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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.

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.

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.

This study examines dissimilar friction stir welding of AA5052-AA6061 aluminum alloys with varying tool offsets. The base metals were positioned and fixed at a slight diagonal positioning such that varying tool offset position from the centreline can also be varied along the length of the weld. After the fabrication process, microstructural and mechanical property characterization was subsequently conducted. The results show that, above a certain threshold for tool offset, incomplete consolidation (i.e. kissing bond defects) will occur. Regardless of the base material positioning, a zero tool offset shows optimum intermixing in the stir zone. EDX mapping confirms the presence of a distinct interface between both materials in the stir zone region. However, enhanced material intermixing and better elongation are observed when AA6061 alloy is positioned at the tool advancing side.

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.

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-...

Aluminum matrix nanocomposites consolidated by powder metallurgy (P/M) have significant potential for practical applications in the aerospace and automotive industries considering their admirable strength to weight ratio and good corrosion resistance. Due to dimensional restrictions in traditional P/M manufacturing routes, similar and dissimilar joining would be essential to design complex structures, however, the stability of Al-matrix nanocomposites during joining process is a crucial issue. In this research, the stability of an Al-6 vol% (50 nm, ex-situ) SiC-2 vol% (15 nm, in situ) Al2O3 hybrid P/M nanocomposite was evaluated during laser joining in comparison to similar and dissimilar joints with commercially pure aluminum (AA1050 alloy). The processing parameters were optimized with a constant focus spot size of 200 μm, laser power of 4 kW, and travel speed of 8 m/min to achieve full-penetration joints. According to the experimental results, the similar P/M nanocomposite exhibited unstable behavior during laser welding due to the formation of porosity associated with clustering and void expansion near SiC nanoparticles. Hence, the fusion zone in both joint designs consisting of similar and dissimilar containing nanocomposite material became heterogeneous due to the presence of clusters and shrinkage micro-voids. The mechanical properties of weldments deteriorated considerably, with large fluctuations in the hardness profiles as well as low joining efficiency (

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.

The tensile flow properties of austenitic (S316-L) and martensitic (S410-L) stainless steel wall structures deposited by powder-fed laser additive manufacturing (LAM) process are evaluated. The properties obtained by the LAM process are compared to commercial rolled sheets of these metals. Strain-rate sensitivity, work hardening, and fracture behavior are assessed by conducting uniaxial tensile testing at different strain rates (0.001, 0.01, 0.1, and 1.0 sec−1). Moreover, a correlation between the final microstructure and mechanical properties is established for the LAM products through detailed analyses of grain structures and hardness indentation measurements. The results indicate a strong dependency for the strain rate in martensitic alloys compared to austenitic alloys produced by the LAM process. Interestingly, the tensile strength of commercial rolled martensitic stainless steel sheet doubles (∼100% increase) by increasing the strain rate, while preserving its elongation to failure. Comparing the two manufacturing methods, a lower strain-rate sensitivity factor is recorded for the additive manufacturing material (m of ∼0.0336) compared to the commercial sheet (m of ∼0.0775). This lower sensitivity is attributed to coarser grain structure and greater microstructural heterogeneity of the LAM product, which stems from directional solidification and cooling phenomenon during the layer-upon-layer deposition process. In contrast, the work hardening exponent (n value) varies little (0.1834–0.2854) for the different materials and manufacturing methods. Fractographic studies reveal that the fracture mode varies from ductile rupture towards ductile-brittle with the formation of greater martensitic phases, which is in combination with the failure component changing from shear to tensile at high strain rates.

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

Strain-based design (SBD) of pipelines allows stress owing to displacement-controlled loads originating from landslides, seismic motions, or frost heaves, to exceed yield stress. In such cases, the distribution of the strain over a large area (pipe) rather than local area (weld) is preferred. Accordingly, in SBD, yield strength of the circumferential girth weld is over matched when compared to the base metal. In this work, API-X80 pipeline steel sections were joined together using the robotic Gas-Metal Arc Welding (GMAW) process; filler metals and shielding gases were varied to achieve three different levels of strength (even, over and under match) between the weld and base metal. Weld metal and HAZ cross-section microstructures were investigated and correlated with micro-hardness maps to ensure that different mismatch levels were achieved. Yield strengths of the welds in the hoop direction were measured using both instrumented indentation and conventional tensile testing. The instrumented indentation technique used a 100-μm diameter nearly-flat indenter to estimate yield strengths for the welds and heat-affected zones from the load-displacement response for each zone. The measured yield strengths were compared with 0.2% offset tensile yield strength obtained through conventional tensile testing of all-weld and base metal specimens. Strength measurements, in the hoop direction, for weld metal from both of the techniques agreed to within 4.6%. The results showed that the nearly-flat tip indentation method can be used as a tool to directly estimate strength mismatch of pipeline girth welds. Measurements of yield strength for narrow coarse and fine grained heat-affected zones, which cannot be measured through any other method, can also be assessed using the techniques described herein.

