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Carbon and low-alloy steel plates clad with stainless steel or other metals are a good choice to meet the demand for cost-effective materials to be used in many corrosive environments. Numerous technical solutions are developed for the production of clad steel plates, as well as for their joining by fusion welding. For thick plates, a careful strategy is required in carrying out the multiple passes and in choosing the most suitable filler metals, having to take into account the composition of the base metal and the cladding layer. The specificity of the different processes and materials involved requires an adequate approach in the study of the metallurgical characteristics of clad steel, thus arousing the interest of researchers. Focusing mainly on ferritic steel plates clad with austenitic steel, this article aims to review the scientific literature of recent years which deals with both the production and the fusion welding processes. The metallurgical issues concerning the interfaces and the effects of microstructural characteristics on mechanical behaviour and corrosion resistance will be addressed; in particular, the effects on the fusion and thermally affected zones that form during the fusion welding and weld overlay processes will be analysed and discussed.

The properties of flux-cored Zn-Al filler metals are prone to deteriorating due to corrosion, making filler metals unusable. In this study, flux-cored Zn-2Al-xGe (x = 0, 0.3, 0.5, and 0.8 wt.%) filler metals are prepared to explore the effect of minor Ge on the corrosion resistance of Zn-2Al filler metals. The salt spray test is carried out on filler metals. A scanning transmission electron microscope is used to identify the phases in filler metals. The electrochemical performance of filler metals was tested by a workstation. The findings indicate that the microstructure of the Zn-2Al filler metal is composed of α-Al and η-Zn. Diamond-Ge forms in the microstructure of the Zn-2Al filler metal due to the introduction of Ge. Zn-2Al-xGe filler metals exhibit pitting corrosion characterized by intergranular corrosion (IGC) in the salt spray environment. Ge improves the IGC resistance of filler metals by changing the distribution of α-Al in the filler metal. The Zn-2Al-0.3Ge filler metal demonstrates the most excellent corrosion resistance. It has 16% elongation after 15 d of corrosion, which is higher than that of Zn-2Al by 13.6%.

Welding high-strength 6xxx aluminum alloys using a commercial ER4043 filler often results in inferior joint strength. This study investigated the effects of newly developed Al-Si-Mg filler metals with varying Mg (0.6–1.4 wt.%) and Mn (0.25–0.5 wt.%) contents on the microstructure evolution and mechanical performance of high-strength AA6011-T6 plates using gas metal arc welding. Two commercial fillers, ER4043 and ER4943, were used as references for comparison. The results revealed that increasing the Mg and Mn contents in the novel fillers resulted in sufficiently high alloying elements in the fusion zone (FZ), leading to higher microhardness. Under as-welded conditions, the weakest region of the joint was the heat-affected zone (HAZ). The joint strength was almost independent of the filler type and was controlled by the HAZ strength, measuring a UTS of 230 and 241 MPa for ER4043 and the other joints, respectively. The higher Mg contents in the novel fillers promoted the precipitation of a large volume fraction of fine β″-MgSi in the FZ during post-weld heat treatment (PWHT), resulting in superior strength and higher welding efficiency relative to the reference fillers. The optimal Mg content of the novel fillers was 0.6 wt.%. Increasing the Mn content of the filler metal had an insignificant effect. The FMg0.6 filler with 0.6% Mg achieved the best combination of strength (UTS of 410 MPa) and elongation (6.7%) as well as the highest welding efficiency (94%) after PWHT, among all of the fillers studied. However, the newly developed fillers adversely affected the impact toughness of the joints.

Ag-based and Cu-based brazing filler metals, which are the most widely used brazing materials in industrial manufacturing, have excellent gap-filling properties and can braze almost all the metallic materials and their alloys, except for the low-melting-point metals such as Al and Mg. Therefore, Ag-based and Cu-based brazing filler metals have attracted great attention. In this review, three series of typical Ag-based filler metals: the Ag-Cu, Ag-Cu-Zn, and Ag-Cu-Zn-Sn alloys; and three series of Cu-based filler metals: the crystalline and amorphous Cu-P filler metals, as well as the Cu-Zn filler metals, were chosen as the representatives. The latest research progress on Sn-containing Ag-based and Cu-based brazing filler metals is summarized, and the influences of Sn on the melting characteristics, wettability, microstructure, and mechanical properties of the selected filler metals are analyzed. Based on these, the problems and corresponding solutions in the investigation and application of the Sn-containing Ag-based and Cu-based filler metals are put forward, and the research and development trends of these filler metals are proposed.

3003 aluminum alloy was widely used for the manufacturing of heat exchangers in the automotive industry by employing controlled atmosphere brazing (CAB) with NOCOLOK flux brazing technology. However, commercially available filler metals for NOCOLOK flux brazing technology are usually required to be carried out at a relatively high temperature, causing the assembled heat exchanger to be partially molten or easily deformed. A new low-melting-point brazing filler metal Al-5.0Si-20.5Cu-2.0Ni was prepared by using melt-spinning technology and then applied to CAB of 3003 aluminum alloy in this research. The solidus and liquidus of brazing filler metal was 513.21 °C and 532.48 °C. All elements were evenly distributed and free from elemental segregation. The microstructure of brazing filler metal was uniform, and the grain size was less than 500 nm. As the brazing temperature reached 575 °C, the void in the joint disappeared completely. The morphology of CuAl2 was sensitive to the brazing temperature and dwell time. The appearance of net-like CuAl2 brazed at 575 °C for 20 min was more beneficial to improve joint mechanical properties. The leakage rate of the joint was qualified to be 10−10 Pa·m3/s when the brazing temperature was 570 °C or higher. The maximum shear strength of 76.1 MPa can be obtained when the joint was brazed at 575 °C for 20 min. More dwell time induced growth of the interfacial layer and reduced joint shear strength. The open circuit potential and corrosion current density test indicated that the brazing filler metal Al-5.0Si-20.5Cu-2.0Ni had better corrosion resistance than that of 3003 aluminum alloy.

