Search

The current investigation of refill friction stir spot welding (refill FSSW) Al alloy to copper primarily involved plunging the tool into bottom copper sheet to achieve both metallurgical and mechanical interfacial bonding. Compared to conventional FSSW and pinless FSSW, weld strength can be significantly improved by using this method. Nevertheless, tool wear is a critical issue during refill FSSW. In this study, defect-free Al/copper dissimilar welds were successfully fabricated using refill FSSW by only plunging the tool into top Al alloy sheet. Overall, two types of continuous and ultra-thin intermetallic compounds (IMCs) layers were identified at the whole Al/copper interface. Also, strong evidence of melting and resolidification was observed in the localized region. The peak temperature obtained at the center of Al/copper interface was 591 °C, and the heating rate reached up to 916 °C/s during the sleeve penetration phase. A softened weld region was produced via refill FSSW process, the hardness profile exhibited a W-shaped appearance along middle thickness of top Al alloy. The weld lap shear load was insensitive to the welding condition, whose scatter was rather small. The fracture path exclusively propagated along the IMCs layer of Cu9Al4 under the external lap shear loadings, both CuAl2 and Cu9Al4 were detected on the fractured surface on the copper side. This research indicated that acceptable weld strength can be achieved via pure metallurgical joining mechanism, which has significant potential for the industrial applications.

The electrical current required for a transition from globular to spray droplet transfer during gas metal arc welding (GMAW) is determined by the specified wire feed speed in the case of constant-voltage power supplies. Generally, in narrow groove welding, spray transfer is avoided, be-cause this transfer mode can severely erode the groove sidewalls. This work compared the globular-to-spray transition mechanism in cold wire gas metal arc welding (CW-GMAW) vs. standard GMAW. Synchronized high-speed imaging with current and voltage samplings were used to characterize the arc dynamics for different cold wire mass feed rates. Subsequently, the droplet frequency and diameter were estimated, and the parameters for a globular-to-spray transition were assessed. The results suggest that the transition to spray occurs in CW-GMAW at a lower current than in the standard GMAW process. The reason for this difference appears to be linked to an enhanced magnetic pinch force, which is mainly responsible for metal transfer in higher welding current conditions.

Cold wire gas metal arc welding (CW-GMAW) has been increasingly used in heavy-gauge manufacturing where high deposition rates are required. In such applications, the thermal efficiency of the CW-GMAW is crucial, yet it is not reported in the literature. Water calorimetry experiments were conducted to assess the thermal efficiency of CW-GMAW for two cold wire feed fractions and three common transfer modes: short circuit, globular, and spray, and these are compared to standard GMAW using the same transfer modes. The welds were produced using ER70S-6 as the electrode and cold wires. AISI 1020 plain carbon steel plates were used as the base metal with thicknesses of 9.53 mm and 6.35 mm. After producing the welds, three cross-sections were cut and analyzed using Vickers hardness maps, where differences were attributed to cooling variation rate across the weld cross-sections in high arc power samples. Results have shown that feeding a cold wire into the arc can re-introduce part of the lost heat back into the weld pool both in the short circuit and spray transfer regimes, suggesting an increase in the heat content in the weld pool.

The hot-wire gas metal arc welding (HW-GMAW) process is widely used to increase the melting rate of a secondary wire through Joule heating without significantly increasing the total heat input to the substrate. Because there is limit-ed knowledge regarding the associated arc dynamics and its influence on bead geometry, the present study considers how these are affected by the hot-wire polarity (negative or positive), hot-wire feed rate, and hot-wire orientation using a two-factor full factorial experiment with three replicates. During welding, high-speed imaging synchronized with current and voltage acquisition to study the arc dynamics. After this, each replicated weld was cut into three cross sections, which were examined by standard metallography. The preliminary results suggest that the arc was stable within the range of process parameters studied. The arc polarity played a role on arc position relative to the hot wire, with a decrease in penetration depth observed when the arc was attracted to the hot wire.

In this study, the progression of martensite tempering as a function of tempering parameter has been investigated using instrumented nanoindentation. Three distinct stages of tempering related to the carbon segregation, carbide nucleation, and Ostwald ripening were identified. During the second tempering stage, the nanohardness achieves a plateau which is attributed to the change of carbide morphology, dissolution of intra-lath carbides, and growth of inter-lath and block boundary carbides.

The increasing demands in productivity and reliability in welding have led to the development of newer welding processes. These processes frequently have superior deposition rates; however, they might lead to a wider heat-affected zone (HAZ) which might impair their mechanical properties. Here, a promising process termed the double cold wire gas metal arc welding (DCW-GMAW) is evaluated, offering higher deposition rates via feeding two cold wires into the weld pool to improve productivity. In contrast to similar processes, the HAZ of DCW-GMAW welds are smaller than gas metal arc welding (GMAW) welds. This work reports a preliminary assessment of the feasibility of the process in terms of electrical arc characteristics and stability. High-speed video acquisition was used to analyze the metal transfer across the arc. Finally, standard metallographic techniques and Vickers hardness were used to assess the weld cross sections and microstructures. The results demonstrate the feasibility of DCW-GMAW based on the process stability at different cold wire feed rates while also offering high deposition (high productivity) of weld deposits with desirable hardness due to increased acicular ferrite content.

