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The use of pulsed gas metal arc welding (P-GMAW) is fundamental to applications were versatility and control of heat input are required during deposition. However, when welding using pure argon shielding gas, a drawback is the instability derived from wandering of the cathode spots on the weld pool. This work investigates an alternative to weld steels using pure argon shielding gas with cold wire pulsed gas metal arc welding (CW-P-GMAW). A mechanism for enhanced stability is revealed in CW-P-GMAW, related to the migration of cathode spots to the cold wire which prevents the cathode spots from wandering around the weld pool. The migration of cathode spots is likely related to charging of oxides on the cold wire surface by ions formed in the arc plasma. The enhanced arc stability smooths the shape of bead profile, since wandering of the arc due to cathode motion is suppressed.

In this critical note, the thermal stability behavior of ultra-fine grained (UFG) and nano-structured (NS) metals and alloys produced through severe plastic deformation (SPD) techniques is reviewed. For this case, the common engineering metals with body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) crystal structures such as aluminum, copper, nickel, magnesium, steel, titanium, and their relating alloys were assessed. Microstructural evolution in these severely deformed materials following post-processing annealing treatment was investigated for various times and temperatures below the recrystallization point. The microstructure development reported in the literature was studied in terms of the stable grain structures correlated with different levels of plastic straining. The stacking fault energy (SFE) is noted to be a key issue which has a critical influence in predicting the coalescence or coarsening behavior of ultra-fine and nanoscale grains after SPD treatment by controlling the cross-slip phenomenon for screw dislocations.

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.

In this work, an instrumented indentation technique with a nearly flat tip indenter was used to measure the yield strength of API-X80 line pipe weld. Using this technique, the yield strength can be estimated directly from the indentation load–displacement response. The yield strength of the weld metal was measured in the transverse (hoop) and longitudinal (long) directions of the weld, and the results indicated that the weld metal exhibits anisotropy in terms of the yield strength by as much as 100 MPa, which could lead to a non-conservative estimate when strength mismatch is considered. Conventional tensile testing obtained strength values to within 4.6% of the indentation measurements and confirmed the results. Hardness maps also show a 20 HV0.5 difference in the two directions of the weld, which supports anisotropy of yield strength. The microstructure analysis using SEM reveals a slight difference in the grain size, which partially explains the differences in properties.

The evolution of processing parameters by varying tool eccentricity in AA6061 alloy friction stir welding was examined using a combination of detailed force/torque measurements and thermal history as well as high-speed camera (HSC) observations. Tri-axial forces show larger oscillations during the steady-state phase of welding if the tool eccentricity is increased. However, the tool torque remains similar for up to 0.4 mm eccentricity versus the aligned tool even with varying weld speed. In situ HSC observation indicates that tool eccentricity is reduced during the welding process for larger eccentric setups. Stir zone thermal measurements reveal that the temperature peaks and stabilizes near the solidus temperature of the AA6061 base material.

A procedure combining computational fluid dynamics modeling/Monte Carlo simulation was implemented to predict grain refinement during friction stir processing (FSP) of an Al–Mg alloy. Based on the critical parameters during FSP treatment such as rotational tool speed (w), and traverse velocity (v), the thermal and strain rate contours were simulated, and used as inputs for a statistical model of dynamic recrystallization. Afterward, the simulated grain structures were verified experimentally by electron backscattering diffraction analysis. FSP generated equiaxed grains with average sizes in the range of 3–10 µm depending on the heat input index in terms of w/v ratios in the range of 4–28 rev.min/mm. A correlation between simulated and experimentally validated grain structures is observed, with crystallographic textures consistent with shear strain induced preferred orientations with a dominant {112} component.

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 thermal efficiency of arc welding influences the cooling rate, peak temperature, and microstructures of a weld, which affects the material properties of the welded joint. This work quantifies the uncertainty of thermal efficiency measurements on gas metal arc welds using low (2.5 kW) and high (9.5 kW) arc powers. In order to understand the effect of calorimeter parameters on the measured efficiency, a two-factor, two-level full factorial design was carried out for each arc power condition. In this study, the factors are flow rate (2 l/min and 5 l/min) and plate thickness (0.25 in. [6.35 mm] and 0.375 in. [9.53 mm]). The results show that the uncertainty in the thermal efficiency measurements increases when both higher flow rates and plate thickness are used. Moreover, the use of a thick plate causes the heat transfer regime to change, increasing the cooling rate and decreasing the observed thermal efficiency. Uncertainty is also influenced by the metal transfer mode during welding.

