Numerous industrial applications, particularly those in the transport industry, require the joining of dissimilar materials which offers considerable benefits in terms of low cost, design ...flexibility, and weight reduction for overall structures. The problems associated with conventional fusion welding processes have stimulated researchers in recent years to develop new joining methods for dissimilar materials which are particularly difficult to join. Friction stir welding (FSW) originally developed for joining difficult-to-weld Al-alloys and FSSW (a variant of FSW for spot welding) have exhibited great potential for obtaining sound joints in various dissimilar alloy systems in different configurations namely butt-, lap- and spot-welding, particularly in dissimilar Al-alloys systems with different properties, which are very difficult to weld using conventional fusion welding techniques. A major difficulty in joining dissimilar Al-alloys by FSW/FSSW lies in the discontinuity in mechanical and technological properties (such as high-temperature strength, plastic deformation capacity, viscosity, etc.) of the materials to be welded across the abutting surfaces. This discontinuity as well as inherent asymmetry in heat generation and material flow of FWS/FSSW processes causes a higher asymmetry in materials flow behavior in dissimilar welding. However, it is relatively easier to implement the FSW/FSSW process to dissimilar Al-alloys in contrast to FSW of dissimilar materials combinations with very differing properties, such as Al-alloy to Mg-alloy or Al-alloy to steel.
The present study deals with laser beam welding (LBW) and friction stir welding (FSW) applied to high-strength aluminum alloys used in aircraft industry and displays their advantages compared with ...the riveting technique regarding structural integrity, weight and material savings. First of all, it is shown with respect to different applications and strength levels which high-strength aluminum alloys represent the state-of-the-art and which aluminum alloys are proposed as substitutes in the future. Furthermore, the respective joining process principles are described and demonstrated on different joint configurations, whereby mechanical and microstructural properties of laser beam- and friction-stir-welded joints are discussed and compared. The current study clearly demonstrates that these two joining techniques are not competing but complementary joining techniques in the aircraft industry.
FSW, as a solid-state joining process, has the advantage that the joining is conducted at temperatures below the melting point of the materials to be joined. Therefore, improved mechanical performance of joints is expected compared to that of fusion joining processes such as LBW. Furthermore, better mechanical properties can be obtained when heat input during joining is reduced by employing stationary shoulder FSW and/or external cooling. On the other hand, LBW offers several advantages such as low distortion, high strength of the joint, and high welding speeds due to its low localized-energy input. Thus, LBW - as a high-speed and easily controllable process - allows the welding of optimized complex geometrical forms in terms of mechanical stiffness, strength, production velocity, and visual quality. Both joining processes have advantages and disadvantages, depending on joint geometries and materials. They both have the potential to reduce the total weight of the structure. The FSW process (particularly lower heat input stationary shoulder FSW process) is more advantageous in producing long-distance straight-line butt joints or overlapped joints of aircraft structures, whereas the high-speed and easily controllable LBW process allows the joining of complex geometrical forms due to its high flexibility, particularly in the new generation high strength Al-alloys (such as AA2198), the strengthening phases of which are more heat resistant.
In this study, the effect of thermochemical boriding on the microstructural properties and high-temperature wear resistance of cobalt-based Haynes 25 superalloy was investigated. Haynes 25 alloy was ...subjected to a pack-boriding process at 850, 950, and 1050 °C for 4 h using a mixture of 90% B4C (boron carbide) and 10% NaBF4 (sodium tetrafluoroborate). Optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray (XRD) analysis, microhardness, and high-temperature wear tests were performed for the characterization of boride layers obtained after the boriding process. Adhesion wear tests were carried out in a ball-on disc wear device with a 20 N load against an alumina (Al2O3) ball and a sliding distance of 250 m at both room temperature and 500 °C. Due to the high hardness and self-lubricating properties of the boride layers obtained, the borided samples exhibited a higher wear resistance than the untreated sample at both test temperatures, i.e. 18 times higher at room temperature and 4 times higher at 500 °C. The decrease in the wear performance of the boride layers at high temperatures is due to the fact that the wear mechanism turns into fracture-type wear besides oxidative wear at 500 °C. On the other hand, the wear mechanism of the untreated Haynes 25 alloy at room temperature is mixed type (abrasive, oxidative, and plastic deformation), while at 500 °C it turns into an oxidative and fracture type wear mechanism. Oxidative supported fracture type wear mechanism was observed in borided samples both at room temperature and at 500 °C, but the oxidative effect was considerably higher at 500 °C than at room temperature.
