Fabrication Of Al/Al And Cu/Al Bimetals By Solid-Liquid Bonding And Study On Their Microstructure And Properties

Bimetallic materials have been widely used in industrial area because of its unique properties,and it has shown broad application prospect and strong momentum of development in materials science and engineering.Al/Al and Cu/Al bimetals are important area in bimetallic materials,and it has been widely studied and applied in the field of power electronics,automobile,aviation and aerospace.However,Al/Al and Cu/Albimetals are prone to defects when fabricated by traditional welding method because of its unique surface properties and physical and chemical properties.This severely impeded the application of Al/Al and Cu/Al in industrial areas.Solid-liquid bonding has drawn great attention in a variety systems because of its simple process procedure,less limit to the dimension of solid substrate,less demand of equipment and high processing efficiency.However,Al/Al and Cu/Albimetal made by solid-liquid bonding has not been comprehensively studied yet.Systematic research on surface treatment,processing technic,optimization of processing parameters has not been conducted.Intensive study on interfacial structure,melting and heat transfer behavior,diffusion behavior and bonding mechanism are still needed.Bimetallic materials provide wide design and research space for manufacturing of bimetallic rotor.Problems that often appear during the centrifugal casting and die casting were well solved by the introduction of bimetallic design.In addition,Al/Al and Cu/Al bimetals could fulfill the requirement of high conductivity,high strength,light weight and economical for rotors.In this study,Al/Al and Cu/Al systems were chosen as the study objects.Al/Al and Cu/Al bimetal were fabricated by squeeze casting method after conducting surface treatment onto insert material.Optical microscope(OM),Scanning electron microscope(SEM)and electron back scattered diffraction(EBSD)analyzing methods were conducted to characterize its microstructure.Hardness,shear strength,tensile strength and electrical conductivity were used to evaluate its properties.For Al/Al bimetallic system,systematic research was carried out to obtain the influence of processing parameters on microstructure and mechanical properties of gravity cast and squeeze cast Al/Al bimetal.Interfacial behavior,solidification behavior and heat transfer phenomenon was analyzed.For Cu/Al bimetallic system,systematic research was carried out to obtain the influence of processing parameters on microstructure and mechanical properties and electrical properties.The composition of intermetallic compounds,formation and growing law of intermetallic compound were studied.The diffusion and solidification behavior was analyzed.Then squeeze casting mould was designed and fabricated,bimetallic rotor manufacturing process was simulated,processing parameters were optimized.At last,Al/Al and Cu/Al bimetallicrotors for electrical motors were fabricated according to the technical mode obtained.6101/A356,6101/6101 and 6101/pure aluminum Al/Albimetals were fabricated by electro-plating zinc coating on 6101 insert material with squeeze casting method.Effect of processing parameters on microstructure and mechanical properties were obtained.With the increase of thickness of zinc coating,the tensile properties of gravity cast and squeeze cast 6101/A356 bimetal barely vary.With the increase of pouring temperature,the ultra tensile strength increases first then decreases.With the increase of applied pressure,the mechanical properties of squeeze cast A356,6101 and pure aluminum matrix increase.The increase is attributed to the elimination of interior defects and the refining of the microstructure.The refining of the microstructure is due to the increaseof solidification rate caused by the optimizing of heat transfer condition and the promotion of nucleation of?-Al casued be the changing of liquidus temperature.For Al/Al bimetal in different systems,the mechanical behaves differently with the increase of applied pressure.With the increase of applied pressure,ultra tensile strengthand elongation stays at around 160MPa and 5%,respectively,which is higher than that made by gravity casting,while yield strength stays at around 90 MPa,barely differs from that of made by gravity casting.With the increase of applied pressure,ultra tensile strength and yield strength of 6101/6101 bimetal bearly changes,but elongation decreases.With the increase of applied pressure,ultra tensile strength,yield strength and elongationof6101/pure aluminum all incerese slightly.It can be iffered that ultra tensile strength and yield strength are determined by the lower value of the two aluminum alloys,while elongation is related to both component by EL=S₁?