Low-angle synchrotron transmission diffraction has been used to create high-resolution 2D residual strain maps of friction stir welds made with three processing conditions. These spatial maps of residual strain reveal local concentrations not detectable by line scans, and confirm that the asymmetric material flow known to produce asymmetric temperature and texture distributions also results in asymmetric residual strain distributions. The experimental set-up permitted simultaneous measurement of both texture and strain, which provides strong evidence against the correlation of these features in magnesium friction stir welds. Mapping diffraction peak width across the weld provides insight into the spatial distribution of dislocations and microstrains, and indicates locations of interest for higher resolution research such as TEM. A diffraction method is presented to determine the solute content of a ternary system using the lower symmetry of a non-cubic system, which can be extended to detecting the onset of precipitation among other applications. Comparison of three friction stir-welding conditions shows how the residual strains at the interface can reverse from compressive to tensile with decrease in the heat input, explaining a significant disparity in the literature results. Lower residual stress values were found to be well-correlated with improved transverse tensile behavior.

Friction stir spot welding (FSSW) is a variation of linear Friction Stir Welding (FSW), which was invented to compete with resistance spot welding (RSW) and riveting of lightweight alloys in the au...

An Al-4.6%Mg-1.5%Mn-0.27%Fe alloy was specially processed with friction stir processing followed by cold rolling. Half of the sheet thickness contains a large number of blocky or irregular-...

The well-known drawbacks of furnace-based technologies used for hot-stamping ultrahigh strength steel (UHSS) parts motivate development of alternative approaches for austenitizing blanks, among them direct contact heating. While the objective of most hot-stamping operations is to produce fully martensitic parts, there is growing interest in developing tailoring techniques for making parts having inhomogeneous mechanical properties for improved crash performance. This work shows how direct contact heating can produce parts having highly controlled microstructures by virtue of the different thermal effusivities of steel and ceramic regions on the heating die, even though the contact surface remains nearly isothermal. The process is designed with the aid of a finite element model that incorporates an austenitization submodel for 22MnB5, a common ultrahigh strength steel. Microhardness measurements and metallographic analysis on heated and quenched blanks are consistent with modeled austenite phase fractions, and highlight the potential of this technique for producing strong and lightweight automotive parts.

Solid-state butt-joining of powder metallurgy (PM) fabricated Al-Al2O3 nanocomposites was assessed using friction-stir welding (FSW), in which the PM materials were prepared from aluminum powder with different particle size distributions of

Defect-free joints were produced in 2.0 mm thick 7075-T6 Al alloy by refill friction stir spot welding using a modified tool. Weld performance was evaluated in terms of microstructure, inte...

The unique combination of very large strains, high temperatures and high strain rates inherent to friction stir welding (FSW) and friction stir processing (FSP) and their dependency on the processing parameters provides an opportunity to tailor the microstructure, and hence the performance of welds and surfaces to an extent not possible with fusion processes. While a great deal of attention has previously been focused on the FSW parameters and their effect on weld quality and joint performance, here the focus is on developing a comprehensive understanding of the fundamentals of the microstructural evolution during FSW/P. Through a consideration of the mechanisms underlying the development of grain structures and textures, phases, phase transformations and precipitation, microstructural control across a very wide range of similar and dissimilar material joints is examined. In particular, when considering the joining of dissimilar metals and alloys, special attention is focused on the control and dispersion of deleterious intermetallic compounds. Similarly, we consider how FSP can be used to locally refine the microstructure as well as provide an opportunity to form metal matrix composites (MMCs) for near surface reinforcement. Finally, the current gaps in our knowledge are considered in the context of the future outlook for FSW/P.

Transformation induced plasticity (TRIP) steel provides enormous potential for auto-body construction in the automotive sector, owing to its enhanced mechanical behavior. In this work, Si-alloyed TRIP steel is joined by employing laser beam welding (LBW) and by utilizing two arc welding processes: gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) in order to assess the effect of the net heat input on the microstructure, the uniaxial tensile properties and the forming response. Results indicate that in spite of the Si content in TRIP steel; precipitation and growth of carbides (tempering) are observed in both: the martensite islands and the retained austenite phase, thus leading to the measurable softening at the sub-critical heat affected zone (SC-HAZ) of the arc welded samples. Although the failure location was predominantly found at the sub-critical heat affected zone of the GMAW samples, the maximum stress and elongation was basically controlled by the total extension of the weldment including fusion zone and heat affected zone. While the limiting dome height upon tension-tension (T-T) and tension-compression (T-C) depended primarily on the fusion zone hardness, weld width and geometry of the sample; the fracture location was outside the weld for T-C, whereas the fracture initiated at the weld in T-T samples. LBW specimens showed optimum forming performance.

Refill friction stir spot welding involves a 3-piece tool consisting of a clamping ring, with rotating inner sleeve and pin components made of tool steel to join thin sheets. During the welding process, the sleeve and pin components rotate to generate frictional heat, and weld two overlapping Al-alloy sheets. The present work examines the first application of the process to join Al 2099 alloy, which has a composition containing a rather high alloy content of 1.69 wt% Li. It was noted that after a few joints were produced, the tool abruptly overheated, glowing yellow while idling at a 300 RPM rotation speed between welds, and then remained seized once rotation stopped. Optical microscopy and secondary ion mass spectroscopy revealed that high concentrations of Li accumulated between the tool pin and sleeve components, which provided an opportunity for possible multi-phase reactions, and liquid metal embrittlement of the steel tooling. This led to damage of the tool sliding surfaces and catastrophic wear, followed by seizure of the tool.

