Recent Progress in Laser Additive Manufacturing Using Copper-Chromium-Zirconium Alloys:Formation,Microstructure,and Comprehensive Properties(Invited)
Significance Laser additive manufacturing(LAM)is renowned for its exceptional accuracy and the ability to produce complex components with intricate geometries,making it widely used across various industries.The LAM technology primarily encompasses two techniques:laser-directed energy deposition(LP-DED)and laser powder bed fusion(L-PBF).Among these,L-PBF is witnessing rapid advancements and gaining popularity in both scientific research and industrial applications.Copper and its alloys are pivotal functional materials.They act as crucial strategic reserves for the country,with a significant position within the national economy.Nevertheless,the exceptional thermal conductivity and high NIR reflectivity exhibited by copper and its alloys present notable challenges for LAM in relation to their effective processing and shaping.However,copper and its alloys have excellent electrical and thermal conductivities,along with exceptional mechanical properties.Because of the growing call for intricate functional copper and copper alloy components,the LAM of copper and copper alloy parts has become a research hotspot in recent years.CuCrZr is a copper-based precipitation hardening alloy.The addition of chromium significantly enhances its mechanical properties when compared to pure copper.Meanwhile,the presence of zirconium effectively hinders the growth of chromium precipitate phases,ensuring a more uniform distribution of precipitates and further strengthening the alloy.Notably,zirconium has minimal impact on the alloy's electrical conductivity.Firstly,CuCrZr's remarkable heat resistance and superior strength enable it to maintain its integrity and stability in high-temperature environments,making it an ideal material for manufacturing components exposed to extreme temperatures,such as aerospace engine nozzles and components for ITER.Secondly,this alloy demonstrates excellent resistance to oxidation,corrosion,and erosion caused by high-temperature gases.This exceptional property has facilitated its widespread application in various corrosive environments,including chemical equipment,marine engineering,and the nuclear industry.Thirdly,CuCrZr alloys are renowned for their outstanding electrical and thermal conductivity,making them highly suitable for the production of electrical components and heat sinks.Finally,CuCrZr alloys exhibit favorable machinability and can be shaped using various additive manufacturing methods,including L-PBF and LP-DED.Furthermore,they can be welded to other metals.Progress This review comprehensively examines the forming behavior,microstructure,and overall performance of CuCrZr alloys across three distinct areas.Firstly,it highlights the need to consider the laser absorption rate in addition to the traditional volumetric energy density when evaluating CuCrZr alloys'response to laser processing.This is because the absorption of copper and its alloys significantly varies with the laser wavelength,as illustrated in Fig.3 and Table 2.Secondly,the review discusses the densities and process parameters of CuCrZr alloys printed using lasers of various wavelengths,further emphasizing the importance of considering absorption rate(Fig.5 and Table 2).Moreover,the review delves into three types of defects commonly encountered in L-PBF,particularly those that tend to occur during the fabrication of CuCrZr alloy components(Fig.6).It also examines the variations in the alloy's microstructure before and after heat treatment,along with the underlying causes of these changes(Figs.8-16).This analysis provides valuable insights into the microstructure evolution and its impact on alloy performance.Additionally,the review explores the impact of an enhanced heat treatment routine and process parameters on the mechanical properties of CuCrZr alloys,as presented in Table 3.Furthermore,it investigates the correlation between densification,heat treatment regimen,and both electrical(Fig.19 and Table 4)and thermal(Fig.20 and Table 5)properties.Conclusions and Prospects This review presents an overview of the current status of the research on CuCrZr alloys in relation to their forming behavior,microstructure,and mechanical,thermal,and electrical properties.1)The majority of EA values obtained using near-infrared lasers are lower than those obtained using green lasers.This difference can be explained by the absorption rate of the CuCrZr alloy,which is significantly higher for green lasers compared to near-infrared lasers.Notably,the absorption rate of the CuCrZr alloy decreases monotonically as the laser wavelength increases.A particularly sharp decrease is observed when the laser wavelength exceeds 550 nm.When comparing the CuCrZr alloy processed with green lasers to that processed with near-infrared