New requirements for the functionality of materials are raised on the conditions of extremely complex working conditions. Compared with traditional manufacturing technology, additive manufacturing technology of metal heterogeneous materials has greater advantages in the manufacture of heterogeneous material parts. It has broad application prospects in many areas, such as aerospace, biomedical and automotive industries, and can meet these areas’ demand for high performance and multifunctionality of components. Based on the research progress of metal heterogeneous material additive manufacturing technology in recent years, the research status and technical characteristics of arc, electron beam and laser additive manufacturing technology on heterogeneous materials are reviewed. In addition, based on the team’s research on heterogeneous material metal additive manufacturing equipment, material technology and interface defects, the key problems and applications of metal heterogeneous material additive manufacturing technology are summarized, and its development trend are prospected.

Advanced high-strength steels have undergone rapid development from the first to third generation, which has considerably contributed to the continuous improvement of lightweight materials and safety in the automotive industry. The third generation representative steels, including quenching-partitioning (QP) and quenching-partitioning-tempering (QPT) steels have rapidly developed in the past 10 years. This article summarizes the preparation process as well as the strengthening and toughening mechanisms of QP and QPT steels from the following perspectives: (1) process design development and principles from QP to QPT, (2) carbon distribution and microstructure evolution during the partitioning process, (3) stability of metastable austenite and its influence on transformation-induced plasticity, (4) microstructure and heat-treatment process design of nanoprecipitation-strengthened QPT steel, (5) the integrated process of hot-forming QPT steel, and (6) the strengthening and toughening mechanisms and the service performance of QP and QPT steels. Finally, future prospects for manufacturing and using QP and QPT steels are discussed.

高强度、高塑性是汽车钢的重要发展方向，本文综述了高强度高塑性第三代汽车钢的“多相(multiphase)、亚稳(metastable)和多尺度(multiscale)” M³组织性能调控理论和技术，以及面临的新挑战。M³组织与性能调控理论为高强度高塑性钢提供了理论支持，亚稳奥氏体的相变诱发塑性(TRIP)效应能够提高加工硬化率并推迟颈缩的发生，从而提高了钢的强度与塑性，同时产生了剪切边裂纹敏感性提高，氢致延迟断裂性能下降，循环载荷下亚稳奥氏体的转变行为复杂等新的问题和挑战。当前，含亚稳奥氏体高强度高塑性钢的质量一致性和应用基础研究缺乏，而汽车钢作为量大面广的产品，需要从它的成分设计和组织调控-冲裁切割-成形制造-连接涂装-服役评价等全链条环节中开展组织演变和性能评估，充分考虑产品的技术适用性和成本，进而为组织调控理论和技术的完善提供依据。

Dissimilar metal weld joints (DMWJ) widely exist in the nuclear power plants to join the different parts which are made of different structural materials. Among these DMWJs, safe-end DMWJ has attracted much attention of researchers and operating enterprises, as premature failures, mainly stress corrosion cracking failures, have occurred in these kinds of joints. However, DMWJ with 52M as filler metal in the nuclear power plants has no in-service experience. To ensure the structural integrity of the weld joint and the safe operation of the future plants, the microstructure and local properties of a domestic safe-end DMWJ by using hot-wire gas tungsten arc welding (GTAW) technology was studied in detail by OM, SEM, micro-hardness testing, local mechanical tensile testing and slow strain rate tests. The tensile tests were performed at room temperature with the tensile speed of 5 μm/s while the slow strain rate tests were conducted in simulated primary water containing 1500 mg/L B as H3BO3 and 2.3 mg/L Li as LiOH with 2 mg/L dissolved oxygen at 325 ℃. A large amount of type I boundaries and type II boundaries which are susceptible to stress corrosion cracking (SCC) exist in 52Mb near the SA508/52Mb interface and result in the highest SCC susceptibility of this interface. Microstructure transition was found in the SA508 heat affected zone (HAZ). In 316LN HAZ, increasing the distance from the fusion boundary, the number fraction of CSL boundaries increase while the residual strain decreases, resulting in the second-highest SCC susceptibility of 316LN HAZ. In 52M, residual strain distributes randomly but not uniformly, the residual strain is prone to accumulate at the grain boundaries. Dramatic changes of mechanical properties are observed across the joint, especially at the SA508/52M interface. The differences of the local microstructure and chemical composition lead to the differences of the local properties of the weld joint.