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.

The demand to join newer and higher strength materials has motivated the development of controlled arc current waveforms in order to control deposition and heat input. Controlled waveforms in pulsed gas metal arc welding (P-GMAW) where the current is pulsed result in reduced nominal heat input, distortion, and residual stresses. A method to further improve these is by cold wire pulsed gas metal arc welding (CW-P-GMAW) to enhance P-GMAW with a cold wire (non-energized) fed into the weld pool. In this work, the feasibility of the process is reported and the influence of cold wire feeding on pulse parameters for low and high background to peak current ratios (Ib/Ip) were investigated through high-speed cinematography with synchronized current and voltage sampling; as well as evaluation of cross-sections via metallography. Moreover, it appears that the cold wire diminishes the heat transferred across the heat-affected zone (HAZ) for low current ratios.

Statistical models were developed to study the effect of gas metal arc welding process parameters (i.e., wire feed speed, voltage, travel speed, and shielding gas chemistry) on the resultant weld bead width, penetration, and reinforcement height, using a factorial design of experiment. Analysis of variance (ANOVA) indicated that the weld width depended on voltage, travel speed, gas type, and the interactions between these factors. The weld penetration depended only on wire feed speed and gas type, as well as the two-way interactions of wire feed speed with travel speed and gas type. Reinforcement height depended on travel speed, wire feed speed, and their two-way interactions with gas type. Residual analysis revealed that all assumptions inherent in the regression analysis were satisfied over the range of welding parameters considered in this study. The predictive power of the statistical models was validated using intermediate process parameter values in the experimental design, and it was found that predicted values were mainly in agreement with the measured values for a 95% prediction interval.

Dissimilar friction stir welding of AA5052-AA6061 was conducted to assess the effect of material positioning on the welds produced using tools with predetermined eccentricity. Microstructural observation, thermal cycle measurement, and mechanical property evaluation was subsequently conducted. Evidence shows that AA6061 is better suited to welding while placed on the advancing side (AS), since better transverse tensile strength (up to 215 MPa) and elongation (up to 7.6%) can be achieved. On the other hand, higher peak temperatures were generally observed in the AS regardless of base material positioning, where peak temperatures of more than 350 °C were recorded by the thermocouple located closest to the stir zone. Metallurgical analysis reveals that tool eccentricity enhances the AS material flow in the stir zone but limits dissimilar material mixing.

In this research, the parameters of the cold spray process were initially assessed for deposition of a pure titanium coating layer with the thickness in the range of 800-850 µm on an AA5083 alloy substrate. Thereafter, to enhance the structural integrity of Ti-coating layer and decrease the coating porosity, friction-stir processing was employed as a post-modification technique by using a flat cylindrical tungsten carbide tool. The plunge depth of the friction-stir tool (in the range of 0.3-0.5 mm) was found to significantly affect the densification of the porous titanium coating layer. Optical microscopy, field emission-scanning electron microscopy, electron backscattering diffraction, transmission electron microscopy analysis and indentation Vickers micro-hardness testing were conducted on the thickness cross-sections of cold-sprayed coatings to characterize the microstructural features and mechanical properties before and after friction-stir modification performed using two different plunge depths. Furthermore, residual stress profiles on the surface were determined by using x-ray diffraction analysis technique. Significant grain refinement, from an initial cold-sprayed coating grain size of less than 25 µm to grain sizes

Gas metal arc welding (GMAW) with direct current electrode negative (DCEN) can offer some advantages over direct current electrode positive (DCEP) welding, such as a higher electrode melting rate and lower heat input imposed onto the workpiece. Despite these advantages, DCEN is not commonly used in field applications due to instabilities caused by arc climb and the volatile repelled metal transfer mode across the arc. This work reports a comparison between DCEP and DCEN in natural spray transfer mode using a welding power source operating in constant voltage (CV). It aims to assess the feasibility of DCEN in cold wire gas metal arc welding (CW-GMAW) comparing the results obtained in DCEN with previous results obtained in DCEP for the same welding parameters. During experiments, the current and voltage were acquired with synchronized high-speed video to study the arc dynamics. Bead-on-plate welds were deposited onto AISI 1020 steel flat bars. Three cross sections for each bead were cut and metallographically examined to compare bead geometry. Results suggest that the cold wire feeding can suppress arc climb in DCEN welding, consequently improving the arc stability and bead finish. Moreover, the DCEN welds suggest that the heat transferred to the weld pool is fundamental to accommodate high feed rates of cold wire. Lastly, the cross section results show that penetration, dilution, and HAZ area are lower in DCEN than in DCEP mode.

