【作者】 陈源；

【导师】 李铸国；

【作者基本信息】 上海交通大学 ， 材料科学与工程， 2017， 博士

【摘要】 镍基高温合金由于优异的高温力学性能及抗氧化性能而被广泛地应用于航空航天、能源、石油等领域。然而镍基高温合金部件生产制造成本高昂,因而周期性的检查及再制造修复成为延长镍基高温合金部件服役寿命的必要措施。激光增材制造技术由于热输入量低、变形小、与基体形成良好冶金结合等优点而被广泛地应用于镍基高温合金部件的再制造修复中。但是,由于反复的快速加热与冷却,合金中的应力大,诱发的裂纹严重,极大地限制了激光增材制造技术在镍基高温合金部件再制造修复中的应用,针对于此,本文就激光增材制造Inconel 718合金中的裂纹问题进行了系统的研究,并提出了一系列抑制裂纹形成的技术方法。激光增材制造Inconel 718合金主要由细长的柱状树枝晶和顶部少量的等轴树枝晶组成。合金的偏析相主要为共晶反应所形成的NbC相和Laves相。在多层熔覆中,树枝晶间的低熔点共晶物可以在激光的热作用下发生多次重熔与偏析。激光增材制造Inconel 718合金中的裂纹包括熔覆层顶部少量的凝固裂纹以及热影响区内由于树枝晶间液化或部分液化所形成的热裂纹在多层熔覆中,热裂纹一般萌生于热影响区内的熔合线附近,并随着熔覆层数的增加而不断扩展变大。通过有限元模拟分析了激光增材制造过程温度场与应力场的演变。温度场模拟结果表明:尽管激光增材制造冷却速率高,但由于热量的不断积累,在连续熔覆至第五层时,第一层熔覆层底部的温度可达近1000°C,依然可将树枝晶间液化或部分液化,诱发热裂纹。应力场模拟结果则表明:在第一层熔覆时,熔覆层内部为压应力;当熔覆层数增加至三层时,第一层熔覆层内部的应力开始转变为拉应力;继续增加熔覆层数,熔覆层内部的拉应力区逐步上移。分析了激光扫描速度及热输入量对激光增材制造Inconel 718合金热裂纹敏感性的影响。激光扫描速度的增加,提高了熔池的温度梯度,增加了凝固过程的热应力;而热输入量的增加则不仅扩大了热影响区还延长了树枝晶间液化膜的高温停留时间。因此,激光热输入一定时,热裂纹随激光扫描速度的增加而增加;激光扫描速度一定时,热裂纹也随激光热输入量的增加而增加。研究了基材底部定向冷却对激光增材制造Inconel 718合金树枝晶生长及热裂纹敏感性的影响。结果表明:在3 mm薄基材底部添加连续定向水冷可以将熔覆的初始冷却速率从2700 K/s提高至3200 K/s,稳定阶段的冷却速率从250 K/s提高至500 K/s。定向冷却的添加可以有效地提高合金的晶体取向性,同时热裂纹总长度可以减少85%。研究了晶界取向差对激光增材制造Inconel 718合金热裂纹敏感性的影响,晶界取向差的减少降低了凝固过程中晶界液化膜的稳定性,避免了凝固最后期晶界液化膜处形成的局部应力集中,进而抑制了热裂纹的形成。系统地研究了激光入射角度对激光增材制造Inconel 718合金树枝晶生长及热裂纹敏感性的影响。当激光束沿激光扫描方向顺时针偏转时,激光能量更多地汇聚于熔池的前端,熔池内部的侧向散热增强。这一方面提高了凝固过程中二次树枝晶的生长,促使合金中形成许多规则的“交错带”组织,提高了树枝晶间的连接,增加了裂纹扩展的阻力;另一方面,侧向散热的提高减少了熔池底部的热作用,缩小了热影响区。相反,当激光束沿激光扫描方向逆时针偏转时,激光能量更多地汇聚于熔池的后端,熔池的垂直散热增强。这一方面抑制了凝固过程中二次树枝晶的生长,削弱了树枝晶间的连接;另一方面垂直散热的提高增加了熔池底部的热作用,扩大了热影响区。因此,当激光束顺时针偏转至10°和20°时,热裂纹分别减少65%和56%,而当激光束逆时针偏转至10°和20°时,热裂纹分别增加51%和21%。成功制备了碳纳米管增强的激光增材制造Inconel 718复合合金。研究发现,通过碳纳米管表面化学镀镍处理,碳纳米管的管状结构可以在合金中少量地保留,同时碳纳米管管壁会部分或完全地打开,形成石墨烯纳米片结构;碳纳米管管壁会坍塌并与附近的碳纳米管及石墨烯纳米片连接形成碳纳米带结构;碳纳米管会蜷曲形成球状的类金刚石纳米颗粒。这些纳米碳结构的引入,一方面提高了合金的强度,减少了同样热应力条件下树枝晶间的局部应变;另一方面,纳米碳结构的引入,有效提高了树枝晶间的连接和应力传递,减少了树枝晶间局部应力集中的形成;同时,通过碳纳米管、石墨烯纳米片及碳纳米带的桥连及拔出机制作用,树枝晶间液化膜本身的抗应变能力得到提高。因此,添加5 wt.%和10 wt.%镀镍碳纳米管,合金的热裂纹可以分别减少78%和90%,同时合金的抗拉强度可以分别提高2.5%和16.7%。通过激光入射角度的优化、扫描路径的控制及镀镍碳纳米管的添加,可以成功制备出多道多层无裂纹的大块Inconel 718合金,为激光增材制造镍基高温合金部件激光再制造修复中热裂纹的控制提供有效的技术支持。更多还原

【Abstract】 Due to the excellent mechanical property and antioxidation at high temperature,nickel based superalloy is widely used in the fields of aerospace,energy,petroleum,etc.While the manufacturing of nickel based superalloy components is very costly and therefore,periodic inspection and remanufacturing become the necessary mesures to extend the lives of the nickel based superalloy components.Laser additive manufacturing has widely been used in the remanufacturing of nickel based superalloy components because of advantages of low heat input,low deformation and well metallurgical bonding with substrate,etc.Unfortunatly,the cyclic rapid heating and cooling during laser additive manufacturing produce extremely high stress in the alloy,resulting severe cracking formation.As a result,the application of laser additive manufacturing in the remanufacturing of nickel based superalloy components is restricted greatly.Therefore,the hot cracking problem in the laser additive manufacturing of Inconel 718 alloy was studied systematically in this paper and a series of technical methods for hot cracking inhibition were also developed.The microstructure of laser additive manufactured Inconel 718 alloy consists most of fine and long