Showing papers on "Weldability published in 2020"

Processing and Properties of Reversion-Treated Austenitic Stainless Steels

Abstract: Strength properties of annealed austenitic stainless steels are relatively low and therefore improvements are desired for constructional applications. The reversion of deformation induced martensite to fine-grained austenite has been found to be an efficient method to increase significantly the yield strength of metastable austenitic stainless steels without impairing much their ductility. Research has been conducted during thirty years in many research groups so that the features of the reversion process and enhanced properties are reported in numerous papers. This review covers the main variables and phenomena during the reversion processing and lists the static and dynamic mechanical properties obtained in laboratory experiments, highlighting them to exceed those of temper rolled sheets. Moreover, formability, weldability and corrosion resistant aspects are discussed and finally the advantage of refined grain structure for medical applications is stated. The reversion process has been utilized industrially in a very limited extent, but apparently, it could provide a feasible processing route for strengthened austenitic stainless steels.

Elemental Effects on Weld Cracking Susceptibility in Al x CoCrCu y FeNi High-Entropy Alloy

Abstract: Weld solidification and weldability of AlxCoCrCuyFeNi high-entropy alloys (HEA) were characterized as a function of Al and Cu molar ratio in the alloy composition. Autogenous gas tungsten arc welding (GTAW) was performed on the as-melted ingots. Fusion zone and heat-affected zone microstructures were investigated using optical and electron microscopy, and compared to thermodynamic CALPHAD-based calculations. Weld cracking susceptibility was compared to cast pin tear test (CPTT) results. It was found that strong Cu segregation on solidification promotes hot cracking in fusion welds on AlCoCrCuyFeNi alloys (y > 0.1). Cu-rich liquid forms during the final stages of solidification and facilitates solidification cracking in the fusion zone. The Cu-rich interdendritic readily remelts during reheating and promotes liquation cracking in the heat-affected zone. The cracking mechanisms in the welds change to brittle intergranular cracking in low and no Cu alloy compositions due to a high-hardness BCC microstructure (> 500 HV). Lowering the Al molar ratio in AlxCoCrCu0.1FeNi alloys (x ≤ 0.5) mitigates brittle cracking by promoting FCC solidification structure.

From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase

Abstract: Austenitic stainless steels are employed in many industrial fields, due to their excellent corrosion resistance, easy formability and weldability. However, their low hardness, poor tribological properties and the possibility of localized corrosion in specific environments may limit their use. Conventional thermochemical surface treatments, such as nitriding or carburizing, are able to enhance surface hardness, but at the expense of corrosion resistance, owing to the formation of chromium-containing precipitates. An effective alternative is the so called low temperature treatments, which are performed with nitrogen- and/or carbon-containing media at temperatures, at which chromium mobility is low and the formation of precipitates is hindered. As a consequence, interstitial atoms are retained in solid solution in austenite, and a metastable supersaturated phase forms, named expanded austenite or S phase. Since the first studies, dating 1980s, the S phase has demonstrated to have high hardness and good corrosion resistance, but also other interesting properties and an elusive structure. In this review the main studies on the formation and characteristics of S phase are summarized and the results of the more recent research are also discussed. Together with mechanical, fatigue, tribological and corrosion resistance properties of this phase, electric and magnetic properties, wettability and biocompatibility are overviewed.

Current research and development status of dissimilar materials laser welding of titanium and its alloys

Abstract: Since its inception, laser beam welding as a high-quality fusion joining process has ascertained itself as an established and state of art technology exhibiting tremendous growth in a broad range of industries. This article provides a current state of understanding and detailed review of laser welding of titanium (Ti) alloys with corresponding dissimilar counterparts including steel, aluminium, magnesium, nickel, niobium, copper, etc. Particular emphasis is placed on the influence of critical processing parameters on the metallurgical features, tensile strength, hardness variation, percentage elongation and residual stress. Process modifications to improve dissimilar laser weldability by virtue of techniques such as laser offsetting, split beam, welding-brazing, hybrid welding and materials modifications by means of the introduction of single or multiple interlayers, fillers and pre-cut grooves are exploited. Detailed and comprehensive investigations on the phenomena governing the formation and distribution of the intermetallic phase, material flow mechanisms, their relations with laser parameters and their corresponding impact on the microstructural, geometrical and mechanical aspects of the welds are thoroughly examined. The critical issues related to the evolution of defects and the corresponding remedial measures applied are explored and the characteristics of fracture features reported in the literature are summarised in thematic tables. The purpose of this review is tantamount to emphasise the benefits and the growing trend of laser welding of Ti alloys in the academic sector to better exploit the process in the industry so that the applications are explored to a greater extent.