•Boriding of Co-based Haynes 25 at 850, 950 and 1050 °C, has been conducted for the first time. (CoFe)B2, CoB, CrB, W3CoB3 phases dominate the structure of boriding layers.•Boriding treatment provided an 18-fold improvement in the abrasion resistance at room temperature and a 4-fold improvement in the abrasion resistance at 500 °C.•The wear mechanism of the untreated alloy at room temperature was abrasive, oxidative and plastic deformation, it turned to oxidative and fracture type at 500 °C.•On the other hand, borided Haynes 25 alloy exhibited only oxidative supported fracture wear mechanism at both room temperature.
Arc-directed energy deposition (Arc-DED), also commonly referred to as wire arc additive manufacturing (WAAM), is a cost-effective 3D metal additive manufacturing process in which large metallic ...parts can be produced due to high deposition rates. Stainless steels, widely used in many areas due to their excellent corrosion resistance, are one of the most produced materials by the WAAM method. However, stainless steels have low surface hardness. Moreover, the high heat input in the deposition process in WAAM sometimes causes the mechanical properties of stainless steels to be lower than casting or wrought stainless steels. These considerations limit the use of WAAM stainless steels, especially in abrasive environments. For this purpose, 307ER stainless steel produced by WAAM method was subjected to homogenization and boriding process simultaneously at 1000 °C for 1 h and the effect of applied heat treatment on microstructure, phase components, hardness and wear resistance was investigated. The results showed that with the boriding process, a 30 μm thick boride layer consisting of FeB, Fe2B, Cr5B3 and MnB phases with a hardness of 21.5 GPa and a modulus of elasticity of 310 GPa was formed in addition to the dissolution of interdendritic regions in the as-built structure and a complete homogenization of the microstructure. Furthermore, owing to the high hardness and elasticity modulus in addition to the boride layer's self-lubrication properties obtained on the WAAM samples surfaces, 31.84 times and 8.06 times increased in wear resistance at room temperature and 500 °C temperature, respectively, and a decrease in friction coefficients was obtained. Moreover, the results showed that the simultaneous homogenization and boriding processes of stainless steels produced by the WAAM method would improve their microstructure and tribological behavior. This way, these steels can be used in wider areas of application.
•GMAW-based WAAM ER307 stainless steel components were successfully manufactured.•Boride layers improved the surface hardness of WAAM ER307 up to 7 times.•The self-lubrication properties of borides provided lower COF at 25 °C and 500 °C.•Increase in surface hardness resulted in an increase in wear resistance up to 500 °C.•Boriding has the potential to open up new possibilities for additive-manufactured metals.
The diversity and never-ending desire for a better life standard result in a continuous development of the existing manufacturing technologies. In line with these developments in the existing ...production technologies the demand for more complex products increases, which also stimulates new approaches in production routes of such products, e.g., novel welding procedures. For instance, the friction stir welding (FSW) technology, developed for joining difficult-to-weld Al-alloys, has been implemented by industry in manufacturing of several products. There are also numerous attempts to apply this method to other materials beyond Al-alloys. However, the process has not yet been implemented by industry for joining these materials with the exception of some limited applications. The microstructures and mechanical properties of friction stir welded Al-alloys existing in the open literature will be discussed in detail in this review. The correlations between weld parameters used during FSW and the microstructures evolved in the weld region and thus mechanical properties of the joints produced will be highlighted. However, the modeling studies, material flow, texture formation and developments in tool design are out of the scope of this work as well as the other variants of this technology, such as friction stir spot welding (FSSW).
Austenitic stainless steels exhibit excellent properties such as outstanding formability even at cryogenic temperatures, high mechanical properties and good corrosion performance. This makes them ...attractive for use in various industries ranging from petro-chemical industries, food industry, medical-dental equipments, and pressure vessels to LNG tanks and welding is often required in most of these applications. Especially with the increase in energy crisis in recent years, the production of LNG tanks using conventional arc welding processes is important. In this study, the weldability of 10 mm thick 316 L stainless steel plates used in the production of LNG tanks by gas tungsten arc welding was investigated. The effect of multi-pass welding on the microstructural evolution and thus on mechanical behavior of the joint produced was studied. Optical microscopy and scanning electron microscopy as well as energy dispersive spectroscopy were employed to investigate the microstructural evolution in the weld region of the joint fabricated. In addition, microhardness measurements and tensile testing were conducted to mechanical characterization. As a result of the study, a slight hardness increase was observed within the weld region of the joint compared to that of the base material. The welded joint exhibited higher yield and tensile strength values, namely 329 and 630 MPa respectively, than the base plate which displayed a yield strength of 308 MPa and a tensile strength of 603 MPa.