In this study, the cold gas dynamic spraying (CGDS) process was employed to modify the surface of AZ31B magnesium alloy with an aluminum-zinc alloy (AA7075). Friction-stir processing (FSP) was subsequently applied as a solid-state localized surface modification technique to improve the structure and integrity of the cold sprayed layer, by enhancing densification, homogeneity, and microstructural features of the as-deposited material. The structure of precipitates and grains were also refined due to the applied severe plastic deformation during FSP, which ultimately affects the indentation hardness resistance of the processed AZ31B-AA7075 bi-metallic structure. Microscopy indicated slight grain coarsening within the AZ31B substrate occurred simultaneously with refinement of the AA7075 coating, along with significant hardness improvements of ~ 80% and 30% respectively for each region in terms of Vickers hardness after FSP.

AA5754 Al alloy and coated steel were successfully joined by refill friction stir spot welding through the eutectic Al-Si coating. The overlap shear strength increased with plunge depth and reached a high strength plateau which was attributed to maximum bonding size and strong Al/AlSi coating joint interface. The AlSi coating in the as-received coated steel consisted of AlSi eutectic phase and two main intermetallic phases (Al7Fe2Si and Al5Fe2(Si)) with an average width of 8.5 μm and around thickness of 1 μm, respectively. The former played a key role in dissimilar welding, the latter effectively restrained the formation of additional IMCs and the growth of exiting IMCs during welding. Microstructural evolution showed the high strength of joints is also associated with fine silicon particles, elimination of defects and control of intermetallics at the joint interface. Fracture of full bonding welds initiated from the partial metallurgical bonds (PMBs) or kissing bonds, then propagated through the AlSi coating and AlSi/Al7Fe2Si interface, final fracture occurred at Al7Fe2Si/Al5Fe2(Si)/steel substrate interfaces.

In the present research, multi-pass friction stir processing (FSP) was employed for the first time to disperse graphene in the form of graphene nano-platelets (GNPs) into an AA5052 aluminum‑magnesium alloy to fabricate a new Al-Mg/3 vol% GNPs nanocomposite. After five cumulative FSP passes, the GNPs were distributed within the metal-matrix. Field emission-scanning (FE-SEM) and transmission electron microscopy (TEM) analyses were used to examine the dispersion of GNPs, and suggested negligible deterioration of the graphene planar structure following FSP. Some clusters of graphene originating from the initial powder remained due to the high surface energy of these GNPs, while grain orientations were evaluated in the nanocomposite using electron back scattering diffraction (EBSD). A fine equiaxed recrystallized grain structure with an average size of 2.1 μm was formed in the stir zone (SZ) after FSP while dispersing GNPs. Indentation revealed the hardness of the nanocomposite increased by 53% compared to the processed Al-Mg alloy. Yield strength also was improved by more than three times while preserving ductility which achieved 20% strain before fracture. Fractographic studies of tensile test specimens revealed a mixed ductile-brittle fracture behavior. Based on the micromechanics theory, three models considering the microstructural parameters (i.e., aspect ratio, mean size, and volume fraction of GNPs, grain size, and clustering during process) were developed to predict the strengthening effects of GNPs in the terms of elastic modulus, yield strength, and indentation hardness. Correlation between these predicted values and experimental data are discussed in detail.

This paper critically assesses the recent trends in aluminum-magnesium dissimilar welding and suggests a key design guideline to successfully improve the weld joint quality through addition of interlayer. First, the paper describes the main issue of incompatibility between these metals and considers the root cause of the problem, i.e., the Al-Mg-based intermetallic compounds (IMCs). It then reviews the recent trends of interlayer addition in various welding processes to mitigate Al-Mg IMCs. Focusing on laser welding, the paper finally proposes a 3-step design guideline in Al-Mg dissimilar welding through addition of an interlayer and presents a case study of using pure Ni foil as a proof of concept. The design guideline has shown to be an effective means to predict and prevent the formation of deleterious intermetallics.

In this research, a fine-grained aluminum matrix hybrid nanocomposite was fabricated by reactive friction-stir processing (FSP) of an Al-Mg alloy containing about 2.5 vol% of embedded multi-walled carbon nano-tubes (MW-CNTs). It was shown that a homogenous distribution of reinforcement is attained throughout the metal matrix by applying parameters involving w = 1400 rpm and v = 50 mm/min during 5 cumulative FSP passes, where a maximum temperature of 510 °C is reached during processing. The survivability of these tangled MW-CNTs is diminished by the thermo-mechanical conditions imposed during FSP. Transmission electron microscopy analysis reveals that in situ solid-state chemical reactions occur at high temperature between the Al-Mg alloy and the nano-tubes during the severe plastic deformation. Consequently, aluminum carbide (Al4C3) and fullerene (C60) nano-phases with an average size of < 50 nm are formed. The structure of precipitates and grains are also changed due to the applied severe plastic deformation, which ultimately improves the indentation and tensile properties of the processed alloy. The formation of a fine-grained structure (with average size of ~ 1.5 μm) and in situ nucleated phases formed from remnant CNT fragments is accompanied by a significant increase in the yield strength (~ 140.7 MPa) and Vickers hardness (~ 95.6 HV). By employing experimental and theoretical models, a relationship between the microstructure and mechanical properties is established. The contributions of different mechanisms to strengthening are explained using prevailing theories based on the role of grain refinement and direct strengthening by Orowan looping, which were found to be more effective compared to the load bearing strength contribution.