A Novel Method for Predicting Tensile Strength of Friction Stir Welded AA6061 Aluminium Alloy Joints Based on Hybrid Random Vector Functional Link and Henry Gas Solubility Optimization

Abstract: Aluminum alloys have low weldability by conventional fusion welding processes. Friction stir welding (FSW) is a promising alternative to traditional fusion welding techniques for producing high quality aluminum joints. The quality of the welded joints is highly dependent on the process parameters used during welding. In this research, a new approach was developed to predict the process parameters and mechanical properties of AA6061-T6 aluminium alloy joints in terms of ultimate tensile strength (UTS). A new hybrid artificial neural network (ANN) approach has been proposed in which Henry Gas Solubility Optimization (HGSO) algorithm has been incorporated to improve the performance of Random Vector Functional Link (RVFL) network. The HGSO-RVFL model was constructed with four parameters; rotational speed, welding speed, tilt angle, and pin profile. The validity of the model was tested, and it was demonstrated that the HGSO-RVFL model is a powerful technique for predicting the UTS of friction stir welded (FSWD) joints. In addition, the effects of process parameters on UTS of welded joints were discussed, where a significant agreement was observed between experimental results and predicted results which indicates the high performance of the model developed to predict the appropriate welding parameters that achieve optimal UTS.

Laser dissimilar weldability of cast and rolled CoCrFeMnNi high-entropy alloys for cryogenic applications

Abstract: This study investigates the effects of welding velocity on weldability of dissimilar cast/rolled high-entropy alloys and the applicability of cryogenic temperatures. Cast HEA side of dissimilar wel...

Experimental and numerical simulation study of Zr-based BMG/Al composites manufactured by underwater explosive welding

Abstract: Using the underwater explosive welding technique, the Zr60Ti17Cu12Ni11 bulk metallic glass (Zr-based BMG) and Aluminum 1060 plates were successfully welded. The interfacial microstructure characteristics was characterized using optical microscopy (OM), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Meanwhile, the Johnson-Holmquist-Ceramics (JH-2) constitutive model was selected as the constitutive model of BMG to verify weldability of explosive welding experimental and reliability of interface structure formation using the Smoothed Particle Hydrodynamics (SPH) method. The experimental results indicated that the Zr-based BMG and Al plates were successfully welded with a slightly wave structure and without visible defects. The numerical simulation results showed that the validity of JH-2 constitutive model for BMG material selection is verified, meanwhile, the weldability of Zr-based BMG and Al was verified, which are in good agreement with experimental results.

The influence of the process parameters on the densification and microstructure development of laser powder bed fused Inconel 939

Abstract: This work aims to investigate the effect of the process parameters on the densification and microstructure of Inconel 939 (IN939) alloy processed by laser powder bed fusion (LPBF) IN939 is a Ni-based superalloy with high creep and corrosion resistance that can be used up to around 850 °C under load, resulting in higher operative temperatures than the ones commonly allowed for Inconel 718 and Inconel 625 alloys (around 650 °C) However, this alloy can suffer from poor weldability involving possible crack formation In order to minimize the residual porosity and the cracking density, specific process parameters were investigated The parameters to generate IN939 samples almost pores-free (porosity ≤022%) with a cracking density ≤136 mm/mm2 as well as samples almost crack-free (≤010 mm/mm2) with limited residual porosity (≤089%) were determined The microstructure revealed fine dendritic/cellular structures with the formation of sub-micrometric phases A high concentration of these phases was also found along the intergranular cracks, suggesting that their presence, coupled to the high thermal stresses, can be the primary reason for crack formation during the LPBF process