columnar dendrites,as well as a small amont of equaxed dendrites at top of the alloy.The segregation phases of the alloy are NbC phase and Laves phase by eutectic reaction.During the multi-layer cladding process,the interdendritic low melting eutectic compounds can be remelted and re-segregated several times by the heat of laser.The cracks in the laser additive manufactured Inconel 718 alloy includ a small quantity of solidification cracking formed at top of the clad and also the hot cracking produced by the liquation or partial liquation of interdendritic region in the heat affected zone.During the multi-layer deposition,hot cracking in the heat affected zone normally initiates from the fusion lines and then propagates larger gradually by the increase of the deposition layers.Throuth the finite element simulation,the evolution of temperature field and stress field during laser additive manufacturing was analyzed.The results of temperature field simulation show that,althrough the cooling rate of laser additive manufacturing is very high,the temperature in the bottom of the first layer can reach 1000 °C in the continuous fifth layer’s deposition attributed to the accumulation of heat.It means that the interdendritic region can still be liquated or partially liquated,resulting the formation of hot cracking.The results of stress field simulation show that the stress inner the first layer clad is compressive stress;when the deposition increases to 3 layers,the stress in the first layer clad turns to tensile;further increases the deposition layers,the tensile stress region expands upward gradually.The effects of laser scanning speed and heat input on the susceptibility to hot cracking in the laser additive manufacturing were analyzed.The increase of laser scanning speed increases the temperature gradient of molten pool and as a result,it generates higher thermal stress in the solidification.The increase of heat input extends the heat affect zone and also prolongs the high temperature time of liquation film in the interdendritic region.Consequently,under the same laser heat input,hot cracking increases with the increase of laser scanning and under the same laser scanning speed,hot cracking increases with the increase of laser heat input.The effect of directional base cooling on the growth of dendrites and the susceptibility to hot cracking in the laser additive manufactured Inconel 718 alloy was studied.The results show that the directional base cooling on the back of 3 mm thin substrate can increase the cooling rate from 2700 K/s to 3200 K/s at the initial stage,and also increase the cooling rate from 250 K/s to 500 K/s at the stable stage.The directional base cooling can effectively improve the crystal orientation of the alloy,and at the same time,reduce the 85% formation of hot cracking.The effect of grain boundary misorientation on the hot cracking susceptibility of laser additive manufacturing was studied.The decrease of grain boundary misorientation reduces the stability of liquation film on the grain boundary in the solidification.The local stress concentration on the grain boundary liquation film in the last stage of solidification can be avoided and the hot cracking formation can thus be depressed.The effect of laser input angle on the