The paper critically assesses the research progress towards aluminium–magnesium dissimilar friction stir welding (FSW). First, the theoretical requirements are explored through the understanding of joining mechanism and heat generation in aluminium–magnesium FSW. Next, the observed trends in microstructural characterisation and mechanical properties are analysed. Finally, the effects of welding parameters and how it influences process variables and materials responses are discussed in detail, and several suggestions are made based on these discussions.

This study evaluates the distribution of residual stress in friction stir weld (FSW) joints in AA7075 alloy using a semi-destructive technique (depth-controlled nanoindentation), and compares the results to a non-destructive method (neutron diffraction). The FSW process offers many benefits in AA7075 alloys such as grain refinement, prevention of solidification cracking, and reduced heat-affected zone softening. To examine the effects of welding parameters on the development of residual stress in FSW joints, different tool rotation speeds were applied on 8-mm-thick sheets of Al–Zn–Mg–Cu alloy. Nanoindentation and neutron diffraction measurements reveal variations in the residual stress within the nugget zone, thermos-mechanically affected zone, and heat-affected zone as compared to the base metal. The trend in residual stress values obtained from nanoindentation agrees with that from neutron diffraction measurements. The application of higher travel speed was found to reduce the residual stresses to negligible values, most likely as the result of an auto-stress relieving mechanism caused by higher temperatures and slower cooling rates.

A tempering cycle was developed via heat treatment to study the decomposition of detrimental martensite–austenite (MA). The heat treatment cycle was found to preferentially decompose hard MA with a critical size of ≥1 µm, which decreases the hardness of these microconstituents after tempering at 300°C for 10 min. The dislocation density inside the MA and in the surrounding matrix was also decreased. Tempering of the X80 weldments containing MA reveals a similar decomposition behaviour. During transverse weld tensile testing of welds, a fracture occurred in the heat-affected zone due to void formation at the MA/ferrite interfaces. However, a fracture occurred in base metal with improved strength and ductility after tempering, in comparison to the fracture in the heat-affected zone of as-welded samples.

Multipass GMAW (Gas Metal Arc Welding) welding was used to join X80 linepipe materials using two weld metals of slightly different compositions. Welding wires with diameters of 0.984 and 0.909 mm were used while applying the same heat input in each pass. The slight difference in the wire diameters resulted in different HAZ microstructures. The microstructures in the doubly reheated HAZ of both welds were found to contain bainite-ferrite. However, etching also revealed a difference in martensite-austenite (MA) fraction in these reheated zones. The MA exhibited twice the hardness of ferrite when measured by nanoindentation. Tensile testing from the reheated zone of both welds revealed a difference in yield strength, tensile strength and elongation of the transverse weld specimens. In the reheated zone of weld A, (produced with a 0.984 mm wire) a higher fraction of MA was observed, which resulted in higher strength but lower elongation compared to weld B. The ductility of weld A was found severely impaired (to nearly half of weld B) due to formation of closely spaced voids around the MA, along with debonding of MA from the matrix, which occurs just above the yield stress.

In this study, multi-step friction-stir processing (FSP) was employed to fabricate an ultra-fine grained (UFG) Al-matrix nanocomposite with simultaneously enhanced indentation hardness and tensile properties. For this aim, about 3.5 vol.% of SiC nanoparticles were incorporated within an Al-Mg alloy matrix by applying up to five cumulative overlapping FSP passes. Dispersion of nanoparticles at the stirred zone (SZ) and their interfaces with the aluminum matrix were studied by using scanning and electron backscattered electron microscopy. The results showed that the grain and sub-grain structures of the SZ were refined down to about 1.4 µm and less than 1 µm respectively, as a result of dynamic recrystallization (DRX) during FSP. The distribution of grains and their orientations was significantly affected by the presence of SiC nanoparticles during FSP. SiC nanoparticles provided both direct and indirect influences on the strengthening of Al-matrix based on the Orowan looping and grain refinement mechanisms, respectively. The morphology and distribution of precipitates were both broken down and partially dissolved during FSP as well. The processed UFGed nanocomposite exhibited drastically improved hardness, yield stress (YS) and ultimate tensile strength (UTS) by up to ~140%, 75% and 60%, respectively, as compared to the annealed Al-Mg alloy. Fractographic features revealed a combined ductile-brittle rupture behavior, while the ductile portion was more significant and preserved the elongation of nanocomposite up to about 30%. Finally, the tensile flow behavior of the processed nanocomposite was described using a dislocation-based model which suggests that grain boundary strengthening is the dominant mechanism involved.

This article evaluates the performance of the cold-wire gas metal arc welding (CW-GMAW) process for narrow gap girth welding. The CW-GMAW process is characterized by the introduction of a continuously fed non-energized wire (cold-wire) into the electric arc/weld pool region. The cold-wire is melted by using the surplus heat available at the arc and molten metal at the weld pool. Narrow grooves 5 mm wide were prepared in 16 mm thick ASTM A131 grade A steel plates and filled using the CW-GMAW process. The feasibility of the process for narrow gap welding was assessed by analysing the joint cross section, microstructure and Vickers hardness. The mechanism by which the cold-wire prevents groove sidewall erosion is identified by using high speed imaging. It was found that for the CW-GMAW, the electric arc attaches to the cold-wire instead of the weld pool leading to a more stable arc, preventing the arc from attaching to the groove sidewall. The additional weld metal deposited by the cold wire, allowed complete filling of the groove with only three passes, demonstrating the productivity potential of the CW-GMAW process for narrow groove welding.

