CuNi14Al3合金收缩环的工艺性缺陷研究

CuNi14Al3合金收缩环通过真空熔铸,锻造,去应力退火等工艺制备而成。在生产工艺过程中常遇到以下问题:铸锭表面质量差,开坯前去除废料较多;锻造开裂;成品件无损探伤到孔洞、裂纹等缺陷。本研究采用金相显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)、能谱仪(EDS)以及维氏硬度仪等对CuNi14Al3合金收缩环每一步加工工艺后的显微组织和性能进行对比分析,观察到整个制备过程中主要存在四类工艺性缺陷,分别是显微孔洞、成分偏析、混晶组织以及裂纹。研究结果显示,这一系列工艺性缺陷的产生归因于合金的凝固特性,包括凝固时间、凝固顺序以及凝固收缩等,使得原始铸态合金枝晶组织粗大,一次枝晶干最长可达10 mm以上,且枝晶偏析、显微疏松严重,通过后续热加工,这些缺陷未能得到完全消除,甚至进一步形成混晶组织、裂纹等。研究表明,要抑制甚至消除以上缺陷的产生,需要从缺陷根源凝固入手,优化加工工艺,严格控制铸件品质。

Mechanism Study of Defect for Mation in CuNi14Al3 Shroud Rings

Strengthened by the L12 precipitates as such Ni3 Al,the CuNi14Al3 alloyis utilized to manufacture the shroud rings which areone of the key components of power generators.However,after the sequent processing procedures including vacuum melting casting,forging and stress relief annealing heat treatment,the poor surface quality,scrap removal induced materials waste,forging cracks and cavities were detected from time to time in the final products.To uncover the mechanism of the formation of these macroscopic defects,the microstructural features and mechanical properties of the CuNi14Al3 alloy after each crucial processing stage were studied by optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),energy dispersive X-ray spectroscope(EDS),and micro-hardness tester.Owing to the solidification characteristics of the CuNi14A13 alloy,including the solidification sequence,rate and shrinkage,the microstructure of the ingots featured of coarse dendritic structures,which was over 10 mm in length,with severe elemental segregation and micro-pores.Since nickel had higher melting point than copper,the precipitationinduced elements like nickel and aluminum to segregate in the dendritic cores with a segregation ratio up to 0.3 and 0.4,respectively.From the dendritic cores to the interdendritic regions,the shape of the precipitate particles changed gradually from large regularshaped cuboids(with the edge length around 200 nm)to fine irregular spherical shapes,with an average diameter of around 20 nm.The hardness values,obtained using the Vickers indenter,at the dendrite cores were higher than those in the interdendritic regions by approximately 13.1%.To investigate the tensile facture behavior of the ingot,the 45 ocone-shaped fractures revealed the brittleness characteristics of the alloy.However,at micro scale,the facture surface was dimple-shaped morphology with mass of pores,and the precipitate particles could be commonly observed at the bottom of the dimples,indicating that the stress concentration around the casting pores and shrinkage led to the fracture.After the sequent forging,the coarse dendrites and the micro-pores were inherited.Moreover,at the first step of the forging,the plastic deformation could not activate sufficient dynamic recrystallization,leading to the bimodal grain size distribution,in which the continuous small crystal grains distributed among the giant grains inherited from the casting organization.The bimodal microstructure led to the heterogeneity of the deformation partitions at further forging steps,which,in turn,induced the cracks to occur at the interfacial areas between the giant grains and the micro-pores.There was still no necking observed at the tensile facture of the forged specimens.Compared with those of the casting ingots,the dimples became shallower,which was an evidence of even more brittleness.A number of pores initiating from the casting cavities and shrinkage could still be found,and the pores were stretched by the large plastic deformation.All the defects,including dendritic structure,bimodal distributed grain size,pores and cracks,were left to the last processing step,stress relief annealing heat treatment.Unfortunately none of them could be eliminated by the annealing.The low temperature stress relief annealing treatment could not activate the recrystallization sufficiently and many giant grains were left over.Due to the high forging temperature,which was higher than the solutionizingheat treatment temperature of the CuNi14Al3 alloy,the cracks were oxidized as Al2O3,and the oxide particles along the cracks impeded the closure during forging.The densities of the alloys were measured to be 8.188 g·cm-3 after casting,8.245 g·cm-3 after forging,and 8.284 g·cm-3 after stress relief annealing,respectively.All values were significantly lower than the theoretical value(8.500 g·cm-3),revealing that the shrinkage cavities always existed and the defects could not be eliminated completely by the subsequent hot processing.Although the cracks emerging during the forging led to the failure of the product,the hardness and yield strength of the crack-free parts still met the requirements,mainly resulting from the dispersed precipitation hardening.From the dendritic cores to the interdendritic regions of the casting ingots,the size of the Ni3Al precipitate particles varied from 200 to 20 nm.The crystal structure of the precipitation was similar to the copper matrix and had the same orientation.After the forging and the annealing,the size of the precipitation became uniform at around 20-50 nm,which guaranteed the higher tensile strength at 855 MPa.The evolution of the precipitate particles proved that the forging and annealing processing was effective to achieve the required microstructure and mechanical properties,although it could not eliminate the casting defects.It could be concluded that,the formation of all the defects was attributed to the solidification characteristics of the alloy,including solidification rate,sequence and thermal shrinkage.Thus,the approach to improve the processing could be raised.Firstly,to improve the alloy composition by adding the grain refiner to promote the heteromorphy nucleation and/or the modifier to hinder the grain growth and the precipitating.Secondly,to optimize the casting processing by the sloping plate casting and the pressure casting to refine the grain size and restrain the shrinkage.The last but not the least,to add another homogenization annealing to eliminate the dendritic segregation and achieve better homogenization.