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.

In this research, the printability of Ti-5553 alloy is assessed using a modulated laser powder bed fusion method. Cylindrical samples were printed with a wide range of volumetric energy density (VED). Density evaluation was practiced by the Archimedes method and X-ray computed tomography (XCT). Surface roughness analysis and hardness mapping were further used to characterize the as-built samples. In addition, the microstructure was studied using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques. It was observed that low and high VED values resulted in an increase in the level of porosity. The highest relative density of 99.92% and surface roughness of

The fabrication friction stir welded lap joints with Al 7075-T6 sheets is examined with a wide range of tool pin geometries and dimensions while comparing microstructures, bonded area geometry, hardness, and fracture load during overlap shear testing. The tool pin geometry and features significantly affected the hook geometry and thus weld strength and failure mode. A hook feature is prone to forming when a larger diameter or helical threaded tool pins are used and is shown to deteriorate overlap shear fracture load by reducing the bonded ligament in the upper sheet. This hook feature can be controlled by removing the helical threads on the tool pin and replacing these with concentric grooves. The overlap shear fracture load is controlled by a combination of the sheet thickness, the width of the bonded area, and extent of the hook feature. These factors are captured in a model which was shown to provide good predictions of the fracture load and can be used to select tool pin sizes for varying overlapping sheet thicknesses.

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.

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.

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.

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.

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.

A novel closed-loop process is demonstrated to control deposition microstructure during laser additive manufacturing (LAM) in real-time. An infrared imaging system is developed to monitor surface temperatures during the process as feedback signals. Cooling rates and melt pool temperatures are recorded in real-time to provide adequate information regarding thermal gradients, and thus control the deposition microstructure affected by cooling rates during LAM. Using correlations between the cooling rate, traveling speed, and the clad microstructure, a novel feedback PID controller is established to control the cooling rate. The controller is designed to maintain the cooling rate around a desired point by tuning the traveling speed. The performance of the controller is examined on several single-track and multi-track closed-loop claddings in order to achieve desired microstructures with specific properties. Results indicate that the closed-loop controller is capable of generating a consistent controlled microstructure during the LAM process in real-time.

Dual-phase steel with ferrite-martensite-bainite microstructure exhibited secondary hardening in the subcritical heat affected zone during fiber laser welding. Rapid isothermal tempering conducted in a Gleeble simulator also indicated occurrence of secondary hardening at 773 K (500 °C), as confirmed by plotting the tempered hardness against the Holloman–Jaffe parameter. Isothermally tempered specimens were characterized by analytic transmission electron microscopy and high-angle annular dark-field imaging. The cementite (Fe3C) and TiC located in the bainite phase of DP steel decomposed upon rapid tempering to form needle-shaped Mo2C (aspect ratio ranging from 10 to 25) and plate-shaped M4C3 carbides giving rise to secondary hardening. Precipitation of these thermodynamically stable and coherent carbides promoted the hardening phenomenon. However, complex carbides were only seen in the tempered bainite and were not detected in the tempered martensite. The martensite phase decomposed into ferrite and spherical Fe3C, and interlath-retained austenite decomposed into ferrite and elongated carbide.

Friction stir processing (FSP) of Al 5059 is studied in which subsequent heat treatment is conducted to investigate its effects on grain size and hardness. It was found that mainly elongated and rhomboidal morphologies of Al6(Mn,Fe) particles are present in the alloy both before and after FSP, where the rhomboidal particles are more effective in pinning grain boundaries during heat treatment. The stir zone reached a temperature of 705 K (432 °C), and ThermoCalc modeling confirmed that the Al6(Mn,Fe) particles will remain stable at this temperature. Negligible grain growth was observed during FSP of the Al 5059 due to low grain boundary mobility resulting from slow diffusion associated with a high Mg content in the alloy. During heat treatment at 448 K (175 °C) grain growth could be correlated with time using a particle-controlled grain growth model. Microhardness values indicate that Hall–Petch behavior occurs in the processed alloy, while dislocation density and particle dispersion play a minor role in strengthening.