Multi-material additive manufacturing enables high-performance heterogeneous design at the mesoscale, through which bulk parts can be engineered to adapt to complex loading conditions. The optimization of multi-material parts relies on accurate forward prediction, which is challenging to achieve owing to the complex processing conditions in additive manufacturing and the resultant uncertainties in material properties. To address these limitations, here we present a new model calibration and model selection framework based on the high dimensional, local-scale deformation data. By matching the pixel-level deformation data from digital image correlation experiments and constitutive modeling, the presented framework enables more accurate prediction and significant reduction of the prediction uncertainties, as compared to the single material calibration approach that is widely used in additive manufacturing. In turn, this enables quantitative comparison of the candidate models, so the most accurate and computationally efficient constitutive model can be selected for forward prediction in heterogeneous material design. The advantages of the framework are demonstrated using a multi-polymer system manufactured by dual-extrusion additive manufacturing, which consists of two constituent materials with dramatically different deformation behaviors.

During fusion welding of the Cu-containing high-entropy alloy (HEA) of CoCrFeNiCu, Cu precipitates at the grain boundary and reduces the welding quality and mechanical properties. Solid-phase diffusion welding is connected with the diffusion of elements, which reduces the segregation of Cu and solves this problem well. In the present work, the study of diffusion welding of CoCrFeNiCu HEA with 304 stainless steel (304SS) was carried by SEM, EBSD, TEM, and XRD. The properties of the microstructure and crystal type were obtained. The results show that element diffusion can achieve high-quality connection between the two metals. As the temperature increases, the diffusion capacity increases. The pores at the interface disappear. The thickness of the diffusion layer is between 10-31 μm, without forming intermetallic compound, which shows that a solid solution microstructure is formed after diffusion. According to the XRD, this microstructure is mainly as CoCrFeNiCuMn, that leads to an improvement in the mechanical properties. The hardness of the CoCrFeNiCu HEA-diffusion layer-304SS shows a gradient increase. The crystal structure type is mainly as substructured and recrystallized structure. The proportion of low-angle grain boundaries is 93% in the diffusion layer. The EBSD test results show that the grain orientation of the welded joint (diffusion layer) has changed, mostly concentrated on the (111) plane. This also causes anisotropy of the mechanical properties. Tensile performance test showed that the material fractured in the CoCrFeNiCu HEA base material area. These show that diffusion welding achieves a high-quality connection of the two materials.

High entropy alloys (HEAs) have superior mechanical properties that have enabled them to be used as structural materials in nuclear and aerospace applications. As a dissimilar joint design is required for these applications, we created a dissimilar joint between CoCrFeMnNi-HEA and duplex stainless steel (DSS) through laser beam welding; a technique capable of producing a sound joint between the two materials. Microstructure examination using SEM/EBSD/XRD analysis revealed that the weld metal (WM) exhibits an FCC phase regardless of the postweld heat treatment (PWHT) temperature (800 and 1000 °C) without forming detrimental intermetallic compounds or microsegregation. The heat-affected zone of the CoCrFeMnNi-HEA showed CrMn oxide inclusions while that of the DSS showed no inclusions. Moreover, a lower hardness was recorded by the WM compared to the base metal after welding. After PWHT, the hardness of the WM, CoCrFeMnNi-HEA, and DSS decreased with an increase in the PWHT temperature. However, the decrease in the hardness of the HEA was more significant than in the WM and DSS. The cause for this reduction in hardness was attributed to recrystallization and grain growth. In addition, a strength of 584 MPa with low ductility was recorded after welding. The obtained strength was lower than that of the BMs, but comparable to that of the welded CoCrFeMnNi-HEA. The application of PWHT resulted in over a 20% increment in ductility, with only a marginal reduction in strength. The deformation mechanism in the as-weld joint was mainly dominated by dislocation while that for the PWHT joint was twinning. We propose laser beam offset welding as a technique to improve the mechanical properties of the dissimilar joint, which will be the subject of future studies.

