激光增材制造沉淀强化镍基高温合金热裂纹研究进展

Research Progress on Hot Cracking in Precipitation-strengthened Nickel-based Superalloys Fabricated by Laser Additive Manufacturing

针对沉淀强化镍基高温合金中的裂纹现象,对比了2类热裂纹(即凝固裂纹和液化裂纹)的典型特征、形成位置和条件。从枝晶生长、元素偏析、强化相析出、固态相变和残余应力应变等角度,系统综述了热裂纹的形成机理和热裂纹敏感性影响因素。在此基础上,从合金成分调控、工艺参数优化、基板预热以及热等静压处理等方面,概述了增材制造高温合金热裂纹调控和抑制的主要措施。最后,针对激光增材制造沉淀强化镍基高温合金热裂纹研究中存在的问题,提出了进一步研究和发展的建议,为实现无裂纹镍基高温合金的增材制造提供了参考。

Due to the high density ofγ′-Ni3(Al,Ti)precipitates and large amounts of solid solution additions,precipitation-strengthened nickel-based superalloys offer exceptional high-temperature mechanical properties and superior resistance to thermal corrosion.As a result,they are widely used in the manufacturing of hot-section components for aerospace engines and industrial gas turbine engines.On the one hand,these materials with high strength and hardness are commonly regarded to be difficult to machine,making the production of complicated superalloy parts via conventional machining methods challenging.On the other hand,when used in extremely harsh service environments(i.e.,high temperature,high pressure,and alternating loads),Ni-based superalloys suffer from a variety of damages and failures,including ablation,corrosion,wear,falling blocks,fatigue cracks,etc.To lower the overall costs instead of direct scrapping or replacement of damaged parts and prolong the service life,the development of remanufacturing and repair technologies for those hot-section superalloy components is urgently required and of great economic and strategic significance.With the rapid development of the aerospace industry,however,it is becoming increasingly difficult to produce complex-shaped precipitation-strengthened nickel-based superalloy parts by conventional manufacturing methods.In recent years,the emergence of laser additive manufacturing(LAM)technology characterized by the advantages of high heat input,strong metallurgical bond between the deposited layers and the substrate,high material utilization rate,and near net forming of complex parts,has great potential for precise forming,remanufacturing,and repair of complex parts.Nevertheless,the LAM of precipitation-strengthened nickel-based superalloys involves extreme high heating/cooling rate,complicated temperature fields,non-equilibrium solidification,and non-stationary solid-state phase transformation processes,as well as different types of solidification defects,among which the most concerning issue is cracking.Cracking in superalloys can be classified as solidification cracking,liquation cracking,strain-aging cracking,and ductility-dip cracking.The first two are also referred to as hot cracking since their formation requires the participation of liquid phase.The latter two are collectively referred to as solid-state cracking.Hot cracking is a cracking phenomenon that occurs when a metal is subject to elemental segregation,grain boundary liquid film,and tensile stresses during rapid melting and solidification.During the LAM process,precipitation-strengthened superalloys are highly susceptible to hot cracking,which is related to the uniqueγ/γ′dual-phase microstructure.The hot cracking sensitivity of the alloy increases significantly with the increase of the total content of Al and Ti,or the increase of theγ′phase volume fraction.In addition,the complex composition of precipitation-strengthened nickel-based superalloys(more than 10 element additions)and the enlarged solidification range due to elemental segregation lead to constitutional supercooling and thus raise the cracking sensitivity of the alloy.This seriously restricts the industry application of LAM technology.To this end,the present review systematically summarizes the formation mechanisms of two types of hot cracking(i.e.,solidification cracking and liquation cracking)and the factors that affect hot cracking susceptibility in the LAM-fabricated precipitation-strengthened nickel-based superalloys,and outlines the main methods to regulate and suppress hot cracking.Finally,the problems and future research directions in the study of hot cracking in the LAM-fabricated nickel-based superalloys are proposed.The review also provides valuable references for laser additively manufactured crack-free nickel-based superalloy components.