A centrifugally cast 20Cr32Ni1Nb stainless steel manifold in service for 16 years at temperatures ranging from 1073 K to 1123 K (800 °C to 850 °C) has been characterized using scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), auger electron spectroscopy (AES), and X-ray diffraction (XRD). Nb(C,N), M23C6, and the silicide G-phases (Ni16Nb6Si7) were all identified in a conventional SEM, while the nitride Z-phase (CrNbN) was observed only in AES. M23C6, Z-phase and G-phase were characterized in XRD. Thermodynamic equilibrium calculations using ThermoCalc Version S, with the TCS Steel and Fe-alloys Database (TCFE6), and Thermotech Ni-based Superalloys Database (TTNI8) were validated by comparing experimental phase fraction results obtained from both EPMA and AES. A computational study looking at variations in the chemical composition of the alloy, and how they affect phase equilibria, was investigated. Increasing the nitrogen concentration is shown to decrease G-phase formation, where it is replaced by other intermetallic phases such as Z-phase and π-phase that do not experience liquation during pre-weld annealing treatments. Suppressing G-phase formation was ultimately determined to be a function of minimizing silicon content, and understabilizing the Nb/(C + 6/7N) ratio.

Low temperature interconnection processes for lead-free packaging and flexible electronics are currently of great interest. Several studies have focused on bonding using silver nanoparticles (Ag NPs) or copper nanoparticles (Cu NPs). However, pressure assistance is generally necessary for bonding with nanomaterial pastes, which limits its industrial applications. Here, a unique method for bonding of copper wires using Ag NP and nanowire binary pastes is examined, in which joining is accomplished from 60 to 200 °C and yet without the application of pressure. Bonding is facilitated by solid state sintering of Ag nanomaterials and metallic bonding between Cu and Ag interfaces. The effects of different additions of Ag nanowires in bonded joints are studied, in which addition of 20 vol% Ag nanowires improves bonding strength after low temperature sintering by 50–80 % compared with Ag nanoparticle paste. A mechanical reinforcement effect due to introduction of Ag nanowires has been confirmed by observation of the fracture path propagation, where necking, breakage and pullout of nanowires occur on loading. This low temperature pressureless bonding technology has the potential for wide use for interconnection in lead-free microcircuits and flexible electronic packaging.

Low-pressure cold spraying was used to deposit aluminum particles (~25 μm diameter) on to low carbon steel, and the particle–particle interactions of the aluminum coating were analyzed. A simplified energy conservation model was developed to estimate the temperature at the interface of the deformed particle during deposition of the powder. The Johnson–Cook model was used to calculate the particle flow stress, which was used to estimate the total energy dissipated via plastic deformation during impact and spreading of the particle. Microstructural analysis was conducted to show that plastic deformation occurred mainly at the interfacial regions of the deformed particles. By coupling microstructural observations of the cold-sprayed particles with the energy conservation model, it was found that the interface between the aluminum particles contained recrystallized ultra-fine and nanocrystalline grain structures that were likely formed at temperatures above 260 °C, but the majority of particles likely achieved interfacial temperatures which were lower than the melting point of aluminum (660 °C). This suggests that local melting is not likely to dominate the inter-particle bonding mechanism, and the resulting interfacial regions contain ultra-fine grain structures, which significantly contribute to the coating hardness.

Accumulative roll-bonding was adapted to fabricate a carbon nanotube (CNT)-reinforced aluminum matrix composite. Its microstructure was investigated by transmission electron microscopy, and it was confirmed that the nanotubes were embedded into the metal matrix while maintaining their multiwalled structure. Measurements revealed that the as-received CNTs had a bimodal diameter size distribution, while only nanotubes with diameters >30 nm and >30 walls were retained during four consecutive rolling operations at 50% reduction.

The present article examines the textural features produced during friction stir spot welding of single-crystal aluminum sheet. The crystal has the {111} plane perpendicular to the normal direction (ND) of the sheet, and the $$ \left\langle { 1 10} \right\rangle $$ direction parallel to the growth direction (GD). Friction stir spot welding was carried out using a rotation speed of 1500 rpm and a dwell time of 2 seconds, and completed spot welds were characterized using a combination of optical microscopy and electron backscatter diffraction (EBSD). The EBSD measurements indicate there are no significant changes in orientation in locations more than 840 μm from the stir-zone extremity. The orientation distribution in the thermomechanically-affected zone (TMAZ) region conformed with the {110}⊥ND orientation within 580 μm of the stir-zone extremity. In the location immediately adjacent to the stir-zone extremity, there was a deviation from the {110}//ND orientation due to a combination of compressive loading perpendicular to the stir-zone boundary and shear loading in the direction of tool rotation. It is suggested a {111}⊥ND texture in the stir zone is associated with material flow imposed by the thread on the rotating pin.