Tensile and Microstructural Characteristics of Fe-24Mn Steel Welds for Cryogenic Applications

Abstract: Herein, we investigated the weldability and relationship between microstructure and tensile properties in 24 wt% Mn steel welds and, specifically, submerged arc welds (SAWs) were produced using these welds for cryogenic applications. The base metal (BM) and weld metal (WM) exhibited a stacking fault energy (SFE) that maintained a stable austenite phase for 27.1 and 17.0 mJ/m2, respectively. Deformation twins were observed after tensile testing of the BM and WM at 298 K. Weld metals using undermatched fillers showed a lower SFE and coarser grain size (~ 86 μm) compared to that of the BM (~ 12 μm). Therefore, the tensile testing at 110 K produced deformation twins and e-martensite. The formation of e-martensite with deformation twins antedated necking during tensile testing and elongation decreased at 110 K. However, the SAWs of high Mn steels maintained excellent low-temperature mechanical properties such as elongation, tensile strength, and yield strength with values of 20%, 1150 MPa, and 617 MPa, respectively.

Effect of Initial Grain Size on Friction Stir Weldability for Rolled and Cast CoCrFeMnNi High-Entropy Alloys

Abstract: This study investigated the influence of the initial grain size on the plastic deformation and tunnel defects that occurred from friction stir welding of CoCrFeMnNi high entropy alloys (HEAs). The rolled and cast HEAs had a grain size of 2.8 and 308 μm, respectively. After friction stir welding, the cast HEA weld had a grain size of 1.8 μm, which was coarser than that of the rolled HEA weld (1.4 μm). Therefore, the dynamic recrystallization ratios were 60.7 and 99.6% for the rolled and cast HEAs, respectively. The cast HEA weld with a large grain size contained a higher density of high-angle boundaries and twins than the rolled HEA weld with the small grain size. The cast HEA had a larger resistance to plastic deformation owing to the larger fraction of ∑3 twin boundaries than the rolled HEA during friction stirring. This was associated with the high strain hardening rate during tensile testing and to the significant amount of W dissolved from the stirring tool in the cast HEA weld. Thus, the cast HEA weld had a higher tunnel defects ratio than the rolled HEA weld. The total unbonded ratios of the rolled and cast HEA welds were 0.2 and 7.2%, respectively, indicating that the rolled HEA had better weldability than the cast HEA.

Role of liquid backfilling in reducing solidification cracking in aluminium welds

Abstract: The solidification cracking susceptibility of aluminium alloys 2024, 2219, 6061 and 7075 was evaluated recently by the transverse motion weldability test (the lower sheet moved in the transverse di...

Laser weldability of TWIP980 with DP980/B1500HS/QP980 steels: Microstructure and mechanical properties

Abstract: Twinning induced plasticity steel (TWIP980) was fiber laser welded with dual phase steel (DP980), hot stamping boron steel (B1500HS), and quenching & partitioning steel (QP980). The microstructures, segregation behaviors and mechanical properties of similar or dissimilar joints were characterized. All the joints exhibited poor weldability because of the segregation of Mn and C in fusion zone, especially in dissimilar joints of TWIP980/DP980, TWIP980/B1500HS, and TWIP980/QP980. The microstructure in fusion zone of TWIP980/TWIP980 and TWIP980/B1500HS joints consisted of austenite, while fully martensite was found in TWIP980/DP980 and TWIP980/QP980 fusion zone. The solidification cracks in fusion zone of TWIP980/DP980 and TWIP980/QP980 joints embrittled corresponding joints, which contributed to the lower tensile strength and elongation.

