Fe-Mn-Si-Cr-Ni composite powders are utilized to form a functional shape memory alloy cladding layer (SMACL) using a laser cladding method. The microstructure, microhardness, and phase composition of the SMACL are measured, and the extent of deformation of the laser cladding samples is determined. The SMACL is composed of planar, cellular, and dendritic crystals, equiaxed grains, and oxides with increasing distance from the substrate surface. The SMACL is further composed of ε-martensite and γ-austenite phases, while the tempered SMACL consists mainly of γ-austenite. Extensive deformation occurs in AISI 304 stainless steel laser cladding samples. By contrast, limited deformation is observed in the SMACL samples.
The paper presents the stress self-accommodation characteristic of Fe-Mn-Si shape memory alloy, namely the alloy causes positive/reverse ε martensitic transformation and accompanied by deformation in order to adapt the variation of the outside macroscopical stress and deformation. From XRD analysis, it is found that the stress-induced γ↔ε martensitic transformation and its reverse transformation would occur in Fe-Mn-Si shape memory alloy under the tension-compression stress and also validated the stress self-accommodation characteristic of Fe-Mn-Si shape memory alloy. In cycles of tension and compression deformation, the stress self-accommodation characteristic of Fe-Mn-Si shape memory alloy can increase fatigue life of the alloy by reducing stress concentration, restraining plastics gliding deformation and delaying the formation and growing of microcracks. The fatigue fracture in Fe-Mn-Si alloy shows quasi-cleavage brittle rupture type.
The crystal morphology and formation of the microstructure of the laser weld of Fe-Mn-Si memory alloy were analyzed by metallographic microscope and scanning electron microscope(SEM) and X-ray diffractometer (XRD). The results show that the welding seam microstructure of Fe-Mn-Si memory alloy crystallizes symmetrically from the fusion zone on both sides to the weld center. The γ→ε martensite transformation occurs in the welding seam and its adjacent tissues due to laser welding. This is because of the residual stress generated in the welding process, and this kind of ϵ martensite transformation and its deformation relaxes the residual stress of the weld, thus affecting the mechanical properties of the weld.
The dissimilar joints of Fe-17Mn-5Si-10Cr-5Ni memory alloy and 304 stainless steel were welded by 5 kw transverse flow CO2 laser. The microstructure, morphology and composition of the welding of the dissimilar joints were analyzed by metallographic microscope, field emission scanning electron microscopy. The results showed that the crystals of dissimilar joints of Fe-17Mn-5Si-10Cr-5Ni memory alloy and 304 stainless steel were symmetrically distributed on both sides of the joint weld center. At Fe-17Mn-5Si-10Cr-5Ni memory alloy close to the side of the weld, ε martensite phase was also found, which was caused by the γ→ε martensite transformation induced by the residual stress. The distribution of dissimilar joint weld elements was in zones obviously, the distribution mode was related to the unique keyhole effect of laser welding, and the weld center metal was still Fe-Mn-Si memory alloy, which had the characteristic of the inverse phase transition of the stress induced martensite.
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