High nitrogen stainless steel (HNS) is a nickel free austenitic stainless steel that is used as a structural component in defence applications for manufacturing battle tanks as a replacement of the existing armour grade steel owing to its low cost, excellent mechanical properties and better corrosion resistance. Conventional fusion welding causes problems like nitrogen desorption, solidification cracking in weld zone, liquation cracking in heat affected zone, nitrogen induced porosity and poor mechanical properties. The above problems can be overcome by proper selection and procedure of joining process. In the present work, an attempt has been made to correlate the microstructural changes with mechanical properties of fusion and solid state welds of high nitrogen steel. Shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW), electron beam welding (EBW) and friction stir welding (FSW) processes were used in the present work. Optical microscopy, scanning electron microscopy and electron backscatter diffraction were used to characterize microstructural changes. Hardness, tensile and bend tests were performed to evaluate the mechanical properties of welds. The results of the present investigation established that fully austenitic dendritic structure was found in welds of SMAW. Reverted austenite pools in the martensite matrix in weld zone and unmixed zones near the fusion boundary were observed in GTA welds. Discontinuous ferrite network in austenite matrix was observed in electron beam welds. Fine recrystallized austenite grain structure was observed in the nugget zone of friction stir welds. Improved mechanical properties are obtained in friction stir welds when compared to fusion welds. This is attributed to the refined microstructure consisting of equiaxed and homogenous austenite grains. © 2016 The Authors

The microstructure and properties of water-cooled and air-cooled friction stir welded (FSW) ultra-high strength high nitrogen stainless steel joints were comparatively studied. With additional rapid cooling by flowing water, the peak temperature and duration at elevated temperature during FSW were significantly reduced. Compared to those in the air-cooled joint, nugget zone with finer grains (900 nm) and heat affected zone with higher dislocation density were successfully obtained in the water-cooled joint, leading to significantly improved mechanical properties. The wear of the welding tool was significantly reduced with water cooling, resulting in better corrosion resistance during the immersion corrosion test.

As an advanced solid state bonding process, plastic deformation bonding (PDB) is a highly reliable metallurgical joining method that produces significant plastic deformation at the bonding interface of welded joints through thermo-mechanical coupling. In this study, PDB behavior of IN718 superalloy was systematically investigated by performing a series of isothermal compression tests at various processing conditions. It was revealed that new grains evolved in the bonding area through discontinuous dynamic recrystallization (DDRX) at 1000-1150 °C. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) results revealed that the bonding of joints is related with interfacial grain boundary (IGB) bulging process, which is considered as a nucleation process of DRXed grain under different deformation environments. During recrystallization process, the bonded interface moved due to strain-induced boundary migration (SIBM) process. Stored energy difference (caused by accumulation of dislocations at the bonding interface) was the dominant factor for SIBM during DRX. The mechanical properties of the bonded joints were dependent upon the recrystallized microstructure and SIBM ensued during PDB.

Hot compression bonding was first used to join oxide-dispersion-strengthened ferrite steels (14YWT) under temperatures of 750-1100°C with a true strain range of 0.11-0.51. Subsequently, the microstructure evolution and mechanical properties of the joints were characterized, revealing that the 14YWT steels could be successfully bonded at a temperature of at least 950 °C with a true strain of 0.22, without degrading the fine grain and nanoparticle distribution, and the presence of inclusions or micro-voids along the bonding interface. Moreover, the joints had nearly the same tensile properties at room temperature and exhibited a similar fracture morphology with sufficient dimples compared to that of the base material. An electron backscattered diffraction technique and transmission electron microscopy were systematically employed to study the evolution of hot deformed microstructures. The results showed that continuous dynamic recrystallization characterized by progressive subgrain rotation occurred in this alloy, but discontinuous dynamic recrystallization characterized by strain-induced grain boundary bulging and subsequent bridging sub-boundary rotation was the dominant nucleation mechanism. The nuclei will grow with ongoing deformation, which will contribute to the healing of the original bonding interface.