Underwater In Situ Local Heat Treatment by Additional Stitches for Improving the Weldability of Steel

Abstract: In this paper the influence of in situ local heat treatment performed by additional stitches on the weldability of high-strength low-alloy (HSLA) S355J2C+N steel was tested. The investigated steel is characterized by high susceptibility to cold cracking. It is necessary to find a method to improve the quality of welded joints. The local heat treatment was applied as an effect of bead-on plate welding made on the face of a Tekken test joint. The specimens were made by the use of covered electrodes in the water environment. For testing weldability, Tekken test specimens were made. Then, the different number of the pad welds with different overlapping were laid on the face of the tested welds. Non-destructive (NDT) visual and penetrant tests were undertaken. During the NDT, imperfections like shape mistakes and spatters were found. Then, metallographic macro- and microscopic testing were performed. The macroscopic observations proved that water environment can generate imperfections like cracking and pores. However, for specimens with additional stitches the number of imperfections decreased. Microscopic tests proved that the proposed technique affected the structure of the heat-affected zone (HAZ). The specimens without the application of additional stitches are characterized by brittle bainitic and martensitic structure. Specimens, in which the additional stitches were applied, contain tempered martensite, fine ferrite and fine pearlite in their HAZ. It was also observed that the number of cracks decreased for in situ local heat-treatment specimens. The final step was Vickers HV10 hardness measurement. These measurements confirmed previous results. The heat from additional stitches affected the steel by significantly decreasing the hardness by 80–100 HV10. The results of experiments showed that the heat from pad welds provided microstructural changes in heat-affected zones and a decrease in the susceptibility to cold cracking, which results in improvement in the weldability of HSLA steel in wet welding conditions.

Weldability of S700MC steel welded with the hybrid plasma + MAG method

Abstract: The paper describes the microstructure of welded joints produced by the plasma+MAG (Metal Active Gas) method of S700MC high yield strength steel (700 MPa). Welded joints of thermomechanical steel have been made with different values of heat input. The results of metallographic research of welded joints, microstructure of the weld and heat affected zone, hardness distribution and impact toughness are presented. The heat affected zone consists of two sub-zones with different grain size and lowered hardness. The tensile test show that strength of welded joints was slightly reduced and the bending test revealed no crack formation in the weld. The impact toughness of measured welded samples with V-notch in HAZ (heat affected zone) reached high values that are higher comparing to samples with notch placed in the weld area. The investigation results show that the use of plasma concentrated heat source together with MAG welding arc does not significantly change the structure and deteriorate properties of welded S700MC thermomechanically treated high strength steel. The hybrid plasma+MAG welding method has a potential to become a beneficial alternative to other welding processes due to its high efficiency, reduced amount of weld metal content or limited requirements for a preparation of edges of welded joints.

Aluminum-to-steel cladding by explosive welding

Abstract: The production of aluminum-carbon steel and aluminum-stainless steel clads is challenging, and explosive welding is one of the most suitable processes to achieve them. The present work aims to investigate the coupled effect of two strategies for optimizing the production of these clads by explosive welding: the use of a low-density interlayer and the use of a low-density and low-detonation velocity explosive mixture. A broad range of techniques was used to characterize the microstructural and the mechanical properties of the welds, specifically, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction, microhardness and tensile-shear testing with digital image correlation analysis. Although aluminum-carbon steel and aluminum-stainless steel have different weldabilities, clads with sound microstructure and good mechanical behavior were achieved for both combinations. These results were associated with the low values of collision point and impact velocities provided by the tested explosive mixture, which made the weldability difference between these combinations less significant. The successful testing of this explosive mixture indicates that it is suitable to be used for welding very thin flyers and/or dissimilar materials that easily form intermetallic phases.

Evaluating susceptibility of Ni-base alloys to solidification cracking by transverse-motion weldability test

Abstract: The transverse-motion weldability (TMW) test, in which the lower sheet in lap welding is moved at a constant speed V in the transverse direction of welding to induce solidification cracking, was ap...

Resistance Spot Welding of Dissimilar Interstitial-Free and High-Strength Low-Alloy Steels

Abstract: The primary aim of this investigation is to examine the resistance spot weldability of dissimilar interstitial-free (IF) and high-strength low-alloy (HSLA) steels. The effect of dynamic contact resistance on the nugget diameter is examined. The mechanical properties of the spot-welded specimens are investigated in both tensile shear and coach peel configurations. These experiments are supplemented by macro- and microstructural examinations, determination of microhardness profiles, and post-failure examinations, including fractography. It is observed that the nugget diameter increases as mean dynamic contact resistance decreases. The heat-affected zone of the HSLA side of the weld joint shows the maximum hardness, whereas the base metal of the IF side shows the minimum hardness. The hardness at the fusion zone of the dissimilar HSLA–IF joint lies in between that of fusion zone of similar HSLA–HSLA and IF–IF joints because of the homogenization of the chemistry of both the grades. The load carrying ability of the IF–HSLA joint is found to be closer to that of IF–IF joint but significantly lower than that of HSLA–HSLA joint; this is because the location of failure for IF–HSLA and IF–IF joint is same, i.e., the HAZ/base metal interface at IF side, while that of HSLA–HSLA joint is the base metal of HSLA steel.

