Microstructural characterization of dissimilar friction stir welds between AA2219 and AA5083

Fusion welding of dissimilar aluminum alloys is very challenging. In the present work, Al-Cu alloy AA2219-T87 was friction stir welded to Al-Mg alloy AA5083-H321. Weld microstructures, hardness, and tensile properties were evaluated in as-welded condition. Microstructural studies revealed that the nugget region was primarily composed of alloy 2219, which was placed on the advancing side. No significant mixing of the two base materials in the nugget region was observed. Hardness studies revealed that the lowest hardness in the weldment occurred in the heat-affected zone on alloy 5083 side, where tensile failure were observed to take place. Tensile tests indicated a joint efficiency of around 90%, which is substantially higher than what can be achieved with conventional fusion welding. Overall, the results show that satisfactory butt welds can be produced between AA2219-T87 and Al-Mg alloy AA5083-H321 sheets using friction stir welding.     

Influence of CO2-Ar Mixtures as Shielding Gas on Laser Welding of Al-Mg Alloys

In this study, AA5083 samples were butt welded under a conduction regime with high-power diode laser (HPDL). Various mixtures composed of Ar and CO₂ were used as a shielding gas. The influence of the shielding gas composition on the microstructure and on the properties of laser welds was analyzed. The weld beads were deeply characterized by metallographic/microstructural studies, X-ray diffraction (XRD), X-ray energy dispersive spectrometry (X-EDS) chemical analyses, X-ray photoelectron spectra (XPS), microhardness, and tensile strength. The corrosion resistance of laser-remelted surfaces with different CO₂/Ar ratios was also estimated by means of electrochemical tests. The addition of CO₂ to the shielding gas results in a better weld penetration and oxidizes the weld pool surface. This addition also promotes the migration of Mg toward the surface of weld beads and induces the formation of magnesium aluminates spinel on the welds. The best corrosion resistance result is achieved with 20 pct CO₂. The overall results indicate that the addition of small percentage of CO₂ to Ar leads to improvements of the mechanical and corrosion properties of the aluminum welds.     

Microstructural factors governing hardness in friction-stir welds of solid-solution-hardened Al alloys

Microstructural factors governing hardness in friction-stir welds of the solid-solution-hardened Al alloys 1080 and 5083 were examined by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles. The weld of Al alloy 1080 had a slightly greater hardness in the stir zone than the base material. The maximum hardness was located in the thermomechanically affected zone (TMAZ). The stir zone consisted of recrystallized fine grains, while the TMAZ had a recovered grain structure. The increase in hardness in the stir zone can be explained by the Hall-Petch relationship. On the other hand, the hardness profiles in the weld of Al alloy 5083 were roughly homogeneous. Friction-stir welding created the fine recrystallized grains in the stir zone and recovered grains in the TMAZ in the weld of this alloy. The stir zone and the TMAZ had slightly higher dislocation densities than the base material. Many small Al₆(Mn,Fe) particles were detected in all the grains of the weld. The hardness profiles could not be explained by the Hall-Petch relationship, but rather by Orowan hardening. The results of the present study suggest that the hardness profile is mainly affected by the distribution of small particles in friction-stir welds of Al alloys containing many such particles.     

Pulsed Nd:YAG laser welding of copper using oxygenated assist gases

The effects of using oxygenated assist gases on the weldability and weld properties of Nd:YAG, pulsed laser welds in copper (Cu) have been evaluated. It was found that the effective absorptivity of the Cu increased as the oxygen content of the Ar assist gas was increased. This facilitated laser welding of Cu at much lower laser powers and increased weld penetration. The use of oxygenated assist gas promoted nucleation and growth of submicroscopic oxide particles within the weld metal. These particles dispersion-strengthened the weld metal, thereby increasing both weld metal hardness and strength. However, when O₂ concentrations in the assist gas were greater than 90 pct, weld metal embrittlement due to excessive volume fractions of oxides was observed. The use of oxygenated assist gas also led to excessive cold lapping and poor bead quality. The bead quality was improved, however, by ramping-down the laser power before terminating each pulse.     

CO2 laser beam welding of 6061-T6 aluminum alloy thin plate

Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because its low heat input minimizes the width of weld fusion and heat-affected zones (HAZs). In the present work, 1-mm-thick age-hardened Al-Mg-Si alloy, 6061-T6, plates were welded with full penetration using a 2.5-kW CO₂ laser. Fractions of porosity in the fusion zones were less than 0.05 pct in bead-on-plate welding and less than 0.2 pct in butt welding with polishing the groove surface before welding. The width of a softened region in the-laser beam welds was less than 1/4 times that of a tungsten inert gas (TIG) weld. The softened region is caused by reversion of strengthening β″ (Mg₂Si) precipitates due to weld heat input. The hardness values of the softened region in the laser beam welds were almost fully recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas those in the TIG weld were not recovered in a partly reverted region. Both the bead-on-plate weld and the butt weld after the postweld artificial aging treatment had almost equivalent tensile strengths to that of the base plate.     