Although it is generally assumed that friction stir seam welds and friction stir spot welds are free of many of the defect formation issues commonly associated with fusion welding, liquid penetration induced (LPI) cracking in the stir zone have been recently found in friction stir spot welds of AZ91. In the present study, cracking during friction stir spot welding of Mg-alloy (AZ91, AM60 and AZ31) sections is examined. Both liquation cracking and liquid penetration induced (LPI) cracking are observed in Mg-alloy friction stir welded joints. Local melting and cracking is also apparent in Al 7075-T6 friction stir spot welds produced with the precise objective of limiting dissolution of melted eutectic films in the high temperature stir zone and when spot welds cool to room temperature. Based on these test results there is no need to assume that the stir zone temperature during friction stir spot welding is less than that required for formation of melted eutectic films or for spontaneous melting of second-phase particles contained in the as-received base material.

The formation of local melted films during friction stir spot welding of as-cast AZ91D and thixomolded AZ91 material is investigated. The average temperatures close to the tip of the rotating pin vary from 438 to 454 °C during the dwell period in friction stir spot welding. These measured temperature values are higher than the melting temperature of α-Mg + Mg17Al12 eutectic (437 °C). It is suggested that the temperature in the stir zone during the dwell period is determined by the relative proportions of α-Mg and (α-Mg + Mg17Al12) eutectic material, which are incorporated during friction stir spot welding. Based on the stir zone temperature measurements and a detailed examination of material located at the root of the pin thread it is suggested that material is moved downwards via the pin thread and into the stir zone during the dwell period in friction stir spot welding. Evidence of local melted film formation is observed in the stir zone of AZ91 spot welds. It is suggested that melted films are retained since their dissolution rate is much slower in the high temperature stir zone than it is when melted films is formed in the stir zone during Al 7075-T6 friction stir spot welding. The spontaneous melting temperature, solute diffusion rate and the thermodynamic driving force for droplet dissolution are much higher during Al 7075-T6 friction stir spot welding.

Local melting and tool slippage during friction stir spot welding of different Al-alloy base materials is examined using a combination of detailed microscopy and temperature measurement. The stir zone peak temperature during welding is limited by either the solidus of the alloy in question or by spontaneous melting of intermetallic particles contained in the as-received base material. When spontaneous melting occurs this facilitates tool slippage at the contact interface. Accurate stir zone temperature and grain size measurements are essential elements when estimating the strain rate using the Zener–Hollomon relation. In Al 2024 and Al 7075 spot welds spontaneous melting of second-phase particles produces a drastic reduction in strain rate values. In Al 5754 and Al 6061 spot welds there is a strong correlation between tool rotational speed and estimated strain values. Local melted films dissolve rapidly in the high temperature stir zone and when the spot weld cools to room temperature following welding. Evidence of local melting is observed in Al 7075 friction stir spot welded joints made using a combination of rapid quenching, high plunge rates, and extremely short dwell time settings.

Liquid penetration induced (LPI) cracking is investigated during friction stir spot weld of AZ91, AZ31 and AM60 magnesium alloys. A combination of stir zone temperature measurement and detailed metallography has revealed differences in the cracking tendencies of different magnesium alloys when the dwell time during spot welding is varied. LPI cracking in AZ91 spot welds involves the following sequence of events: the formation of \(\alpha-\hbox{Mg}\,+\,\hbox{Mg}_{17}\hbox{Al}_{12}\) eutectic films in the thermo-mechanically affected zone (TMAZ) region immediately adjacent to the stir zone extremity, engulfment of melted eutectic films as the stir zone width increases during the dwell period, penetration of α−Mg grain boundaries and crack propagation when torque is applied by the rotating tool. Cracking occurs early in the dwell period during AZ91 spot welding and almost the entire stir zone is removed when the rotating tool is withdrawn. However, crack-free AZ31 and AM60 spot welds are produced when a dwell time of 4 s is used since the stir zone temperatures are much higher than the α-Mg + Mg17Al12 eutectic temperature (437 °C) and melted eutectic films dissolve rapidly following their engulfment by the growing stir zone. In contrast, the temperature during the dwell period in AZ91 spot welding is close to 437 °C and melted eutectic films are not completely dissolved so that spot welds produced using a dwell time of 4 s exhibit LPI cracking.