In this study, a new manufacturing process, accumulative fold-forging (AFF), based on the severe plastic deformation (SPD) concept was implemented for fabrication of nanostructured metal-matrix nanocomposites. This process was accomplished at room temperature for up to 26 fold-forging steps with an intermediate annealing treatment at a temperature of 150 °C between the passes, and produces 67,108,864 layers to incorporate a ~10 vol% of boron carbide nanoparticles homogenously within the Al-Fe-Mn aluminum alloy matrix. The results indicate a structure is formed with well-bonded interfaces/inter-layers with uniformly dispersed precipitates and nanoparticles between them, leading to formation of an ultra-fine grain structure in the matrix with a mean grain size

In this study, SiC nanoparticles (∼50 nm, 3 vol%) are homogenously incorporated within an Al–Mg alloy metal matrix during multi-step friction-stir processing (FSP) to fabricate an Al-matrix surface...

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.

A novel severe plastic deformation (SPD) process termed accumulative fold forging (AFF) is introduced to fabricate a homogenous ultra-fine grained (UFG) layered metal structure by repetitive folding and forging aluminum alloy foil. The present work studies AFF applied to thin foils of AA8006 Al-Fe-Mn aluminum alloy after 26 folding steps to produce a UFG structure containing 67,108,864 layers across a 2 mm thickness. The structure of the layers and grain refinement are studied using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and scanning-transmission electron microscopy (STEM) analysis. The results indicate a well-bonded inter-layer structure with an average grain size of about 200 nm parallel and 250 nm perpendicular to the forging direction, while dislocation density increased to ~ 7.2 × 1015 m− 2 following AFF. The mechanical strength of the aluminum foil is evaluated in the terms of indentation hardness testing before and after AFF process. The processed UFGed layered material exhibited an average hardness value of ~ 61.5 Vickers as compared to the initial value of ~ 30.4 Vickers for the annealed foil alloy, which indicates an improvement of ~ 100% due to the contributions of grain refinement, work hardening and interfacial strengthening of the bonded layers.

This paper investigates the effect of tool eccentricity on material flow of friction stir welded (FSW) AA6061 aluminum alloy. Samples were butt joined using a threaded conical shaped tool pin with 1200 rpm rotational speed and 63 mm/min travel speed with and without 0.2 mm tool offset. A 0.1 mm thin Al foil was inserted at the faying surface prior to welding process to enhance the observation of faying surface material flow. Results show that tool eccentricity enhances nugget zone's material flow. Evidence show upward and horizontal expansion of the soft region in the nugget zone due to the eccentric setup. In addition, the pin-driven region of the nugget zone experiences increased softening with the soft region expanding upwards owing to the eccentric setup. However, tensile tests have shown minimal effect of the tool eccentricity to the strength and elongation of the weld joint. It can be concluded that the tool eccentric motion due to the 0.2 mm tool offset can enhance material flow in the nugget zone and expand the soft region of the nugget zone, but has shown no effect on the mechanical properties of AA6061 alloy.

In this study, a novel two-step method consisting of powder metallurgy (PM) and subsequent friction stir processing (FSP) was used for preparing SiC/Al-Mg-Sc-Zr nanocomposites integrating multiple beneficial microstructural factors, including high densification, uniform distribution of SiC nanoparticles, good SiC/Al interfacial bonding and recrystallized fine Al grains. These favorable microstructure factors enables the resultant AMCs to overshadow the classical ductility loss in the particle reinforced AMCs, and achieve good strength-ductility synergy. At room temperature, with increasing the SiC nanoparticle content, the strength of the FSPed samples increases, and the corresponding ductility decreases, but still maintains at a high level. The FSPed AMC with 10 wt% SiC nanoparticles has a highest YS and UTS of about 227 MPa and 329 MPa respectively while maintaining a uniform elongation of nearly 20%. At high temperature (523 K), the strength and ductility variation of the FSPed samples with SiC nanoparticle content is similar to that at room temperature. The YS and UTS of the FSPed AMC with 10 wt% SiC nanoparticles reach about 148 MPa and 166 MPa respectively while the fracture elongation approaches 0.3. The fracture of the FSPed samples shows ductile fracture characteristics at both room and elevated temperatures caused by the microvoid coalescence mechanism. The novel two-step preparation method will assist the development of particle reinforced AMCs with balanced strength and ductility.

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.