The Mechanism of Grain Coarsening in Friction-Stir-Welded AA5083 after Heat Treatment

Friction stir welding (FSW) takes place in the solid state, thus providing potential advantages of welds of high strength and ductility because of fine microstructures. However, post-FSW heat treatment can create very coarse grains, potentially reducing mechanical properties. AA5083-H18 sheets were friction-stir butt welded using three sets of welding parameters representing a wide range of heat input. They were then heat treated for 5 minutes at 738 K (465 °C), producing grain sizes exceeding 100 μm near the top weld surfaces, with the coarse grains extending toward the bottom surface to various degrees depending on the welding parameters. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical metallography, inductively coupled plasma–mass spectrometry, and Vickers hardness testing were used to characterize the regions within welds. Particle pinning was determined quantitatively and used with Humphreys’ model of grain growth to interpret the behavior. The mechanism responsible for forming the large grains was identified as abnormal grain growth (AGG), with AGG occurring only for regions with pre-heat-treatment grain sizes smaller than 3 μm. Second-phase particle volume fractions and sizes, textures, and solute concentrations were not significantly different in AGG and non-AGG regions. Ultrafine grain layers with grain diameters of 0.3 mm were characterized and had high densities of pinning particles of MgSi₂, Al₂O₃, and Mg₅Al₈. Strategies to eliminate AGG by alloy and weld process design were discussed.     

Microstructural Characterization of Internal Welding Defects and Their Effect on the Tensile Behavior of FSW Joints of AA2198 Al-Cu-Li Alloy

Internal features and defects such as joint line remnant, kissing bond, and those induced by an initial gap between the two parent sheets were investigated in AA2198-T851 friction stir welded joints. They were compared with the parent material and to defect-free welds obtained using a seamless sheet. The cross-weld tensile strength was reduced by the defects by less than 6 pct. The fracture elongation was not significantly affected in view of experimental scatter. Fracture location, however, changed from the thermomechanically affected zone (retreating side) to the defect in the weld nugget for the welds bearing a kissing bond and for some of the gap welds. The kissing bond was shown by EBSD to be an intergranular feature; it fractured under a normal engineering stress close to 260 MPa during an in situ SEM tensile test. Synchrotron tomography after interrupted tensile testing confirmed opening of the kissing bond. For an initial gap of 23 pct of the sheet thickness, intergranular fracture of copper-enriched or oxide-bearing grain boundaries close to the nugget root was evidenced. The stress and strain state of cross-weld specimens loaded under uniaxial tension was assessed using a 3D finite element, multi-material model, determined on the basis of experimental data obtained on the same specimens using digital image correlation.     

Influence of Tool Offsetting on the Structure and Morphology of Dissimilar Aluminum to Copper Friction-Stir Welds

In this work, a systematic analysis of the effect of tool offsetting on the morphological, structural, and mechanical properties of 6082-T6 aluminum to copper-DHP friction-stir welds was performed, enabling full understanding of Al-Cu bonding structure and failure mechanisms. Important relations between tool positioning and the thermomechanical phenomena taking place during welding were established. Tool offsetting was revealed to be an effective way of solving one of the most important concerns in Al/Cu friction-stir welding, i.e., the formation of large amounts of intermetallic-rich structures, which deeply influence the final strength and surface morphology of the welds. Actually, for welds produced without tool offsetting, it was found that the formation of fluidized intermetallic-rich structures promote the formation of internal decohesion areas inside the nugget, which have a detrimental effect on weld strength. For welds carried out with tool offsetting, intermetallic formation is almost suppressed, but important metallurgical discontinuities in the vicinity of large copper fragments, dispersed over the nugget, and at the nugget/copper interface were also found to have a detrimental effect on weld strength.     