•10 mm thick AISI 316L plates was welded byGTAW using a filler rod of ER316L with a diameter of 2.4 mm in five passes•No weld defects such as porosity or crack formation were obtained in the fusion zone•Microstructure in the HAZ is heterogeneous since the variation in the heat input in multi-pass welding from region to region•Tensile test specimens obtained from the welded joint were ruptured within the BM far away from the FZ
The demand for dissimilar joining of steel grades, namely austenitic stainless steels to low-alloy steels, may increase in near future owing to the fact that the storing of LNG is currently becoming ...a necessity, particularly in Europe, due to the shortage of supply or interruptions in the supply. Therefore, successful dissimilar joining of steel grades using traditional fusion welding techniques in such applications is required. In this study, butt-welded joints of AISI 316L austenitic steel and low-alloy steel plates (containing 9% Ni) of 10 mm thickness were fabricated by gas tungsten arc welding employing a Ni-based filler wire. The microstructure and mechanical properties of the weldment were examined by detailed optical microscopy, extensive micro-hardness measurements, and tensile tests. Further, fracture toughness of the joint at cryogenic temperatures (− 196 °C) was also determined by Charpy impact test. The dissimilar joint exhibited a high tensile strength of 633 MPa, which is higher than that of the lower-strength AISI 316L base plate (about 600 MPa), while its elongation (21%) was much lower due to confined plasticity. The lowest impact energy was displayed by HAZ-F notched specimens, namely about 62.6 J (0.83 J/mm
2
). However, it is still reasonably above the minimum impact energy specified for the LNG storage tanks, i.e., 0.75 J/mm
2
.
The use of the friction stir welding (FSW) process as a relatively new solid-state welding technology in the aerospace industry has pushed forward several developments in different related aspects of ...this strategic industry. In terms of the FSW process itself, due to the geometric limitations involved in the conventional FSW process, many variants have been required over time to suit the different types of geometries and structures, which has resulted in the development of numerous variants such as refill friction stir spot welding (RFSSW), stationary shoulder friction stir welding (SSFSW), and bobbin tool friction stir welding (BTFSW). In terms of FSW machines, significant development has occurred in the new design and adaptation of the existing machining equipment through the use of their structures or the new and specially designed FSW heads. In terms of the most used materials in the aerospace industry, there has been development of new high strength-to-weight ratios such as the 3rd generation aluminum-lithium alloys that have become successfully weldable by FSW with fewer welding defects and a significant improvement in the weld quality and geometric accuracy. The purpose of this article is to summarize the state of knowledge regarding the application of the FSW process to join materials used in the aerospace industry and to identify gaps in the state of the art. This work describes the fundamental techniques and tools necessary to make soundly welded joints. Typical applications of FSW processes are surveyed, including friction stir spot welding, RFSSW, SSFSW, BTFSW, and underwater FSW. Conclusions and suggestions for future development are proposed.
Heat input is extremely important for achieving joints with high mechanical properties in ferritic stainless steels. Thus, one of the objectives of this study is to show the effect of heat input on ...microstructural evolution and thus on the mechanical behavior. For this aim, 5 mm thick AISI 430 plates were butt welded using two heat input levels. Extensive optical microscopy and SEM investigations, microhardness measurements, mechanical tests (i.e., tensile and bending) were conducted to investigate the microstructures evolving in the weld area and to determine the mechanical properties of the joints produced. Strength overmatching was obtained in the weld region of both joints fabricated with both heat inputs employed. In addition, the HAZ region is much wider in the joint fabricated with higher heat input. Although the higher heat input joint exhibited a wider HAZ region the hardness values were similar in the HAZ regions of both joints. As a result, both joints exhibited similar tensile properties, i.e., a strength performance of about 99%. In contrast, the root bend specimen of the higher heat input joint did crack in the bending test while both surface and root bend specimens of the lower heat input joint did not fail.
•No weld defects such as porosity or crack formation were obtained in the fusion zone.•The width of the HAZ and the grain size in the HAZ region increased significantly by an increase in heat input.•A hardness increase was detected in the weld region of both joints indicating existence of strength overmatching in the weld region.•Both joints exhibited a high strength performance, i.e., about 99%. thus, grain coarsening in the HAZ and the thickening of the HAZ region did not diminish the joint performance in tensile testing due to strength overmatching.•However, root bend specimen of the higher heat input joint cracked indicating that the loss of toughness in the HAZ region diminished the joint performance in bending.
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