The stir zone temperature and microstructures are compared in friction stir spot welds produced in Al 5754 and Al 6061 alloys. Electron backscattered diffraction was used to determine the relationship between tool rotation speed during welding and final stir zone grain size. Comparison of the grain sizes in rapidly quenched welds with those in air-cooled joints confirmed that grain growth occurred only in Al 6061 spot welds. There was no evidence of abnormal grain growth in the stir zones of Al 6061 welds; the final grain size could be represented using an Arrhenius equation. The strain rates during welding were determined by incorporating the stir zone temperature and average subgrain sizes in quenched spot welds in the Zener–Hollomon relation. When the tool rotation speed increased from 750 to 3000 RPM, the strain rate values ranged from 180 to 497 s−1 in Al 5754 spot welds and from 55 to 395 s−1 in Al 6061 spot welds. It is suggested that a no-slip boundary condition may be appropriate during numerical modeling of Al 5754 and 6061 friction stir spot welding. This is not the case during Al 7075, Al 2024, and Mg-alloy AZ91 spot welding because spontaneous melting facilitates slippage at the tool contact interface.

The stir zone microstructure, crystallographic texture, temperature and strain rate in the stir zones produced during Al 2024 spot welding using different tool rotational speed settings are investigated. The calculated strain rate during spot welding decreases from 1600 to 0.6 s−1 when the tool rotational speed increases from 750 to 3000 rpm. The low strain rate values are associated with tool slippage resulting from spontaneous melting of S phase particles at temperatures ≥490 °C. However, the calculated strain rate is 1600 s−1 in Al 2024 spot welds made using tool rotational speed of 750 rpm since the temperature never reaches 490 °C. Material transfers downwards via that pin thread during the dwell period in Al 2024 spot welding. It is proposed that this downward transfer of material provides a continuous supply of undissolved S phase particles, which melt spontaneously when the welding parameter settings produce stir zone temperatures ≥490 °C. A weak crystallographic texture where the {100} planes are oriented at about 45° to the θ-direction exists in the stir zones of spot welds made using different tool rotational speeds (from 750 to 3000 rpm). Another crystallographic texture where the {100} planes are parallel to the Z-direction (to the tool axis) is stronger in spot welds made using higher tool rotational speed settings. Also, material located at the root of the pin thread has a quite different crystallographic texture from that in the bulk of the stir zone.

The factors determining the temperature, heating rate, microstructure, and strain rate in Al 7075-T6 friction stir spot welds are investigated. Stir zone microstructure was examined using a combination of transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) microscopy, while the strain rate during spot welding was calculated by incorporating measured temperatures and the average subgrain dimensions in the Zener-Hollomon relation. The highest temperature during friction stir spot welding (527 °C) was observed in spot welds made using a tool rotational speed of 3000 rpm. The stir zone regions comprised fine-grained, equiaxed, fully recrystallized microstructures. The calculated strain rate in Al 7075-T6 spot welds decreased from 650 to about 20 s−1 when the tool rotational speed increased from 1000 to 3000 rpm. It is suggested that the decrease in strain rate results when tool slippage occurs when the welding parameter settings facilitate transient local melting during the spot welding operation. Transient local melting and tool slippage are produced when the welding parameters produce sufficiently high heating rates and temperatures during spot welding. However, transient local melting and tool slippage is not produced in Al 7075-T6 spot welds made using a rotational speed of 1000 rpm since the peak temperature is always less than 475 °C.

The mechanism of tool penetration during friction stir spot welding of Al-alloy and Mg-alloy sheet materials is investigated and is explained as a progression of wear events, from mild wear to severe wear and then to melt wear in material beneath the base of the rotating pin. Melt wear can also occur under the rotating tool shoulder provided that sufficient penetration of the upper sheet occurs during the spot welding operation.

Metallic bonding at an interface is determined by the application of heat and/or pressure. The means by which these are applied are the most critical for joining nanoscale structures. The present study considers the feasibility of room-temperature pressureless joining of copper wires using water-based silver nanowire paste. A novel mechanism of self-generated local heating within the silver nanowire paste and copper substrate system promotes the joining of silver-to-silver and silver-to-copper without any external energy input. The localized heat energy was delivered in-situ to the interfaces to promote atomic diffusion and metallic bond formation with the bulk component temperature stays near room-temperature. This local heating effect has been detected experimentally and confirmed by calculation. The joints formed at room-temperature without pressure achieve a tensile strength of 5.7 MPa and exhibit ultra-low resistivity in the range of 101.3 nOhm·m. The good conductivity of the joint is attributed to the removal of organic compounds in the paste and metallic bonding of silver-to-copper and silver-to-silver. The water-based silver nanowire paste filler material is successfully applied to various flexible substrates for room temperature bonding. The use of chemically generated local heating may become a potential method for energy in-situ delivery at micro/nanoscale.