This assessment considers recent work on friction stir processing (FSP), which has been demonstrated to be an effective method for grain refinement and synthesis of new alloys and composites. The grain refinement is attributed to high strain rates leading to recrystallisation, while external cooling suppresses grain growth during cooling. The technique is capable of producing nanocrystalline alloys, and also able to disperse nanoparticles into alloys. The mechanical properties of processed materials agree with a combination of existing models for grain refinement, and precipitate reinforcement theory. Further improvements in the technique may help deal with severe tool wear during the FSP of composites, and reduce the complexity of composite fabrication using novel processing methods and tooling.

The characteristics of a pulsed gas metal arc welding (GMAW-P) process that simultaneously achieves streaming transfer and low arc length are described. The process uses a nearly square shaped current profile. It was found that when high pulse frequency pulses are used with high peak and low background current, stream metal transfer is achieved at short arc lengths. Stable streaming transfer was accomplished at a wire feeding speeds of 13.97 m/min with heat input as low as 0.58 kJ/mm. Therefore, a high productivity welding process with potential advantages for narrow groove welding of heat sensitive materials is presented.

Intermetallic compounds (IMCs) formation is a key factor which limits the strength of dissimilar friction stir welded joints. To suppress the formation of brittle phases at the interface, a cold sprayed Ni interlayer was introduced to the copper surface before applying friction stir butt welding of Al alloy to Cu. The results indicated that when no Ni interlayer was added, the IMCs formed at the interface were Al2Cu and Al4Cu9, each of which had a thickness of ~0.6 μm. When a Ni interlayer was used, a single Al3Ni2 layer was generated with a thickness of ~200 nm. With the addition of Ni interlayer, the average tensile strength of the joint was increased from 152 MPa to 190 MPa. The joint ductility was also significantly improved from ~5.5% to ~10.5%. The improvement of ductility is a result of a fracture path change from the Al/Cu interface, to through the Al alloy stir zone instead.

This article deals with the corrosion behavior of friction stir processed aluminum-magnesium alloys undergoing electrochemical measurements, including potentiodynamic polarization and electrochemical impedance spectroscopy. The effects of friction stir processing (FSP) critical parameters such as the rotational tool speed (w in the range of 800–1400 rpm) and traverse velocity (v in the field of 50–200 mm/min) along with varying cooling rates imposed using submerged cooling medium (air, water/dry-ice mixture, and liquid nitrogen) on the microstructural features and mechanical properties of Al-Mg alloys in two tempering states of annealed and wrought were characterized. Then, by assessing the electrochemical behavior of the friction stir-modified stirred regions produced using varying cooling rate and initial temper, a correlation between the microstructural details with the mechanical and chemical properties was established. The experimental results revealed the significant grain refinement down to ~1 μm following FSP treatment by controlling the processing conditions, which had a detrimental impact on the electrochemical behavior due to the activation of grain boundary corrosion phenomenon. The primary tempering state of alloy in the form of stored strain energy played a critical role by controlling the fraction of sub-boundaries and recrystallized grains upon FSP modification. The inferior electrochemical property reported for the FSP treated annealed Al-Mg alloy under processing conditions of w = 800 rpm and v = 200 mm/min, as the less noble structure with the highest corrosion rate (CR) of 0.01814 mm/year.

Many studies of friction stir welding have shown that periodicity of metal flow around the tool pin may result in the formation of periodic differences in microstructure and texture in the weld nugget area correlated with the weld pitch. The current work investigates the periodicity of magnesium weld microtexture in the nugget region and its association with material flow using optical and electron microscopy. Two welds created in AZ80 at different processing conditions are presented in detail, one illustrating periodic longitudinal texture change, and one showing for the first time that periodic variations in texture, grain size, or composition are not defining features of periodic nugget flow. While nugget texture is dominated by shear deformation, it was found here to be affected to a lesser degree by compaction of material behind the welding tool, which led to reduction in intensity of the shear texture fiber. The decreased tendency for magnesium based alloys to form periodic patterns as compared to aluminum based alloys is explained with reference to the shear textures.

Owing to the advantages of nanocomposites for structural applications, we present microstructural evolutions and texture development during dissimilar friction stir welding (DFSW) of an Al-matrix hybrid nanocomposite (Al-2 vol.-% Al2O3-2 vol.-% SiC) with AA1050. It is shown that DFSW can successfully be performed at a rotating speed of 1200 rev min−1 and a transverse speed of 50 mm min−1 while locating the nanocomposite at retreating side. Formation of macro-, micro-, and nano-mechanical interlocks between dissimilar base materials (BMs) as a result of FSW tool stirring action possessed an impact influence on the mechanical performance of dissimilar welds. Electron microscopy revealed formation of a three-modal grain structure from microscale (>1 µm) to nanoscale (

Mg/Mg similar and Mg/steel dissimilar spot welds were produced using refill friction stir spot welding. For comparison, the Mg/Mg and Mg/steel welds were evaluated in terms of microstructures, microchemical analysis, interfacial reactions, tensile-shear and fatigue fracture. Acceptable Mg/steel dissimilar spot weld strength can be obtained by plunging the sleeve to a depth 0.05 mm above the Mg/steel interface. The presence of a 10 μm zinc coating on the DP600 steel surface is beneficial to the bond formation between Mg/steel sheets, which also contributes to brazing outside the weld boundary. Hence, this increases the bonded area for Mg/steel welds compared to Mg/Mg spot welds. Consequently, Mg/steel dissimilar welds exhibit superior overlap shear static and fatigue behavior compared to Mg/Mg similar spot welds. When stress Pmax ⩽ 1450 N, all of the welds fractured at the Mg parent sheet. Most fatigue fractures in Mg/steel welds occurred at the sheet interface, however when 1087 N ⩽ Pmax ⩽ 1812 N, Mg/steel welds occasionally fractured through the Mg parent sheet. When the fatigue fracture propagation occurred through the parent Mg sheet in Mg/steel welds, superior fatigue behavior is observed and attributed to zinc brazing at the surrounding weld interface. The SEM and fracture analysis show that fatigue cracks at the parent Mg sheet initiated from multiple sites located at the Mg side and then propagated into the base material.

