Study On The Microstructure And Properties Of CuZnAl Shape Memory Alloys Fabricated By Selective Laser Melting

Cu-based shape memory alloys(SMAs)are promising materials for intelligent components because of their good shape memory properties,superelasticity,wide sources of raw materials and low cost.At present,the separation of materials preparation and forming as well as the difficulty to fabricate the complex components are prominent problems in Cu-based shape memory alloy components.As an important representative of metal additive manufacturing technology,selective laser melting(SLM)can melt selectively the powder in the preset powder bed to realize the integration of materials preparation and forming,which can produce the components with arbitrary complex geometry.Starting with the control of shape and properties of selective laser melted CuZnAl shape memory alloys,this study carried out three aspects of research:the forming process and microstructure evolution of CuZnAl alloys,the composition optimization design of CuZnAl alloys and the effect of thermal cycles on the microstructure and properties of CuZnAl alloys.The micro-defects and the microstructure evolution during SLM process were revealed.It was found that the process parameters can change the content of?'martensite by affecting the content of Zn,which can further result in the difference in properties.A composition design theory of CuZnAl alloy powder for SLM process was established as well,through which the problem of?'phase absence in CuZnAl alloys prepared by SLM can be solved.In addition,the effect of thermal cycles on the microstructure and properties of CuZnAl alloys was clarified and a method of heat treatment to eliminate the effect of thermal cycles was proposed,which can help improve the superelasticity.The findings of this research provide theoretical guidance for the application of selective laser melted CuZnAl alloys in intelligent components.The main contents and conclusion are as follows:(1)Formation mechanism of defects in CuZnAl alloys prepared by SLM.The main defects in the selective laser melted CuZnAl alloys with low Zn content include porosity and element evaporation.The formation of pores is mainly caused by the following three reasons:spheroidization and insufficient overlapping tracks caused by the low laser energy input;the uneven powder spreading caused by the splashing of melt and powder on the surface of molten track;the small spherical pores caused by the evaporation of Zn and the residual gas.In addition,due to the large amount of Zn evaporation during the SLM process,there is a predominance of?phase and a small quantity of?'phase in the alloys.What's more,the?'phase is absent when the energy density increases to 194.4 J/mm~3.(2)The effect of energy density on the microstructure and properties of CuZnAl alloys prepared by SLM.The phase constitution is significantly different resulted from the different content of Zn under the conditions of different energy densities.The Zn content is regarded as the key factor for the formation of martensite.When the energy density is low,there are mainly brittle and hard martensite(?'phase)in alloys with relative higher Zn content.With the increase of energy density,the Zn content experiences a decrease and the?'phase decreases as follows while?phase predominates.As for the microstructure morphology,the needle-like martensite transforms into lamellar shape and then into a rod-like?phase.The rod-like shape decreases in length and finally changes into an equiaxed or elliptical shape.In addition,the microhardness decreases from 236.8 HV to 167.6 HV with increasing energy desnity.The intensity of the exothermic and endothermic peaks in the DSC curves decreases as well.What's more,there are two pairs of exothermic and endothermic peaks in the DSC curves for each thermal cycle.The first pair of exothermic and endothermic peaks are regarded as normal martensitic transformation and reverse transformation.The second ones are attributed to the martensite stabilization resulting from more thermal cycles.(3)The effect of thermal cycles on the microstructure and properties of CuZnAl alloys prepared by SLM.Subjected to the thermal effect caused by the subsequent printing,the bottom surface has more?phase and less?'phase compared with the top surface,which results in the decreased intensity of endothermic and exothermic peaks in the DSC curves.In addition,the microstructure of the side surface overlaps with each other resulting from complex thermal cycles.The needle-like martensite decreases in length and gradually evolves into the lamellar?phase when observed along the opposite direction of building.A lot of stacking faults appear on the martensite resulting from thermal cycles,which would destroy the matching relationship of interface and lead to martensite stabilization.(4)The elimination effect of heat treatment on thermal cycles.The?phase caused by the thermal cycles can be solutionized into to the parent phase at high temperature.Therefore,the heat-treated alloys exhibit higher superelastic strain compared with the as-fabricated alloys.In addition,with the ambient temperature from 100?to 200?,the superelastic strain of the alloys increases first and then decreases.The former is due to the complete austenitization of the alloys,while the latter is due to the formation of?phase caused by the bainite transformation during the aging process of parent phase.The superelastic strain of the alloys reaches the highest at 100?and 9%prestrain,which is 2.497%.