growth of dendrites and the susceptibility to hot cracking in the laser additive manufactured Inconel 718 alloy was studied systematically.When the laser beam turns clockwise to the laser scanning direction,most of the laser energy focuses on the front of molten pool,as a result,the lateral heat dissipation in the molten pool increases.On the one hand,this improves the growth of secondary dendrites in the solidification,producing many regular “cross bands” microstructure in the clad.The interdendrtic bonding is improved and therefore,the resistance to hot cracking is increased.On the other hand,the increase of lateral heat dissipation decreases the heating effect in the region beneath the molten pool and then narrows the heat affected zone.On the contrary,when the laser beam turns anticlockwise to the laser scanning direction,most of laser energy focuses on the back of molten pool,as a result,the vertical heat dissipation in the molten pool increases.On the one hand,this inhibits the growth of secondary dendrties and then weakens the interdendritic bonding.On the other hand,the increase of vertical heat dissipation improves the heating effect in the region beneath the molten pool and then extends the heat affected zone.Consequently,hot cracking decreases 65% and 56% when laser beam turns clockwise to 10° and 20°,respectively.Whereas,hot cracking increases 51% and 21% when laser beam turns anticlockwise to 10° and 20°,respectively.The carbon nanotube reinforced laser additive manufactured Inconel 718 composite alloy was fabricated successfully.Through the electroless plating treatment of nickel on the surfaces of carbon nanotube,the tube-like structure of carbon nanotube can be silightly mantainted.At the same time,the tube of carbon nanotube can be patrtially or fully opened,forming the structure of graphene nanosheet;the tube of carbon nanotube can be collapsed and also inter-bonded with the carbon nanotube and graphene nanosheet in the ambient region,forming the structure of carbon nanoribbon;the tube of carbon nanotube can also be curled and transformed into spherical diamondlike nano particle.The incorporation of these carbon nano structure increases the strentgh of the alloy and then reduces the local interdendritic strain under the same condition of thermal stress.It also improves the interdendritic bonding and stress transfer,avoiding local stress concentration formation in the interdendritic region.At the same time,through the bridging and pull-out mechanism of carbon nanotube,graphene nanosheet and carbon nanoribbon,the intrinsic resistance of interdendritic liquation film to thermal strain can also be improved.In the consequence,hot cracking in the alloy can be depressed 78% and 90%,and the tensile strength of the alloy can also be improved 25% and 16.7% attributed to the addition of 5 wt.% and 10 wt.% nickel plated carbon nanotube,respectively.Through the optimization of laser input anlge,modification of laser scanning path and addition of nickel coated carbon nanotube,large multi-pass and multi-layer crackfree bulk Inconel 718 alloy was manufactured successfully,offering more effective technical supports on the control of hot cracking during the remanufacturing of nickel based superalloy components by laser additive manufacturing.更多还原