The texture evolution of Ti/Al2O3 nanocomposite fabricated using friction stir processing (FSP) was investigated at both macroscopic and microscopic levels employing X-ray diffraction and electron backscattering diffraction techniques. The developed textures were compared with ideal shear textures of hexagonal close-packed (hcp) structure, revealing that the fabricated nanocomposite is dominated by the P 1 hcp (fiber $$ \{ 10\bar{1}1\} \langle 1\bar{2}10\rangle $$ (and relatively weak B (fiber $$ \{ 10\bar{1}1\} \langle \bar{1}\bar{1}23\rangle $$ ) textures. The analyses of macro- and microtextures showed that the presence of nanosized Al2O3 particles activated the pyramidal $$ \{ 10\bar{1}1\} \langle \bar{1}\bar{1}23\rangle $$ slip system in addition to dominant $$ \{ 10\bar{1}0\} \langle 1\bar{2}10\rangle $$ prism, basal $$ \left\{ {0002} \right\}\langle 1\bar{2}10\rangle, $$ and pyramidal $$ \{ 10\bar{1}1\} \langle 1\bar{2}10\rangle $$ slip systems which normally govern plastic deformation during FSP of commercially pure titanium alloy. Moreover, the presence of nanoparticles promoted the occurrence of continuous dynamic recrystallization as well as increasing the fraction of high-angle grain boundaries within the developed microstructure.

Fusion zone microstructures of fiber laser welded Al-Si coated press-hardened steels at pre- and post-press-hardened conditions were analyzed using transmission electron microscopy (TEM). TEM and nanoindentation studies suggest that dislocation-free δ-ferrite phase formed in the weld metal (by ingress of the Al-Si surface coating into the fusion zone) exhibits lower nanohardness compared to α-ferrite transformed after press hardening. The nanoscale properties of δ-ferrite, α-ferrite, and martensite and their crystallographic orientation relationships are reported.

The fatigue strength of weld joints manufactured using gas metal arc welding and cold wire GMAW (CW-GMAW) was evaluated under stress-controlled cyclic loading. The material used in this study was class ASTM 131 grade A steel, joined using ER70S wire filler metal. The addition of cold wire led to a decrease in the amount of intergranular ferrite and an increase in hardness in the heat-affected zone. The assessment of fatigue life was performed by using the Weibull distribution and the results revealed that with a 99% reliability the joints produced using the CW-GMAW process have a longer fatigue life, especially as the stress amplitude increases.

Friction stir welding (FSW) of dissimilar joints has recently attracted great interest for the fabrication of bimetallic and layered composite structures. In this paper, dissimilar joining of an aluminum-matrix hybrid nanocomposite to commercial pure aluminum is reported for the first time. An aluminum hybrid nanocomposite reinforced with Al2O3 (2 vol%; 15 nm) and SiC (2 vol%; 50 nm) nanoparticles was prepared by powder metallurgy routes including mechanical milling and hot powder consolidation techniques. Different joint designs at various ranges of rotational (w) and traverse velocities (v) were evaluated to determine process window for the dissimilar solid-state welding. Macro- and micro-structural studies showed that sound joints, which were free of voids, cracks or un-bonded areas, could be attained by applying w=1200 rpm and v=50 mm/min, while locating the nanocomposite at retreating side. Optical microscopy revealed that the stir zone consisted of micro-scale mechanical inter-locks. Mechanical characterization indicated that the mechanical properties of the dissimilar joint were superior to that of commercial aluminum but lower that of the hybrid nanocomposite by an order of 128–70% (in terms of tensile strength) and 135–70% (hardness). Fractographic analysis showed a ductile fracture mode around the interface of aluminum/stir zone and a combined ductile-brittle fracture mode for the interface region of the nanocomposite/stir zone. The role of nano-scale hard particles on the weldability, microstructural development, mechanical strength, and fracture behavior was elaborated.

An infrared thermal imaging system is developed to compare the effects of cooling rate and melt pool temperature on the microstructure of 316L stainless steel produced by Laser Additive Manufacturing (LAM) with injected powder melting. The system captures the real-time cooling rate to provide online monitoring and control of the microstructure at the onset of solidification. Further study is conducted to evaluate cooling rate effects on grain size, phase transformations and hardness. Cooling rate changes are also studied with respect to the traveling speed, in order to identify a suitable controlling action for a microstructural controller. The study provides a strong experimental justification of the current theories in solidification and microstructural evolution during the LAM with injected powder melting process.

Zirconium is commonly used in nuclear, chemical processing and biomedical applications due to its low thermal neutron cross-section, relatively high corrosion resistance and great biocompatibility. In this study, powder spray laser additive manufacturing technology was used to deposit commercially pure zirconium on zirconium alloy substrates. The clad quality was assessed based on deposition rate, dilution, geometrical circularity and presence of defects. The effect of laser power, laser scan speed, laser spot size and powder feed rate on the clad quality was investigated. Defect-free clads, suitable for laser additive manufacturing were selected based on the analysis of process parameters. Zirconium clad could be deposited at a deposition rate of 0.80 g/min with a dilution rate of 50%.

