| The emerging heterogeneous additive manufacturing overcomes the limitation of single material,and compared with traditional processes containing welding and rolling,multimaterial additive manufacturing can be fabricated and joined together,and is also suitable for the fabrication of complex parts with high degree of freedom.316 L stainless steel has excellent hardness,strength and corrosion resistance,while Cu Sn0 tin bronze has excellent conductivity,thermal conductivity and wear resistance.Both materials have good formability of selective laser melting.Therefore,SLM steel/copper can obtain the advantages of double alloy and save the material cost of the whole structure.However,the research of SLM steel/copper dissimilar material now is limited by the problems of unclear interfacial bonding mechanism,poor bonding quality and low joint strength.Therefore,it is urgent to carry out the investigation of interfacial bonding mechanism,interfacial optimization and strengthening.Based on the above research background and current situation,study on the interfacial bonding and interfacial strengthening of SLM copper/steel dissimilar material is carried out in terms of bonding mechanism,microstructure and properties,interfacial process optimization,heat treatment,ultra-precision turning,application and separation of copper/steel mixed powder.The main contents include are as follows:(1)The single-material process parameters were optimized by multi-material SLM equipment,and a dual-material AM method was proposed with the preferred deposition sequence of steel first and then copper.Then,single-track formation of copper-steel was carried out,and the influence of laser energy input on the geometry and formability of the single-track was clarified.By analyzing the cross-sectional characteristics of the molten pool under different process parameters,the effect of process parameters on the Cu content in the molten pool was studied,and the relationship between the Cu content in the molten pool and its defect characteristics was revealed.The composition distribution and microstructure characteristics of the heterogeneous molten pool were analyzed.It is found that the center of the molten pool after solidification was Fe-rich area and the surrounding area was Cu-rich area.Among them,the Ferich region has coarse grains,and the Cu-rich region has a large number of fine grains due to the high undercooling of the molten pool and heterogeneous nucleation.In the research of multiple tracks formation of copper-steel,the crack distribution characteristics under different overlap ratios are summarized,and the effect of process parameter on the surface roughness of multi-track was clarified.(2)The microstructure characteristics of the copper/steel multilayer samples were clarified,and it was found that there were a large number of cellular grid microstructures in the "grey layer" of "interpenetrating interface",and the structure would undergo segregation evolution with the increase of Cu content.The copper infiltration microcracks induced by Kirkendall effect and capillary effect are distributed at the interface between the "grey layer" and the steel matrix.The mechanisms of "nucleation growth" and "spinod decomposition" promote the primary or secondary liquid phase separation at the interface,and finally the microstructures of steel to copper after corrosion are cell-like grid,island-like and umbrella-like distributed columnar crystals,respectively.By testing the hardness and phase of copper-steel dissimilar materials,the micro-Vickers hardness is obtained as steel > interface > copper,and the interfacial phase composition includes γ-Fe,Cu(Sn)and Cu41Sn11,but no intermetallic compounds were found to exist.The tensile and flexural properties of single material and multimaterial are studied,the ultimate tensile strength(UTS)of steel and copper are 673.1 MPa and578.7 MPa,respectively.Moreover,the UTS of copper-steel horizontal bonding and vertical bonding are 459.1 MPa and 423.3 MPa,indicating that the interfacial bonding of copper-steel dissimilar materials prepared by SLM is stronger than that prepared by welding.On the other hand,the ultimate flexural strength(UFS)of steel and copper single material are 729.1 MPa and 657.1 MPa,respectively.Then,the UFS of coper at the bottom of the composite plate specimen is 590.5 MPa,the UFS of steel at the bottom is 638.6 MPa,both indicating good metallurgical bonding between steel and copper.(3)The effect of interfacial forming process parameters on the interfacial bonding quality and performance was investigated.By comparing the interfacial bonding properties of samples prepared by different interface process parameters,it is found that elongated holes,horizontal cracks and unmelted powder are the major factors that weaken the interfacial bonding.Based on the optimization of the interfacial process parameters,the ideal mechanical properties of the heterostructure were obtained,the interfacial bonding strength was increased by 8.56% and the heterointerface strength coefficient reached 0.79.Comparing and observing the fracture morphology,the fractures with the highest UTS are "gully-shaped" and "smooth surface",while the fractures with the lowest UTS are mostly "lingual pattern" and "fan-shaped pattern".The grain refinement effect increases the interfacial nanohardness,so the order of nanohardness is:interface> steel>copper.The EBSD grain orientation distribution map shows that the microstructure evolution from steel to copper are coarse grains,mixed grains of fine grains and columnar grains.The reverse pole diagram shows that there is <001> preferred orientation with texture strength of 8.793 in the ND direction.Through the research on the electrochemical corrosion mechanism and electrical conductivity of copper-steel heterogeneous materials,it is found that the corrosion resistance and electrical conductivity of copper/steel dissimilar material are between the two single materials.(4)The post-forming heat treatment of copper/steel dissimilar material formed by SLM was carried out.Then,the influence of heat treatment on the elements’ distribution,microstructure and bonding properties of interface was analyzed,and the relationship between the microstructure and mechanical properties of the heterointerface was established.With the increase of the annealing temperature,the element transition zone becomes wider and the copper infiltration microcracks become more serious,but the Fe-rich or Cu-rich "islands"-like characteristics of the interface are alleviated because the element diffusion promotes the homogenization of the microstructure.Using 550℃ for 0.5h,the copper-steel heterostructure obtained the best interfacial bonding,the joint strength was improved by 30.38% compared with that before heat treatment,and the heterointerface strength coefficient reached 1.04.When the annealing temperature is higher than 750 °C,it is found that the UTS of the sample decreases significantly due to the microstructure coarsening and element segregation in the interface and the copper area.The grooving experiment was carried out on the side of the copper-steel heterostructure by the ultra-precision turning machining center,and the influence of the turning parameter change on the interfacial quality,groove surface roughness and cutting force was investigated,and the shape characteristics of the copper-steel heterogenous chip flow was also analyzed.The application of the copper-steel heterogeneous coaxial powder feeding hole nozzle is realized,which overcomes the problem that the original nozzle is easy to burn and causes the powder hole to be blocked.In addition,the effectiveness of the magnetic separation method for screening the copper and steel mixed powder is verified. |
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