Research On Hot Extrusion Microstructure Heterogeneity Of Large Size Cu-15Ni-8Sn Bar

Cu-15Ni-8Sn alloy possesses excellent mechanical properties,wear and corrosion resistance,good electrical and thermal conductivity and outstanding stress relaxation resistance,having been considered as the most potential substitute for Cu-Be alloy.This alloy has a broad application prospect in aircraft landing gear,aerospace engineering,marine engineering equipment and electrical engineering.Nowadays,many researchers have done numerous studies on the phase transition,alloy composition design,manufacturing engineering and heat treatment of Cu-15Ni-8Sn alloy,and prepared Cu-15Ni-8Sn alloy with high strength and high plasticity under laboratory conditions.However,the previous research are mainly aimed at small-size samples under laboratory conditions,few reports about the engineering application of the alloy has been reported.During the production of large size bar,the problems such as composition segregation,surface and internal defects,non-uniformity of microstructure will be more serious.It is of great significance to explore the production process of large size bar and realize the engineering application of the alloy.This research,aiming at the engineering application of Cu-15Ni-8Sn,carried out the hot extrusion numerical simulation of large size Cu-15Ni-8Sn alloy and optimized the deformation parameters by analyzing the multiple physical fields.Moreover,actual production of large size Cu-15Ni-8Sn bar was carried out,and the microstructure and properties of bar were analyzed.The main conclusions of this research were drawn as follows:(1)Based on the flow stress data obtained from the hot compression test,three constitutive models,strain-compensation Arrhenius model,modified JC model and modified ZA model,were established.And the prediction accuracy was verified by relative error,correlation coefficient and AARE.The results show that: strain-compensation Arrhenius model has the highest accuracy,with correlation coefficient of 0.997 and AARE of 0.034.And the relative error values of more than 95% of samples are within ±10%.Moreover,the dynamic recrystallization model was established based on microstructure characterization of hot compression samples.By carrying out relevant experimental and comparing the results with simulation results,the dynamic recrystallization model was proved to have great accuracy.(2)Based on the models established above,hot-extrusion simulation of large size Cu-15Ni-8Sn alloy was carried out.The results show that: physical fields of the extrusion bar have heterogeneity,and are greatly affected by the initial ingot temperature and extrusion speed.From the surface to center of bar,strain rate and stress decrease,but temperature and grain size increase gradually.Moreover,as extrusion speed increasing,the strain rate and stress gradually increase and the grain size heterogeneity is improved,but it has a little effect on the average grain size;as initial ingot temperature increasing,the stress gradually decreases,average grain size gradually increases and the grain size heterogeneity is worsen,but it has little effect on the strain rate.(3)Based on the results of hot-extrusion simulation,the hot-extrusion parameters was optimized in this paper: initial ingot temperature should in the range of 900-920 ? and extrusion speed should in the range of 3-4 mm/s.Based on the parameters optimized above,?100 mm Cu-15Ni-8Sn bar was manufactured,and microstructure characteristics of the bar were analyzed.The results show that: from the center to 1/2 radius of the bar,the microstructure is composed of fine grains formed by dynamic recrystallization and long elongated grains,and the volume fraction of long elongated grains increases gradually;from 1/2 radius to surface of the bar,the microstructure is composed of fine grains and coarse grains,resulting from the recrystallize of long elongated grains.And the volume fraction of the coarse grains increases gradually.Near surface,the microstructure is completely composed of the coarse grains.(4)The Cu-15Ni-8Sn bar was heat-treated under 820? for 1 hour and then 400? for 4hour.And the effect of heat treatment on microstructure and properties was analyzed.The results show that: heat treatment improved the microstructure and properties,and effectively reduced the heterogeneity.After heat treatment,the average grain size of fine grains decreased from 6.87 ?m to 5.15 ?m,and heterogeneity parameter decreased from 2.45 to 0.4.The average grain size of coarse grains decreased from 31.1 ?m to 28.2 ?m,and heterogeneity parameter decreased from 5.7 to 1.9.Moreover,the average hardness increased from 261 HB to 342 HB,and heterogeneity parameter decreased from 11.7 to 5.4.After heat treatment,the tensile strength of the extruded bar is between 940-980 MPa,and the elongation is between 12%-16%.