| Cu-Cr-Zr alloy is one of the structural and functional integrated precipitatehardened alloys,containing a large number of sub-stable Cr-rich precipitates.Cu-Cr-Zr alloy is widely used in electronic information,energy and electric power,transportation,and military industries due to its high strength,good electrical and thermal conductivity.With the development of related fields,higher requirements have been put forward for the performances of Cu-Cr-Zr alloy.Severe plastic deformation(SPD)is an effective way to enhance the strength of material,but the introduction of crystal defects such as high-density of grain boundaries and dislocations will inevitably reduce the electrical conductivity and the thermal stability.How to reconcile the strength-electrical conductivity-thermal stability of Cu-Cr-Zr alloy is a classical problem.In addition,precipitate coarsening and dissolving often occur during the welding process with high heat-input,which seriously decrease the mechanical and electrical properties of the joint,hindering its further application.In this dissertation,the microstructure of a Cu-Cr-Zr alloy was designed and controlled by means of cold rolling,friction stir processing(FSP),friction stir welding(FSW)and heat treatment to meet the application requirements.The objectives are to(a)synergistically optimize the mechanical properties and electrical conductivity of the Cu-Cr-Zr alloy;(b)to investigate the connection among microstructure,mechanical properties,electrical conductivity,and thermal stability;(c)to provide a new perspective for the design and preparation of the Cu-Cr-Zr alloy;(d)and also to solve the long-standing problem of softening during welding.The main research aspects and results are as follows:Firstly,samples with four precipitate states(solid solute,under-aged,peak-aged,and over-aged)were subjected to cold rolling(CR)and post-aging,respectively.The results showed that,the under-aged sample with less precipitates accumulated the similar deformed structure as the peak-aged one during rolling and relieved the negative effect caused by recovery during post-aging,retaining more deformed structure.After rolling and post-aging,the under-aged sample exhibited the highest dislocation density(9.72×1014 m-2),the highest grain boundary density(4.1 μm-1),the "hardest" grain orientation(M=2.99),and the finer precipitation(3.1 nm).Therefore,its ultimate tensile strength reached to 588 MPa,higher than those with other pre-aging treatments.The interaction between precipitates and dislocations during CR and aging was elucidated.A new process of under-aging pre-treatment+CR+aging treatment was proposed to enhance the strengthening effect and shorten the process.Secondly,because of the restricted strain and insignificant strength improvement after cold rolling,the peak-aged Cu-Cr-Zr alloy was subjected to one-step FSP at room temperature.The peak temperature on the advancing side was about 240℃ during FSP.The precipitates played a key role in the process of grain refinement,and the ultrafine grains(UFG)with an average size of 250 nm were produced.The FSP process caused neither coarsening nor dissolution of the precipitates,and the subsequent aging process was omitted.As a result,UFG structure,nanoscale precipitates(～3.1 nm),low solid solution Cu matrix were obtained.Finally,the FSP sample exhibited excellent comprehensive properties of high tensile strength(702 MPa),good elongation(16%)and electrical conductivity(74.3%IACS),which provide a direction for microstructure optimization of this alloy.Thirdly,for the demand of Cu-Cr-Zr alloy in high-temperature service,the thermal stability behaviors of FSP samples(mainly strengthened by high-angle grain boundaries)and CR samples(mainly strengthened by dislocation)were contrastively investigated.At the same strength increment,the pure copper rolled sample without precipitates has better thermal stability than the FSP sample.While,the thermal stability differences disappeared in the FSP and CR Cu-Cr-Zr samples with the same onset softening temperature(400℃).The hardness of the FSP samples was higher than that of the rolled samples when the annealing temperature below 500℃.On the one hand,precipitates effectively stabilized the deformed microstructure and hindered the onset of softening;on the other hand,the deformed microstructure accelerated the precipitate coarsening,resulting in softening.Finally,based on the results and principles of obtaining the high-performance CuCr-Zr alloy via low heat input FSP,FSW joints with high joint strength and high electrical conductivity comparable to that of peak-aged Cu-Cr-Zr alloys were obtained for the first time.For the FSW of thin Cu-Cr-Zr alloy plates,it is found that the threaded cermet stirring tool not only reduced the heat production under the shoulder but also promoted the material flow around the stirring pin.The lower heat input inhibited the coarsening and dissolution of the precipitates in the nugget zone(NZ)and heat affected zone(HAZ),avoiding the appearance of softening zones and low electrical conductivity in the joint.For the FSW of medium-thick Cu-Cr-Zr alloy plates,through softening the Cu-Cr-Zr plates to the solid solution state prior to FSW,the peak temperature during FSW under water was reduced to a level close to the aging temperate of the Cu-Cr-Zr alloy.The hardening caused by the grain refinement(0.65 μm)compensated for the softening caused by the precipitate coarsening(10.5 nm)in the NZ.The formation of softened zones was avoided as the peak welding temperature at the locations out of the NZ was lower than the peak aging temperature of the alloy.Therefore,the softening zone in the joint disappeared.This work revealed the key factors that affected the mechanical properties of the FSW joints of age-hardened Cu-Cr-Zr alloys,resolving the contradiction between high heat input induced joint softening and low heat input induced welding defects and providing a new solution for high-quality welding of CuCr-Zr alloys. |
PDF Full Text Request