A WC–12%Co coating was thermally sprayed on a 52100 steel substrate and subsequently friction stir processing (FSP) was performed on this layer. The wear resistance and hardness was compared before and after FSP. Optical and SEM revealed that FSP intermixes the sprayed layer with the substrate, reduces porosity, and enhances both hardness and wear performances. 3D profilometry mapping was conducted to evaluate the wear track depth and its morphology. Refined grain structures and a martensitic structure with retained austenite are promoted by the FSP treatment. This leads to formation of new intermetallic and carbides which were detected by X-ray diffraction, thus accounting for the increased hardness and improved wear resistance. Keywords: Friction stir processing, Sliding wear, Thermal spray coatings, Wear testing, Wear mechanism

Two welding consumables suitable for joining X80 linepipe steel are compared in terms of weld metal microstructures, hardness, impact toughness, and tensile properties. The chemical compositions of the consumables were similar, where one of the consumables had a rich wire chemistry containing higher C, Ni, Ti alloying additions compared to a lean one. Beads on plate welding were performed using the gas metal arc welding (GMAW) process set to achieve the same heat input of 0.66 kJ/mm. The results revealed that for both wires the weld metal microstructure was mainly consist of acicular ferrite. The consumable with richer chemistry (C, Ni and Ti) exhibited higher strength and hardness due to its finer final weld metal microstructure; however, Charpy impact tests results revealed the lean chemistry wire had higher toughness at low temperature. Since both weld metals exhibited similar acicular ferrite structures, the lower toughness of the richer chemistry weld was attributed to the presence of titanium inclusions which may provide crack initiation sites.

A fine-grained Al–Mg/Al 3 Ti nanocomposite was fabricated by friction stir processing (FSP) of an aluminum-magnesium (AA5052) alloy with pre-placed titanium powder in the stirred zone. Microstructural evolutions and formation of intermetallic phases were analyzed by optical and electron microscopic techniques across the thickness section of the processed sheets. The microstructure of the nanocomposite consisted of a fine-grained aluminum matrix (1.5 µm), un-reacted titanium particles ( 3 Ti ( 2 Si ( 3 Ti intermetallic phase. The hard inclusions were then fractured and re-distributed in the metal matrix by the severe thermo-mechanical conditions imposed by FSP. Evaluation of mechanical properties by hardness measurement and uniaxial tensile test determined significant enhancement in the mechanical strength (by 2.5 order of magnetite) with a high ductility (~22%). Based on a dislocation-based model analysis, it was suggested that the strength enhancement was governed by grain refinement and the presence of hard inclusions (4 vol % ) in the metal matrix. Fractographic studies also showed a ductile-brittle fracture mode for the nanocomposite compared with fully ductile rupture of the annealed alloy as well as the FSPed specimen without pre-placing titanium particles.

Plates of AA5052 (Al–Mg) alloy in both annealed (solution-treated) and wrought (rolled) temper conditions were subjected to friction stir processing (FSP) at various w/v pitch ratios from 4 to 28 rev.min/mm. The role of stored strain energy on the evolution of restoration mechanisms and crystallographic texture components were assessed in terms of microstructural features evaluated using electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) analysis. The results revealed that FSP significantly refined the grain structure and changed the crystallographic micro-texture components. The grain size of the annealed and wrought alloy was reduced from 49.4 and 9.7 μm initial values to 3.3 and 3.6 μm, respectively when w/v=4. Also, the formation of a { 112 } ⟨ 110 ⟩ crystallographic shear texture with a strong B / B ¯ component intensity were observed. The microstructural changes in the annealed alloy were related to the occurrence of discontinuous dynamic recrystallization (DDRX) mechanism, while operation of a static recrystallization (SRX) prior to a continuous (CDRX) mode in the wrought one, revealing the effect of stored strain energy. Evaluations of the mechanical properties also determined enhanced ultimate tensile strength, elongation, and indentation Vickers hardness while preserving the yield stress.

An aluminum–magnesium alloy was friction-stir processed in the presence of TiO2 nanoparticles which were pre-placed in a groove on the surface to produce a composite. Field emission-scanning and transmission electron microscopy studies show that solid state chemical reactions occur between the Al–Mg matrix and the ceramic particles upon the severe plastic deformation process. The microstructure of the aluminum alloy consists of a coarse grain structure, large complex (Fe,Mn,Cr)3SiAl12 particles, and small Mg2Si precipitates. After friction stir processing, a deformed grain structure containing rod-like Al–Fe–Mn–Si precipitates is attained, along with cuboidal (~100 nm) Cr2 precipitates and spherical (~100 and 5 nm) Mg2Si particles. In the presence of TiO2 nanoparticles, magnesium oxide (MgO) and titanium aluminide (Al3Ti) nanophases are formed. It is shown that these microstructural modifications lead to a significant enhancement in the hardness and tensile strength of the aluminum alloy. The relationship between the microstructural evolution and mechanical properties and the role of hard inclusions are presented and discussed. An analysis based on strengthening models indicates that the yield strength of the nanocomposite is mainly controlled by dislocations and grain boundaries rather than the nano-scale inclusions.