₁+S₂?₂.Depend on the system composition and processing parameters,the fractured location could be inside 6101 insert materials(squeeze cast 6101/6101),inside casting materials matrix(gravity cast 6101/A356,squeeze cast and gravity cast 6101/pure aluminum)and transition zone(squeeze cast 6101/A356).The temperature variation of the surface of 6101 insert bar during gravity casting solid-liquid bonding was obtained and analyzed,the relationship between the temperature of the surface of 6101 insert bar and interfacial reaction was studied.In order to achieve sound metallic bonding,the temperature of the surface area of6101 insert need to fulfil two requirements.First,it should stays above the solidus temperature of insert material for a certain time to guarantee ansufficient solid-liquid reaction between the casting material and 6101 insert.Second,it should stay below the liuqidus temperature of insert material to avoid severe deformation or melting.By analyze the heat transfer between casting aluminum,mold and the enviroment,it was found that the relationship between the tempearture of the surface of 6101 insert material and thermal physical properties of both component could be determined by equation:_?0^?(?),=exp(-?₁²F₀),?=t(x,?)-t_?.As a result,in order to achieve a sound metallic bonidng between the two aluminum alloys,certain thermal physical properties need to be taken into cosideration and choose carefully.For insert material,the larger the solidfication range the better,for casting material,ones with low heat transfer coefficient,low specific heat capacity and liquidus temperature lower that insert material's solidus temperature are suitable for making Al/Al bimetal.Cu/Al bimetals were made by conduct thermal spray zinc coating onto Cu insert meterial with squeeze casting method.Good metallurgical bonding were achieved between cast pure aluminum and Cu insert materials.Effect of processing parameters on microstructure and mechanical properties were obtained.It was found that there are three different layers in the transiton zone from Cu side to Al side,intermetallic layer,?-Al/Al₂Cu eutectic layer and?-Al solid solution layer.Intermetallic layer could be further divided into Al₄Cu₉(?₁)layer and Al₂Cu(?)layer.With the increase of pouring temperature,the thickness of Al₄Cu₉(?₁)layer and Al₂Cu(?)layer increase.The increase is due to the increase of diffusion coefficient and diffusion time.With the increase of applied pressure,the thickness of Al₄Cu₉(?₁)layer and Al₂Cu(?)layer bearly varies while for eutectic layer it decreases.The decrease is due to the decrease of diffusion because of the increase of solidification rate and the shifting of eutectic composition in the diagram to the Cu(Al₂Cu)side.The relationship between processing parameters and mechanical behaviors of Cu/Al bimetal,interfacial microstructure and mechanical properties were obtained.With the increase of temperature,the tensile strength increases at first then decreases to and stays steady at 16 MPa,the highest value appears at pouring temperature of 700°C.With the increase of applied pressure,it barely changes.The tensile strength and fracture mechanism is determined by the thickness of Al₂Cu(?)layer.When Al₂Cu(?)layer is thin,the crack attacks eutectic region during the propagation,gives a river look fractured surface and higher tensile strength.When Al₂Cu(?)layer is thick,the crack propagates inside the Al₂Cu(?)layer,give a flat fractured surface and lower tensile strength.Squeeze casting mold was designed and fabricated to manufacturing Al/Al and Cu/Al bimetallic rotor.Anycasting was used to simulate the manufacturing process.With the increase of the dimension of ingate and pouring temperature,the peak temperature of aluminum bar increases.With the increase of filling speed,the peak temperature of aluminum bar decreases.The difference of peak temperature of 56aluminum bars was used to characterize the uniformity.It decreases with increase of end ring outer diameter and increase of filling speed,while increase with the increase of the dimension of ingate.Then Al/Al and Cu/Al bimetallic rotors for electrical motors were manufactured by squeeze casting.Sound metallic bonding could be achieved.This study provides a new technical method to manufacture rotors for electrical motors.