Laser melting deposition (LMD) combines the laser cladding and rapid prototyping manufacturing technologies, and can be used for swift prototyping of complex parts with excellent comprehensive properties. However, due to its unique metallurgical conditions, it is easy to develop penetrating columnar crystals and coarse primary grains along the building direction. This remarkably reduces the mechanical properties of the alloy. The root cause of this issue can be traced back to the thermodynamic and dynamic metallurgical processes. Thus, this study proposes an oscillating laser melting deposition (OLMD) based on laser oscillating welding technology, and aims to elucidate the metallurgical structure and defects of laser melt deposition. OLMD modifies the motion trajectory of the molten pool using a laser in situ oscillation, and directly impacts the temperature gradient and solidification rate, thus improving the microstructure of titanium alloy by LMD. Furthermore, the microstructure evolution and mechanical properties of TC4 titanium alloy produced using OLMD were studied using OM, SEM, EBSD, and a Vickers hardness tester. The results indicate that the optimum process parameters of laser melting deposition without oscillation are as follows: the laser power is 1000 W, scanning rate is 8 mm/s, and powder feeding rate is 6.92 g/min. The optimum technological parameters of linear oscillation are as follows: the frequency is 200 Hz and the oscillation amplitude is 1.5 mm. Addition of linear laser oscillation considerably improved the morphology of the molten pool, and defects such as porosity and cracks were not observed. The overall number and size of columnar crystals reduced, and the grains were equiaxed. When compared to the sample without oscillation, the average grain size of Ti-6Al-4V alloy with linear oscillation decreased from 5.20 μm to 4.37 μm, while hardness increased from 418.00 HV to 428.75 HV.

针对激光熔化沉积冶金组织与缺陷，借鉴激光摆动焊接技术，提出一种激光摆动送粉增材制造TC4钛合金工艺，借助激光原位摆动改变熔池运动轨迹进而影响温度梯度和凝固速率，改善增材制造钛合金的微观组织。利用OM、SEM、EBSD和Vickers硬度计研究了激光摆动送粉增材制造工艺对TC4钛合金微观组织演变及力学性能的影响。结果表明，无摆动激光熔化沉积实验的最佳工艺参数为：激光功率1000 W，扫描速率8 mm/s，送粉速率6.92 g/min；直线型激光摆动的最佳工艺参数为：摆动频率200 Hz，摆动幅度1.5 mm。直线型激光摆动对熔池形貌改善显著，气孔和裂纹等缺陷较少，柱状晶数量和尺寸均有所减小，并且晶粒出现了等轴化的现象。相比无摆动样品，激光摆动后Ti-6Al-4V合金单道区域平均晶粒尺寸从5.20 μm减小到4.37 μm；硬度从418.00 HV提升到428.75 HV。

The phase-type of a stainless steel is generally predicted by equivalent equations in terms of a major austenitic (γ) or ferritic (α) stabilizer Ni or Cr. The present paper attempts to understand the equivalent methods in stainless steels via the slopes of the phase boundary lines separating γ and γ + α phase zones. The prevailing equivalent coefficients are well interpreted using the slope ratios of the alloying elements divided by that of Ni or Cr, after analyzing over one hundred common stainless steels. Different from traditional composition equivalents which evaluate γ stabilizers and α stabilizers separately; the new equivalent scheme provides a unified phase stabilizing parameter for all alloying elements in stainless steels. This parameter is defined as γ stabilizing efficiency. Its negative or positive sign indicates γ stabilizer or α stabilizer, and its value represents the stabilizing efficiency.

Variant selection during the martensitic transformation in steels may play an important role in determining the transformation kinetics and the resulting mechanical properties. In this study, the variant selection and crystallographic features of deformation-induced martensite were investigated by quasi in situ electron backscatter diffraction (EBSD) in grade SUS321 during tensile deformation. Significant differences in variant selection between austenite (γ)→hcp-martensite (ε)→bcc-martensite (α’) and γ→α’ transformation routes were observed and reported in detail, which demonstrated that ε-martensite plays an important role in the variant selection of α’. Variant selection at different deformation stages was also analysed and revealed that α’ variants with the highest priority and variant pairs were preferred at the initial and last deformation stages in the γ→ε→α’ sequence, respectively. Meanwhile, the single α’ variant nucleated at the thin slip band keeps its crystallography feature upon further deformation in the γ→α’ sequence. In addition, the strain work of the martensitic transformation for applied loads was quantitatively estimated to explain the variant selection and associated mechanism. When these calculations are compared to the experimental results it is found that they are not able to predict which α’ variant is forming preferentially during either during the γ→α’ or the ε→α’ sequences, while only accurate predictions are obtained for the γ→ε transformation which indicates that the γ→α’ variant selection is more complex.