Study On Isothermal Extrusion Forming Of Different Kind Of Shape Memory Alloys Composite Tube

Shape memory alloy is widely used in fields of material science and engineering due to its outstanding shape memory effect.At present,NiTi-based shape memory alloy,Fe-based shape memory alloy and Cu-based shape memory alloy are three kinds of shape memory alloys which are well studied.It is generally accepted that the pipe coupling for pipe connection is the typical application of shape memory alloy.The pipe coupling manufactured by NiTi-based shape memory alloy has the advantages of convenient installation,high reliability,no leakage and light weight,but it is expensive and is difficult to be manufactured.In contrast,the pipe coupling manufactured by Fe-based shape memory alloy has the advantages of high strength and low cost,but it possesses relatively small recoverable strain and stress,as well as relatively weak corrosion resistance.In terms of the pipe coupling manufactured by Cu-based shape memory alloy,it possesses relatively low strength and small recoverable strain,but it has the advantages of low cost and good formability.Therefore,it has broad prospect to manufacture the composite pipe with low cost and high reliability,where the outer tube is made of Cu-based shape memory alloy or Fe-based shape memory alloy,and the inner tube is made of NiTi based shape memory alloy.In the present study,three kinds of shape memory alloys,including CuAlNi,NiTiNb and NiTiFealloys,are studied and the finite element simulations in terms of isothermal extrusion of composite tube are conducted in three different cases,including the case with outer tube of CuAlNiand inner tube of NiTiNb,the case with outer tubeof CuAlNi and inner tube of NiTiFe and the case with outer tubeof NiTiFe and inner tube ofNiTi Nb.Specific research work and achievements are as follows.By means of metallographic microscope,X-ray diffraction?XRD?,differential scanning calorimetry?DSC?and scanning electron microscopy?SEM?,the microstructure and phase transition temperature of CuAlNi alloy in the original state and after heat treatment are analyzed.Experimental observation confirms that there are a lot of brittle phase?₂ and serious composition segregationwithin the as-cast CuAlNi alloy.Moreover,large coarse grains are distributed within as-cast CuAlNi alloy and there is no change of transformation temperature.After heat treatment,there arestill a lot of brittle phase?₂ within CuAlNi alloy,but composition segregation and grain size are reduced.DSC experimentconfirms that phase transformationoccurs in the CuAlNi alloy after heat treatment and the transformation temperatures are as follows:Ms=-59.3?,Mf=-84.5?,As=-29.3?,Af=-6.7?.Under different temperatures?600?,700?,800?,900?and 1000??and different strain rates(0.0005 s^-1,0.005 s^-1,0.05 s^-1 and 0.5s^-1),hot compression experiments are conducted on CuAlNi alloy after heat treatment,and consequently the true stress-true strain curves at different temperatures and different strain rates are obtained.Based on the observation of stress-strain curves,it is obvious that at the same temperature,the true stress of CuAlNi shape memory alloy increases with the increase of strain rate,and at the same strain rate,the true stress of CuAlNi shape memory alloy decreases with the increase of temperature.Based on the true stress-true strain curves,the constitutive equation of CuAl Ni shape memory alloy based on the Arrhenius equation is established.Furthermore,the effect of strain compensation on the constitutive equation is considered in the present study.Based on the dynamic material model and the true stress-true strain curves at different temperatures and different strain rates,hot processing map of CuAlNi shape memory alloy is constructed and it lays the theoretical foundation for the study of hot workability of CuAlNi shape memory alloy.According to the hot processing map of CuAlNi shape memory alloy,it is found that the instability zone of plastic flow mainly occurs in the region with high strain rate at high tempeture.In addition,rage of the instability zoneincreases with the increase of strain value.Based on the results of hot processing map and microstructure observation,it is found that the optimalworking temperature of Cu AlNi shape memory alloy ranges from850?to 975?.Based on the Arrhenius constitutive equations of CuAlNi,NiTiNb and NiTiFe shape memory alloys as well as the rigid viscoplastic finite element method,finite element simulations in terms of isothermal extrusion of composite tube are conducted by using the ABAQUS subroutine UHARD in three different cases,includingthe case with outer tube of CuAlNi and inner tube of NiTiNb,the case with outer tubeof CuAlNi and inner tube of NiTiFe and the case with outer tubeof NiTiFe and inner tube of NiTiNb.Simulation results indicate that in the case with outer tube of CuAlNi and inner tube of NiTiNb and the case with outer tubeof CuAl Ni and inner tube of NiTiFe,plastic deformation is mainly distributed in the outer tube,while no plastic deformation occurs in the inner tube.The resultshows that when the mechanical properties of the inner tube and outer tube possess great difference,incompatible deformationnear the interface will occur during the extrusion process,which makes it difficult to form composite pipe with high quality.In the case with outer tubeof NiTiFe and inner tube of NiTiNb,simulation results confirm that becausethe mechanical properties of the inner tube and outer tube possess small difference,the finally manufactured composite tube possesses good forming quality.At the same time,the influence of friction coefficient,extrusion speed and extrusion temperature on the forming quality in the case with outer tube of NiTiFe and inner tubeof NiTiNb was studied and it lays a theoretical foundation for optimizing the process parameters.