不可焊镍基高温合金是制造航空发动机关键热端部件不可替代的材料。激光熔覆在实现其铸造缺陷修复方面具有巨大潜力，但修复过程中诱发的熔覆热影响区液化裂纹一直无法避免。实现无液化裂纹的激光熔覆修复技术，是国内外同行关注的热点问题，具有重要的学术意义和工程应用价值。本文系统研究了激光熔覆工艺参数对K447A熔覆热影响区液化裂纹形成的影响规律，厘清了液化裂纹的形成机理。液化裂纹不连续地分布在熔覆热影响区的晶界区。控制熔覆热输入可降低液化裂纹倾向，但最优工艺参数下液化裂纹仍然大量存在。晶界区在1120-1267℃相继发生Ni21Hf8/γ共晶液化、M5B3/γ共晶液化和γ'/γ共晶液化，形成厚度不均匀的液膜。从应变角度建立了液膜开裂评判准则，发现液膜厚度是影响其开裂的关键因素，较厚的液膜优先开裂。考察了热处理和表面喷丸2种母材预处理方法对熔覆热影响区液化裂纹的影响规律及作用机制。熔覆前对母材进行整体热处理或者表面喷丸处理，均能够减少但无法完全避免液化裂纹。最优的热处理制度（1200℃/4 h）下，母材晶界区的Ni21Hf8和大部分M5B3发生固溶，初熔行为由此发生显著变化，由约1120℃发生的Ni21Hf8/γ共晶液化转变为1200-1250℃发生的M5B3/γ共晶液化，导致液膜的深度和厚度显著减小，液膜开裂倾向降低，但局部液膜因承受较高应变仍然发生开裂。喷丸预处理积累大量畸变能，驱动热影响区的高温区在激光熔覆过程中发生再结晶，细化了熔覆热影响区晶粒。同时，Hf在再结晶过程中向晶界扩散，诱发再结晶晶界在1260-1267℃发生大面积液化，形成细密的液膜网，导致液膜承受的应变显著降低，进而减小了液膜压降，抑制了热影响区高温区的液膜开裂，但热影响区其他区域因未发生再结晶，液膜形态没有发生改变，仍表现出高的开裂倾向。提出并实现了一种基于母材复合预处理（先整体热处理，再表面喷丸处理）的不可焊镍基高温合金表面激光熔覆新技术，在优化工艺条件（1200℃/4 h+0.7 MPa/(30-70 min)）下，熔覆热影响区液化裂纹被抑制。其原理是：整体热处理改变了母材晶界区的微观组织结构，提高了母材的初熔温度；接下来的喷丸预处理积蓄的畸变能促使熔覆热影响区发生静态再结晶，细化了晶粒，使得液膜所受应变降低，液膜开裂被抑制。所获得的无缺陷多层多道熔覆层抗拉强度达到984MPa，延伸率达到5.38%，高于母材。

Non-weldable nickel-based superalloys are irreplaceable materials for the manufacture of key hot-end components of aero-engines. Laser cladding has great potential in realizing the repair of its surface casting defects, but the liquation cracking induced in the heat-affected zone (HAZ) during laser cladding have always been unavoidable. To develop a laser cladding repair technology without liquation cracking on non-weldable nickel-based superalloy is a hot issue that domestic and foreign counterparts pay attention to, and it has important academic significance and engineering application value.This paper systematically studied the influence of laser cladding parameters on the HAZ liquation cracking during the repairing of non-weldable nickel-based superalloy K447A, and clarified the formation mechanism of liquation cracking. Liquation cracking are discontinuously distributed in the grain boundary of the HAZ. The liquation cracking can be reduced by reducing heat input, but there are still a large number of liquation cracking under the optimal process parameters. Ni21Hf8/γ eutectic liquefaction, M5B3/γ eutectic liquefaction and γ'/γ eutectic liquefaction occurred successively in the grain boundary of the base material at 1120-1267℃, forming a liquid film with uneven thickness. The criterion for liquid film cracking was established based on the strain, and it was found that the thickness of liquid film is the key factor of cracking, and the thicker liquid film cracks preferentially.The influence of heat treatment and surface shot peening on liquation cracking were investigated respectively, and the mechanisms were revealed. The overall heat treatment or surface shot peening of the base material before cladding both can reduce but cannot completely avoid liquation cracking. Under the optimal heat treatment technology with 1200℃/4 h, Ni21Hf8 phase and most of the M5B3 phase in the grain boundary dissolved. The incipient melting behavior of the base material changes from the Ni21Hf8/γ eutectic liquefaction at about 1120℃ to the M5B3/γ eutectic liquefaction at 1200-1250℃, which leads to a significant reduction in the depth of the liquid film in the HAZ. However, the local liquid film were reduced to cracking due to excessive pressure drop because they still bore high strain,. The large amount of distortion energy accumulated by the shot peening pretreatment drove the high-temperature zone of the HAZ to recrystallize during the laser cladding process, which refines the grains. At the same time, Hf diffuses to the grain boundary during the recrystallization process, which induces large-area liquefaction of the recrystallized grain boundary at 1260-1267℃, forming a fine liquid film network to reduce the strain applied on the liquid film, as a result, the liquid film is inhibited from cracking because of the low pressure drop. However, the low-temperature area of the HAZ did not undergo recrystallization and the morphology of the liquid film did not change, and still showed a high tendency to crack.A new technology of laser cladding of non-weldable nickel-based superalloy surface based on composite pretreatment of parent material (heat treatment combined with surface shot peening) was proposed and realized. Under the optimal process conditions (1200℃/4 h+0.7 MPa/(30-70 min)), the HAZ liquation cracking are completely suppressed. The main reasons are: on the one hand, the overall heat treatment changes the microstructure of the grain boundary zone of the base material and increases the incipient melting temperature of the base material; on the other hand, shot peening pretreatment promotes the base material in the heating stage of laser cladding to static recrystallization, as a result, a liquid film network forms. Within the optimal process window, because the incipient melting temperature of the alloy is higher than the recrystallization temperature, the liquid film only appears in the recrystallized layer, which leads to the suppression of liquation cracking. After the composite pretreatment of the base material, a defect-free multilayer and multi-pass cladding layer can be obtained on its surface. The tensile strength reaches 984MPa and the elongation reaches 5.38%, which are better than the base material.