| As the the development of new matrrials and technologys in the ship and engine repairing filed, the life improvement of vehicle parts and the repair of worn and corrosion parts become possible due to the development of advanced manufacture technology. In order to improve the metallic surface corrosion and wear resistance,304 stainless steel coating and 304 stainless steel/Al2O3 composite coating are obtained on the surface of 45 steel by laser cladding method, and its technology and performance are studied. The results show that crack sensitivity of composite coating is big because of the considerable residual stresses. For reducing or eliminating the residual stresses of the cladding samples, the principle of γâ†'ε phase transformation in Fe-Mn-Si shape memory alloy releasing the residual stresses is used for designing a kind of Fe17Mn5SilOCr5Ni shape memory alloy. The coating which possesses a low residual stress and good creak resistance is successfully prepared on the surface of 304 stainless steel substrate by laser cladding, and this thesis focuses on the following works.The 304 stainless steel laser cladding coating possesses smooth surface when the laser spot diameter is 3 mm, scanning speed is 600～1000 mm/min, laser power is 2-3 kW and the overlapping rate is 50%. Based on this, put Al2O3 powders whose chemistry compositions is 5wt.%, 10wt.%,15wt.% respectively into the 304 stainless steel coating, the 304 stainless steel/Al2O3 laer cladding composite coating is prepared, and possesses good surface property under the conditions of the laser spot diameter 3 mm, scanning speed 800 mm/min, laser power 2.5 kW, and the overlapping rate 50%. Micro structure of the composite coating is in sequence of planar crystal, cellular crystal, dendritic crystal and fine equiaxed grain from the the bottom of molten pool to surface, heat affected zone includes quenched area and normalized zone which is composed of fine martensite and ferrite respectively. In comparison with laser cladding 304 stainless steel coatings, the composite coating possesses higher microhardness and better wear resistance, and its wear mechanism is abrasive wear. Polarization curves indicate that the composite coating is easier to be corroded than laser cladding 304 stainless steel coating, but much harder than the substrate. Besides, the results show that, as the content of Al2O3 is increased, the surface roughness is decreased, microhardness is a little improved, wear and corrosion resistance doesn’t change. However, adding Al2O3 hard particle induces the improving residual stresses and crack sensitivity in the composite laser cladding coating.The study of preparation technology of Fe17Mn5Si1OCr5Ni shape memory alloy laser cladding coating shows that, chemistry compositions of Mn and Si in the cladding layer reduces greatly, while chemistry compositions of Cr and Ni improves when the Fe-Mn-Si coating is obtained in the air environment, that’s because the burning loss rate of Mn and Si is bigger than Cr and Ni, and a part of Cr and Ni move from the substrate to the cladding layer. The coating is obtained on surface of 304 stainless steel in natural environment under the conditions of the powder mixture ratio Mn:Si:Cr:Ni:Fe=32:9:4:3:Bal. (wt.%), laser spot diameter 3 mm, scanning speed 600 mm/min, laser power 2 kW, and overlapping rate 50%.Fe17Mn5Si10Cr5Ni shape memory alloy laser cladding coating is composed of ε martensitic and γ austenite phases containing transition from planar crystal, to (?), dendretic crystal, equiaxed crystal and the oxide with an increase in distance from the the bottom of molten pool to surface, and the tempering (1000℃×1 h) coating is composed of coarse austenite. The wear stress induced γâ†'ε phase transformation and its transformation deformation can inhibit the slip deformation and the formation and extension of the dislocation, thus the coating shows better wear resistance and contact fatigue strength compared with the 304 stainless steel substrate. The phase transformation was driven by the residual stresses, thus the residual stresses and the deformation in the coating is small.The shape memory alloy coating possesses higher stress relaxation ratio than 304 stainless steel. Stress relaxation ratio of the coating decreases with the increasing pre-strains, increases sharply in the first 5 minutes then slow down with the increasing suspending loading time. In the process of tension and suspending loading, γâ†'ε phase transformation occurs, εâ†'α’ and γâ†'α’ phase transformation occurs in the terminal phase. With the increasing suspending loading time, the shape recovery ratio of the coating reaches a maximum in the frist 10 minuts and then decreases, and the maximum shape recovery ratio decreased with the increasing pre-strains. |
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