Additive manufacturing processes play a disruptive role in several industrial sectors. Among them, wire arc additive manufacturing (WAAM) is a very promising process for the production of large-scale steel components and structures. As a common characteristic of innovative technologies, this process requires additional research and experimental work in order to understand how its thermal cycles will modify the mechanical and metallurgical properties of the manufactured components. Considering the lack of literature on the properties of WAAM components from specific steel alloys, the present work proposes a benchmark evaluation of five different steel filler metals based on their mechanical properties and operational parameters when applied for additive manufacturing. The mechanical properties, such as the tensile strength, yield strength, elongation, Charpy impact toughness, and hardness, were evaluated together with operational parameters such as cost, printability, spatter, and fume formation during manufacturing, defect-free printing, and heat input. These were analyzed in order to obtain a full evaluation and comparison of the five filler metals. The experimentally determined results of every above-mentioned aspect were higher or similar to the values found in the literature for generic steel filler metals used in additive manufacturing. This means that a designer or welding engineer can select the WAAM steel filler metal for further testing based on the welding wire datasheet. Besides that, a comparison was performed between blocks manufactured by WAAM with some frequently used base metals, in plate format, confirming its applicability.

Motivated by the loss of tensile strength in 9%Ni steel arc-welded joints performed using commercially available Ni-based austenitic filler metals, the viability of retaining tensile strength using an experimentally produced matching ferritic filler metal was confirmed. Compared to the austenitic Ni-based filler metal (685 MPa), higher tensile strength in gas metal arc (GMA) welded joints was achieved using a ferritic filler metal (749 MPa) due to its microstructure being similar to the base metal (645 MPa). The microstructure of hard martensite resulted in an impact energy of 71 J (−196 °C), which was two times higher than the specified minimum value of ≥34 J. The tensile and impact strength of the welded joint is affected not only by its microstructure, but also by the degree of its mechanical mismatch depending on the type of filler metal. Welds with a harder microstructure and less mechanical mismatch are important for achieving an adequate combination of tensile strength and notched impact strength. This is achievable with the cost-effective ferritic filler metal. A more desirable combination of mechanical properties is guaranteed by applying low preheating temperature (200 °C), which is a more practicable and economical solution compared to the high post-weld heat treatment (PWHT) temperature (580 °C) suggested by other research.

The performance of flux-cored Zn-Al filler metal is susceptible to corrosion-induced degradation, thereby impairing its brazability. In this study, flux-cored Zn-2Al filler metals are prepared, and the salt spray test is subsequently carried out on the prepared filler metals. Scanning transmission electron microscope is used to identify the phases in filler metals. An electrochemical workstation was employed to test the electrochemical performance of the filler metals. The corrosion pathways and evolution patterns of filler metals are analyzed. The findings demonstrate that the corrosion type of the filler metals is electrochemical corrosion, characterized primarily by the corrosion modes of pitting corrosion and intergranular corrosion. The cathode is the α-Al phase, which undergoes an oxygen-absorption corrosion reaction, while the anode is the η-Zn phase, which experiences corrosion and subsequent dissolution. The continuously distributed α-Al phase bands and discontinuously distributed large-sized rod-like α-Al phases accelerate the corrosion rate, and the corrosion propagation rate along the extrusion direction is higher than that in the radially inward direction. After 15 days of salt spray corrosion, the tensile strength of filler metals decreases by 16.2%, and the elongation rate decreases to 3.73%.

To achieve the brazed diamond tools with low thermal damage, the diamonds were brazed using Ni–Cr base filler metals containing up to 1.5 weight percent Nd. The microstructure of Ni–Cr base filler metals and the surface morphology and interfacial microstructure of brazed samples were characterized. The results show that by the addition of Nd, the microstructure of filler metals is obviously refined and becomes uniform. Meanwhile, Nd doping effectively reduces the graphitization degree and thermal residual stress of brazed diamonds. In addition, a variety of carbides (such as Cr 3 C 2 , Cr 7 C 3 , and SiC) which have low hot cracking tendencies are formed through occurring chemical/metallurgical reactions. The microstructural NdNi 4 B particles contribute to reducing the thermal damage of the brazed diamonds after brazing. At a doping level of 1.0 wt.% of rare-earth Nd, the maximum residual stress of brazed diamond was reduced by 20% and the material removal increased by 45.1%.

Composite filler metal refers to the traditional filler metal by adding a certain proportion of various forms of superalloy, carbon fiber and ceramic particles as reinforcement phase. Due to the addition of the reinforcement phase, the filler metal can have a suitable thermal expansion coefficient, which can effectively reduce the residual stress at the brazing joint caused by the different thermal expansion coefficients of the base metal and improve the comprehensive performance of the brazing joint. In recent years, with the progress of science and technology, the research on nanomaterials has been deepening, and nanomaterials are widely used in the modification of composite filler metals because of their special surface effect, small size effect, quantum size effect and macroscopic quantum tunneling effect. The modification performance of different composite solders by nanoparticles in recent years is reviewed, the advantages and disadvantages of nano-reinforced composite solders are analyzed, and the future research direction of composite solders is prospected.