Lap joint friction stir welding (FSW) between dissimilar AZ31B and Al 6061 alloys sheets was conducted using various welding parameters including tool geometry, rotation and travel speeds. Tapered threaded pin and tapered pin tools were applied to fabricate FSW joints, using different rotation and travel speeds. Metallurgical investigations including X-ray diffraction pattern (XRD), optical microscopy images (OM), scanning electron microscopy equipped with an energy-dispersive X-ray spectroscopy (SEM–EDS) and electron probe microanalysis (EPMA) were used to characterize joints microstructures made with different welding parameters. Intermetallic phases were detected in the weld zone (WZ). Various microstructures were observed in the stir zone which can be attributed to using different travel and rotation speeds. Mechanical evaluation including lap shear fracture load test and microhardness measurements indicated that by simultaneously increasing the tool rotation and travel speeds, the joint tensile strength and ductility reached a maximum value. Microhardness studies and extracted results from stress–strain curves indicated that mechanical properties were affected by FSW process. Furthermore, phase analyses by XRD indicated the presence of intermetallic compounds in the weld zone. Finally, in the Al/Mg dissimilar weld, fractography studies showed that intermetallic compounds formation in the weld zone had an influence on the failure mode.

A preliminary study compares the feasibility and microstructures of pure copper claddings produced on a pressure vessel A516 Gr. 70 steel plate, using friction stir welding versus gas metal arc welding. A combination of optical and scanning electron microscopy is used to characterize the grain structures in both the copper cladding and heat affected zone in the steel near the fusion line. The friction stir welding technique produces copper cladding with a grain size of around 25 μm, and no evidence of liquid copper penetration into the steel. The gas metal arc welding of copper cladding exhibits grain sizes over 1 mm, and with surface microcracks as well as penetration of liquid copper up to 50 μm into the steel substrate. Transmission electron microscopy reveals that metallurgical bonding is produced in both processes. Increased diffusion of Mn and Si into the copper cladding occurs when using gas metal arc welding, although some nano-pores were detected in the FSW joint interface.

Joining of ZEK100 Mg alloy and Zn coated DP600 steel sheets is studied using refill friction stir spot welding (RFSSW). The RFSSW process involves a tool with independently moving sleeve and pin components, which rotate at a constant speed and penetrate into only the top sheet. Welds could be achieved, which exceeded 4.7 kN shear strength, using the following process parameters: 1800 rev min− 1 tool speed, 3.0 s welding time and 1.5 mm of penetration into the upper ZEK100 sheet. Scanning electron (SEM) and transmission electron microscopy (TEM) are used to characterise the Mg/steel interface. It is revealed that a continuous layer of FeAl2 particles accommodate bonding of the sheets, which appears to have originated from the galvanised coating on the DP600.

The present work investigates strengthening mechanisms and mechanical assessment of Ti/Al 2 O 3 nano-composites produced by friction stir processing of commercially pure titanium using nano-sized Al 2 O 3 with different volume fractions and particle sizes. Microstructural analyses were conducted to characterize the grain size of matrix, size and dispersion of reinforcing particles. The mean grain size of the composites ranged from ~0.7 to 1.1 μm that is much lower than 28 μm of the as-received material. Reduction of grain size was found to be in agreement with Rios approach (based on energy dissipated during the motion of an interface through particle dispersion), and showed deviation from Zener pinning model. Scanning and transmission electron microscopies revealed a near uniform dispersion of Al 2 O 3 nano-particles, with only a small fraction of widely spaced clusters. The maximum compression yield strength of the fabricated nano-composite (Ti/3.9%vol of 20 nm-Al 2 O 3 ) was found to be ~494 MPa that is ~1.5 times higher than that of the as-received material. Strengthening analyses based on grain refining (Hall–Petch approach), load transfer from matrix to reinforcements, Orowan looping, and enhanced dislocation density due to thermal mismatch effects were carried out considering Al 2 O 3 reinforcement with different volume fractions and sizes. However, Hall–Petch approach was found to be the dominant mechanism for the enhancement of yield strength.

Friction stir welding is an efficient manufacturing method for joining dissimilar alloys, which can dramatically reduce grain sizes and offer high mechanical joint efficiency. Lap FSW joints between dissimilar AZ31B and Al 6061 alloy sheets were made at various tool rotation and travel speeds. Rotation and travel speeds varied between 560–1400 r/min and 16–40 mm/min respectively, where the ratio between these parameters was such that nearly constant pitch distances were applied during welding. X-ray diffraction pattern (XRD), optical microscopy images (OM), electron probe microanalysis (EPMA) and scanning electron microscopy equipped with an energy-dispersive X-ray spectroscopy (SEM-EDS) were used to investigate the microstructures of the joints welded. Intermetallic phases including Al12Mg17 (γ) and Al3Mg2 (β) were detected in the weld zone (WZ). For different tool rotation speeds, the morphology of the microstructure in the stir zone changed significantly with travel speed. Lap shear tensile test results indicated that by simultaneously increasing the tool rotation and travel speeds to 1400 r/min and 40 mm/min, the joint tensile strength and ductility reached a maximum. Microhardness measurements and tensile stress–strain curves indicated that mechanical properties were affected by FSW parameters and mainly depended on the formation of intermetallic compounds in the weld zone. In addition, a debonding failure mode in the Al/Mg dissimilar weld nugget was investigated by SEM and surface fracture studies indicated that the presence of intermetallic compounds in the weld zone controlled the failure mode. XRD analysis of the fracture surface indicated the presence of brittle intermetallic compounds including Al12Mg17 (γ) and Al3Mg2 (β).