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.

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.

A 100-µm diameter, nearly flat, instrumented, indenter is used to indent aluminum AA-6463 and X80 pipeline steel. In contrast to sharp and spherical indenters, a rising load-displacement response is followed by a concave-downwards response during indentation. The substrate materials are characterized using tension and compression tests. Yield strengths measured under compression are within +/− 10% of the tensile values thereby providing partial support for assuming symmetric tension-compression response. Based on imaging of the actual indenter using a Scanning Electron Microscope, a model of the indenter that accounts for the curved contact profile was created, assumed to be rigid and used in the finite element simulations. In the simulations, tensile yield strength and flow properties, obtained by tensile testing are used to describe the behavior of the substrate and good agreement with measured indentation force-displacement curves was obtained when the exact shape of the indenter was used. The agreement is poor when the contact profile of the indenter was idealized as flat. In the context of the inverse approach, using the Efficient Global Optimization technique, fits to the stress-strain curves of both of the alloys were obtained, and again the curvature of the indenter contact profile is found to be crucial. This work sets the stage for a large-scale deployment of the inverse approach to map the stress-strain response of heterogeneous microstructures such as welds.

Accumulative fold-forging (AFF) as a newly developed severe plastic deformation (SPD) process based on the repetitive fold-forging steps is implemented for the production of the layered UFG (~200 nm) AA8006 alloy and AA8006-B4C nanocomposite (~35 nm, 10 vol%) materials from the initial AA8006 alloy foil. The remarkably refined grains and nanoparticles can control metallic materials' mechanical properties, including the strength, strain rate dependency, and thermal stability behavior. In this context, nano-grains' local mechanical response during nanoindentation can vary considerably depending on the testing temperature, and this has yet to be discussed. In this research, after materials characterization of produced nanostructured materials according to the AFF route, the relating depth-sensing thermal stability of them assessed by conducting the nanoindentation testing at different temperatures in the range of 300–523 K. Depth sensing softening behavior is elaborated to identify the low-temperature thermal stability of processed materials. The results enunciated the occurrence of thermal softening by refining the grain structure. However, introducing the reinforcing nanoparticles lead to a pinning action that stabilized the grain boundaries.

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.

Powder metallurgy (PM) is a versatile technique to fabricate the particulate reinforced aluminum matrix composites (AMCs) by combining Al alloy powders with various types of reinforcement particles. However, the AMCs fabricated by PM generally lack ductility due to processing-related factors such as porosity, oxygen content, and undesirable microstructural features initiated by the contamination during powder preparation or sintering. In this study, the 5 wt% SiC/Al–Mg-Sc-Zr nanocomposite produced by PM was subjected to friction stir processing (FSP) to obtain the dense, uniform and refined microstructures and the attendant improved mechanical properties. The effect of FSP passes on the microstructural features and mechanical properties of as-PM nanocomposite was investigated, and the strengthening and toughening mechanism of FSPed nanocomposite was revealed. The results showed that the FSP passes with the opposite moving direction of FSP tool between the consecutive passes, largely affected the microstructure of the stir zone. Increasing FSP passes significantly reduced the microdefects and homogenized the microstructure. After 4-pass FSP, the whole stir zone consisting of the dense, uniform and refined microstructure, was obtained. This resulted in an increase in yield strength from 92 MPa in the as-PM condition to 161 MPa after 4-pass FSP, and a simultaneous nearly double enhancement in fracture strain from 0.11 to 0.29.

This article presents the experimental and modeling results of nanoindentation testing at ambient and elevated temperatures (i.e. 300 to 523 K), examining the small-scale plasticity of ultra-fine grained (UFG) alloy and nanostructured nanocomposite (NS) materials. The complex small-scale deformation mechanisms are highlighted to assess the material flow behavior under the indenter, which involves the interaction of dislocations and nanoparticles restricting the slip/glide of dislocations. Combined with UFG band propagation and transition stages, these lead to a stabilized mechanical strength at elevated temperatures. Accordingly, the activation energy for softening during localized deformation behavior of the nanocomposite is increased compared to the UFG alloy. It is attributed to a shift in the dislocations activation volume and energy aided by altered grain boundary diffusion and sliding mechanisms. It is shown that the straining role of nanoparticles leads to substantial grain size refinement to the nano-scale with enhanced generation of geometrically necessary dislocations. However, these nanoparticles' presence on the boundaries of nano-sized sub-grains can reduce the migration energy and restrict the sliding-based deformation mechanism.

