladding layer,and reduces the friction coefficient of the cladding layer.With the increase of load,the average friction coefficient also decreased.The wear mechanism was mainly adhesive wear and abrasive wear,but the friction and wear property was become better.Finally,Laser cladding deposited nickel-based alloy remanufacturing application research was carried out.Through microstructure analysis,it was found that the microstructure obtained by multi-layer and multi-channel laser cladding was different from that obtained by single multi-layer and multi-channel laser cladding.The repaired layer did not orderly present planar crystal,cellular crystal,columnar crystal and equiaxed crystal from bottom to top,and the growth of dendrite was random.In the middle of the repaired layer,the dendrite growth direction could be roughly divided into three directions:horizontal,vertical and inclined 30^o.There are high density dislocations in the repaired layer,a larger number of nanophases in the grain,and some metal compounds around the grain boundary,which can improve the strength of repaired layer effectively.Based on the above experimental research and mechanism analysis,some innovative achievements of this work were listed as follows:(1)The orthogonal experiment method was used to study the influence of laser cladding process parameters on the quality of single-layer single-pass nickel-based cladding layer,and it was revealed that laser power and scanning speed were the main factors affecting the quality of cladding layer.The range analysis method was used to determine the optimal process parameters of laser cladding,which provided theoretical and experimental basis for large area laser cladding deposition.(2)The effect of laser power on microstructure and properties of multi-layer multi-channel cladding was studied.It was found that with the increase of laser power,the dendrite size increased,the microhardness of the cladding layer decreased,and the influence of laser power on the microstructure and properties of the cladding layer was mastered.(3)Using laser peening technology combined with laser cladding deposition technology to prepare nickel-based alloy coatings was proposed,and the microstructure evolution law of the cladding layer under the action of laser shock wave was preliminarily explored.It was found that laser shock strengthening could effectively improve the surface hardness and wear resistance,and solve the problem that the mechanical properties of parts decreased due to the uneven temperature field in the process of laser cladding deposition.