应用Gleeble-3500热模拟机对7075铝合金进行了热变形连接试验,利用金相显微镜和扫描电镜对不同温度和保温时间的接头界面组织进行了观察和分析,并对接头进行了拉伸性能测试。结果表明:当连接温度为460℃,保温时间为24 h时,对界面进行EDS分析,界面处已经没有氧化物的偏聚,此时的力学性能最佳,抗拉强度达到444 MPa,屈服强度为265 MPa。

Superalloys with excellent high-temperature resistance and oxidation resistance have been widely used in aviation and energy fields. The new Ni-Co base superalloy is considered a candidate for the next generation of turbine discs due to its higher performance of mechanical properties and microstructure stability at high temperatures. However, tungsten inert gas (TIG) welding, metal inert gas (MIG) welding, and other welding techniques are not suitable for welding the new Ni-Co base superalloy because the Al + Ti content of the alloy reaches 7.5%, while traditional welding techniques (electron beam welding, friction welding, and diffusion welding) also have some disadvantages. For example, friction welding has certain requirements on the shape of the sample, and it is not suitable for welding large-volume alloys. Diffusion welding requires a long heat retention period and a harmful precipitation phase exists at the interface. A new welding method is applied in this study to solve the problem of welding nickel-based superalloy, achieving a better bonding effect. The Gleeble 3500 thermal simulator was used to study the plastic deformation bonding of Ni-Co base superalloys in a temperature range of 1000-1200oC and a strain range of 5%-40% with a strain rate of 0.001 s-1. The recrystallization behavior of the interface was studied by OM, EBSD, and TEM, and the bonding mechanism of the interface was clarified. The results showed that the resistance to deformation of the alloy was low when the plastic deformation bonding was performed at 1150oC, and there was no risk of cracking of the alloy. Plastic deformation bonding experiments with different deformations had shown that the alloy can achieve complete bonding of the interface under the condition of 40% deformation, and its mechanical properties can reach the same level as the matrix. The tensile fracture analysis showed that the fracture profile of the 40% deformed joint was consistent with the base material, showing a ductile fracture pattern. The results of EBSD and TEM showed that the coarse grains near the interface were first refined during the plastic deformation. With the increase of deformation, the refined grain removed the original interface by the migration of the interfacial grain boundaries with the assistance of a continuous dynamics recrystallization process and ultimately led to the bonding of the interface.

为解决镍基高温合金的焊接难题，以新型Ni-Co基高温合金为实验材料，采用塑性变形连接的新方法，实现了新型Ni-Co基高温合金的连接。通过OM、EBSD、TEM等分析手段探究了界面的再结晶行为及界面的愈合机制。结果表明，1150℃进行塑性变形连接时，合金的变形抗力小，不易开裂。不同变形量的连接实验表明，在40%的变形量下，合金可以实现界面的完全愈合，其力学性能达到基体同等水平。在塑性变形过程中，界面附近的粗大晶粒首先发生细化，随着变形量的增加，细化的晶粒在连续动态再结晶的辅助作用下，通过界面晶界的迁移消除了原始连接界面，实现界面的愈合。

During hot deformation, discontinuous dynamic recrystallization (DDRX) taking place by nucleation and growth in materials with low to medium stacking fault energies (SFEs), plays a crucial role in grain refinement, especially for the material with coarse grains. In order to study the formation mechanism of typical microstructure (necklace structure) during DDRX, the behavior of Incoloy 028 alloy at temperature range of 1000~1150 ℃ and the strain rates of 0.001~1 s-1 was investigated by means of thermodynamic simulation, EBSD and TEM. The results show that with the decrease of deformation temperature or the increase of strain rate, the mechanism of DDRX is transformed from the traditional type nucleating at triple junctions, into necklace structure which dominated by the multilayer nucleation mechanism. The first strand of recrystallized grain is nucleated through the bulging of serrated grain boundaries which is assisted by twinning at the back of the fluctuation. With the increase of the true strain, the large strain gradient in the deformation band develops rapidly resulting in the transformation of the subgrain boundary into a high angle grain boundary, and then the second/followed layer nucleation occurs by the rotation of subboundaries accompanied with nucleation at triple junction. Twin boundaries are formed by strain-induced grain boundaries migration and disappeared after nucleation to enhance the recrystallization grain boundary mobility, and then formed again during growth to lower the interfacial energy of the system.

利用热力模拟、EBSD和TEM等方法,研究了Incoloy 028合金在1000~1150 ℃和0.001~1 s^-1应变速率条件下的不连续动态再结晶(DDRX)行为,分析了DDRX过程中链状组织的形成机理。结果表明,随着变形温度降低或应变速率升高,体系发生传统型向链状型DDRX转变,其中传统型DDRX过程由晶粒长大主导,主要在三叉晶界处形核;链状型DDRX发生多层形核长大,其第一层形核机制为孪晶界辅助的原始晶界弓出形核,后续层为亚晶扭转与三叉晶界形核;孪晶界在辅助形核后消失以提高界面移动性,晶粒长大时再次形成以降低体系能量。