A closed-loop directed energy deposition (CDED) system is developed to control the microstructure and mechanical properties of laser additive manufactured material in real-time. An austenitic stainless-steel grade of S316-L, which avoids eutectoid phase transformation, is examined to evaluate this unique control technique's performance. A set of walls are constructed in an open-loop manner by the laser additive manufacturing process to study the thermal dynamics and microstructure variations of a multi-layer structure under fixed processing conditions. The microstructure and subsequent mechanical properties of different thicknesses are affected mainly by the cooling rate change, resulting in a graded microstructure profile and mechanical properties in the deposition cross-section. The CDED system is implemented to eliminate cooling rate variations and locally control the deposition cooling rate to the desired setpoint value in similar five-layer depositions. This control is achieved by adaptively adjusting the traveling speed in real-time rather than keeping it constant. The closed-loop control of the cooling rate results in a uniform microstructure and hardness throughout the deposition cross-section and different layers. Although improved uniformity is observed in closed-loop samples, some microstructural and hardness deviations are still present, caused by post-solidification heating cycles of subsequent layers.

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.

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.

Procedures to determine yield stress based on two loads from load-displacement curves obtained using nearly-flat, instrumented indenters are presented. Using measured loads in cavity expansion and slip-line theory, estimates of yield of steel and aluminum alloys were obtained. Magnitude of the relative error in the estimated yields is bounded by 16%.

Tempered martensite obtained from four different tempering modes were characterized using transmission electron microscopy, high-angle annular dark field scanning transmission electron microscopy, and nanoindentation techniques. Crystallographic analysis of tempered martensite revealed that ferrite (α) and cementite (θ) obtained via furnace and Gleeble heat treatment obeyed the Isaichev orientation (or close to it) with [ 3 1 1 ] θ 0.91° from [ 1 1 1 ] α and the [ 1 12 ] α || [ 001 ] θ Bagaryatsky orientation relationship. A strict orientation relationship between ferrite and cementite could not be determined on the tempered structure extracted from the sub-critical heat affected zone of two different laser beam welded samples. Extensive recovery and reduction of boundary regions was identified on the structure tempered slowly, whereas rapidly tempered structures retained a high density of dislocation and less decomposition of the lath structure. The relationship between dislocation density and modified tempering parameter was determined and their contributions on tensile strength were evaluated.

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.

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.

In this study, the combined effect of dislocation density and rapid thermal cycling on the tempering kinetics of fully martensitic steel has been investigated by straining the sheets to different deformation levels. It was found that the strain-induced dislocations created more nucleation sites for carbide precipitation, and decreased carbide growth time. Strain-enhanced precipitation caused the carbide morphology to change from elongated to small quasi-spherical precipitates. Plastic deformation also accelerated the decomposition of carbides and film-like retained austenite in the as-received martensite laths. After tempering the deformed martensite had higher microhardness than tempered unstrained martensite, resulting from finer and semi-coherent precipitates, a high retained dislocation density, and the presence of untempered and partially tempered martensite blocks.

The newly developed severe plastic deformation (SPD) process of accumulative fold forging (AFF) was implemented for the production of multilayered composite structures by incorporating magnesium powder between nickel foil layers. The process was carried out up to 20 folding steps to produce a composite structure with more than a million number layers. By controlling the content of introduced magnesium powder and cyclic repeating of fold-forging steps, a reverse composite of magnesium matrix reinforced by fragments of nickel foil layers was produced. Microstructural characterization according was conducted using channeling contrast imaging by field emission-scanning electron microscopy (FE-SEM) and focused ion beam (FIB)/transmission electron microscopy (TEM) analyses, which revealed the formation of ultra-fine grains (UFG) with the average sizes of ~900 and 400 nm in the magnesium matrix and nickel foil layers, respectively. Moreover, a well-bonded structure was characterized at the interface of nickel folded layers and magnesium matrix was noted as a result of severe plastic shear straining during the AFF process as well as the deformation-assisted elemental inter-diffusion across the interface. Mechanical behavior of the produced multilayers composite structure was interrogated by indentation micro-hardness testing while displayed a fluctuation of hardness profile across the layers depending on the location of the micro-indenter. Considerable hardness improvements up to maximum values of ~300 and 60 HV are noted for the nickel layers and magnesium matrix, respectively, i.e., increases of around 100% compared to the primary annealed state of these metals as a result of the microstructural refinements.

In this work, the microstructure development in the stir zone of Al-Mg-Si alloy is evaluated while employing tool eccentricity during friction stir welding. Low dislocation density with dispersoids were observed in the inner band region of the stir zone produced with aligned tooling. On the other hand, a high dislocation density with Mg2Si precipitates can be observed in the same region of the stir zone when a tool eccentricity of 0.2 mm was utilized. The discrepancy is attributed to the enhanced shearing activity imposed on the material during the welding process.