Superior-tensile property of CoCrFeMnNi alloys achieved using friction-stir welding for cryogenic applications

Abstract: The friction-stir weld (FSW) was investigated based on the relationship between microstructural and mechanical properties at room and cryogenic temperatures for rolled and cast Co0.2Cr0.2Fe0.2Mn0.2Ni0.2 high entropy alloys (HEAs). The rolled and cast HEA welds exhibited good weldability without welding defects. The grain size (GS) of the stir zone (SZ) for rolled and cast HEAs was smaller than that of the base metal (BM). The W, Cr, and C particles caused by the wear of the WC-Co tool formed fine M23C6 carbides, due to the heat generated during the FSW, and accelerated the recrystallization via the particle stimulated nucleation (PSN) effect. Therefore, the area with M23C6 carbide exhibited a finer grain size compared with the area without M23C6 carbide. The rolled HEA was fractured in the SZ owing to the thinning phenomenon, and the cast HEA was fractured in the BM owing to the GS of the SZ, which was much finer than the GS of the BM. However, both HEA welds had larger room temperature strength than BM, and the cryogenic strength was also improved owing to the primary twin, secondary twin, and tangled dislocation. The PSN effect due to the carbides contributed to the increase in strength. Therefore, the FSW of the rolled and cast HEAs produced in this study is suitable for cryogenic applications.

Effect of Al-Si Coating on Weldability of Press-Hardened Steels

Abstract: Resistance spot welding (RSW) of Al-Si-coated PHS was undertaken, and the effect of Al-Si coating on nugget formation and mechanical properties was investigated. Press-hardened steel (PHS) has long been applied to automotive body structure construction to support mass and corresponding greenhouse gas emission reductions. PHS materials are often combined with an Al-Si coating applied as an oxidation barrier though unfortunately, the Al-Si coating poses a challenge to the resistance spot welding (RSW) of PHS containing stack-ups. As a newly developed coating for the hot stamping process, the property of Al-Si coating is different from base metal and traditional coatings, and the influence mechanism of Al-Si coating on the welding process is not clear. It would remain at nugget edge and might cause severe stress concentration. To investigate this problem, RSW of 1.5-mm Al-Si-coated PHS was undertaken, and the results indicate that a large portion of the Al-Si coating is extruded and forms a sharp notch close to the nugget edge during the welding process. During the post-weld cooling stage, a thin layer of residual coating is formed between the nugget and notch root. The mechanical performance of the welded joints is limited by the thin residual Al-Si layer which acts as a preexisting crack and supports the interfacial fracture. The presence of the Al-Si coating at the faying interface also significantly delays nugget formation, though it contributes to a larger nugget size by inhibiting expulsion events at the faying interface.

Fusion Weldabilities of Advanced High Manganese Steels: A Review

Abstract: A large amount of manganese has been added to next-generation advanced high strength steels for automotive applications. The increased manganese content changes the microstructural and mechanical characteristics by varying both the stacking fault energy and the austenite stability, but it is known to deteriorate weldability. The current review provides a general strategy to address several problems in fusion welds of advanced high manganese steels from the viewpoint of microstructural transitions, plasticity mechanism, and mechanical properties based on the welding metallurgy. The importance of research on the weldabilities of advanced Mn steels was highlighted. Among the representative types of advanced manganese steels, this review focuses on high Mn twinning-induced plasticity steel, high Mn austenitic lightweight steel, and medium Mn transformation-induced plasticity steel. Specifically, this review suggests fundamental concepts for designing each steel with improved weld characteristics by presenting many research results on the correlations between the microstructure and mechanical properties of welds and heat-affected zones for these three different steel groups.