lasers,it is evident that the former exhibits a narrower range of density fluctuations.However,the overall density is lower,indicating the potential for further optimizing the green laser process parameters.Common and challenging defects encountered during the LAM of CuCrZr alloys include pores,cracks,and unfused powder.2)ST modifies the melt pool boundary and grain morphology of the alloy,whereas DAH generates precipitates that bolster its mechanical properties.The L-PBF method yields a CuCrZr alloy rich in supersaturated Cr and Zr atoms within the matrix,owing to the solid solution.Subsequently,DAH triggers precipitation within this matrix,primarily forming Cr,CuxZry,and Cr2Zr precipitates.These micro-and nano-sized precipitates significantly enhance the alloy's mechanical properties.However,as the treatment temperature rises,the precipitate distribution transitions from uniform to partially concentrated.Specifically,aging treatments at 500 ℃ for 2 hours or 550 ℃ for 1 hour attenuate the {111} and {200} crystal plane orientations while strengthening the{220} crystal plane orientation in the CuCrZr alloy.3)In as-built state,the CuCrZr alloy demonstrates relatively weak mechanical properties,with yield strengths ranging from 175.2 MPa to 400.0 MPa,ultimate tensile strengths between 254.6 MPa and 447.0 MPa,and elongations varying from 10.0％to 49.4％.However,it is noteworthy that by utilizing green lasers,it is possible to fabricate an as-built CuCrZr alloy with superior mechanical properties.Among various methods,direct aging treatment stands out as the most effective means to enhance these mechanical properties.This treatment achieves optimal results when conducted at approximately 500 ℃.Subsequently,the CuCrZr alloy undergoes significant improvements in its mechanical properties after this aging treatment.Specifically,yield strengths increase to range from 361.0 MPa to 527.0 MPa,ultimate tensile strengths improve to fall between 466.0 MPa and 612.0 MPa,and elongations enhance to vary from 12.3％to 21.8％.These remarkable improvements in the mechanical properties of the CuCrZr alloy can be attributed to the formation of precipitates during direct aging treatment,along with a reduction in both dislocation density and thermal residual stress.4)The CuCrZr alloy,in its as-built state,demonstrates electrical conductivity ranging from 21％IACS to 30.0％IACS and thermal conductivity varying between 100.0 W/(m·K)and 307.0 W/(m·K).This range of conductivities is primarily attributed to the presence of numerous oversaturated Cr and Zr atoms within the alloy matrix.These atoms cause lattice distortion in the grains,which enhances the scattering effect on free electrons.Consequently,the alloy exhibits lower electrical and thermal conductivities.To enhance both electrical and thermal conductivities,the most effective heat treatment process is SAAH.This process,performed at temperatures ranging from 950 ℃ to 1000 ℃ followed by an additional 500 ℃,significantly improves the conductivity values.Specifically,it elevates the electrical conductivity to a range of 84.0％IACS to 88.1％IACS and boosts the thermal conductivity to levels between 297.0 W/(m·K)and 350.0 W/(m·K).Additionally,the combination of solution annealing and age hardening treatment effectively reduces dislocation density and residual stresses within the alloy.This treatment also produces precipitates,which collectively contribute to further enhancing the electrical and thermal conductivities of the CuCrZr alloy.The following are anticipated to be the future research prospects and development directions for CuCrZr alloys.1)Optimization of the process parameters of green laser processing and conducting a comparative analysis of the microstructure and properties achieved through green laser processing and near-infrared laser processing.Fabrication of CuCrZr through the implementation of hybrid laser systems(blue/green laser+NIR laser).2)The traditional volumetric energy density used in the optimization study of the process parameters still has large limitations because it does not take into account the characteristics of the material.There is an urgent need for a method that can comprehensively consider the material properties and LAM process parameters.3)Currently,the ideal equilibrium between tensile strength and ductility has yet to be determined,and the amalgamated thermal and electrical characteristics remain unclear.Moreover,the mechanical,electrical,and thermal features have not been sufficiently and comprehensively explored.4)The EB-PBF method has been used to produce an equiaxed copper alloy containing nickel,aluminum,and bronze(C63000).This alloy boasts isotropic mechanical properties and high levels of strength and elongation.There is potential in the future to produce CuCrZr alloys with an equiaxial grain structure.
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