Properties of friction-stir-welded 7075 T651 aluminum

Friction stir welding (FSW), a new welding technique invented at TWI, was used to weld 7075 T651 aluminum, an alloy considered essentially unweldable by fusion processes. This weld process exposed the alloy to a short time, high-temperature spike, while introducing extensive localized deformation. Studies were performed on these solid-state welds to determine mechanical properties both in the longitudinal direction, i.e., within the weld nugget, and, more conventionally, transverse to the weld direction. Because of the unique weld procedure, a fully recrystallized fine grain weld nugget was developed. In addition, proximate to the nugget, both a thermomechanically affected zone (TMAZ) and heat affected zone (HAZ) were created. During welding, temperatures remained below the melting point and, as such, no cast or resolidification microstructure was developed. However, within the weld nugget, a banded microstructure that influences room-temperature fracture behavior was created. In the as-welded condition, weld nugget strength decreased, while ductility remained high. A low-temperature aging treatment failed to fully restore T651 strength and significantly reduced tensile ductility. Samples tested transverse to the weld direction failed in the HAZ, where coarsened precipitates caused localized softening. Subsequent low-temperature aging further reduced average strain to failure without affecting strength. Although reductions in strength and ductility were observed, in comparison to other weld processes, FSW offers considerable potential for welding 7075 T651 aluminum.     

Influence of Filler Wire Diameter on Mechanical and Corrosion Properties of AA5083-H111 Al–Mg Alloy Sheets Welded Using an AC Square Wave GTAW Process

In this work, the influence of filler wire diameter on AA5083-H111 weldments was studied. For that, square butt joints were made using an AC square wave gas tungsten arc welding process with the addition of filler wires of diameter 1.2 and 2.4 mm separately. The experimental results revealed that changing the filler wire diameter influenced the bead geometry and a complete penetration was achieved in both welds. The weldment processed with smaller diameter filler wire consisted of a wider heat affected zone with recrystallized grains and a fusion zone with coarser grain structure, thus reducing the mechanical properties and corrosion resistance. However, the use of larger diameter filler wire assisted in faster torch speed, resulting in lower heat input and thus finer equiaxed grains were produced in fusion zone. Also, finer grains along with the dispersion of finer Al₆(Fe,Mn) particles supported in obtaining the superior tensile and corrosion properties.     

Producing of AA5083/ZrO2 Nanocomposite by Friction Stir Processing (FSP)

In this study, friction stir processing (FSP) was used to produce AA5083/ZrO₂ nanocomposite layer. Optical microscopy and SEM were used to probe the microstructures formed in the composite layer. In addition, the mechanical properties of each sample are characterized using both tensile and hardness tests. Results showed that FSP is an effective process to fabricate AA5083/ZrO₂ nanocomposite layer with uniform distribution of ZrO₂ particles, good interfacial integrity, and significant grain refinement. On processing, in the proper combination of process parameters, the metal matrix composite layer was observed to have increased tensile and hardness properties.     

Effect of Multiple-Pass Friction Stir Processing Overlapping on Microstructure and Mechanical Properties of As-Cast NiAl Bronze

As-cast Cu-9Al-4.5Ni-4Fe NiAl bronze alloy (NAB) was subjected to multiple-pass friction stir processing (FSP) with a 50 pct overlap. After FSP, the coarse microstructure of the base metal (BM) was transformed to defect-free material with fine microstructure. While the torchlike patterns in the stir zone (SZ) and the uplifted grains in the transitional zones (TZs) between two passes were observed in the multiple-pass FSP region, no grain coarsening was found in the remnant zone of the previous SZ after subsequent FSP pass. The hardness value of the FSP materials was higher than that of the BM and was homogeneously distributed throughout the entire multiple-pass FSP region. The FSP materials showed greatly improved tensile properties compared to the BM, and the TZs showed similar tensile strength and ductility to the single-pass FSP materials. The BM broke in a mixture mode of brittle cleavage and microvoid coalescence fracture, whereas the FSP and TZ samples failed in the latter fracture mode. The results showed that the multiple-pass overlapping (MPO) FSP was feasible to modify the microstructure of large-sized plate of the NAB.     

Dilution control in gas-tungsten-arc welds involving superaustenitic stainless steels and nickel-based alloys

Fusion welds were prepared between a superaustenitic stainless steel, (the AL-6XN alloy) and two Ni-based filler metals (IN625 and IN622) using the gas-tungsten-arc welding (GTAW) process. Fusionzone compositions over the full range of dilution levels (0 to 100 pct) were produced by varying the independent welding parameters of arc power and volumetric filler-metal feed rate. Microstructural characterization of the welds was conducted via light optical microscopy, with quantitative chemical information obtained through electron-probe microanalysis (EPMA). The dilution level of each weld was determined from the EPMA data as well as through geometric measurements of the weld cross-sectional areas. The dilution level was observed to decrease with increasing filler-metal feed rate and decreasing arc power. These effects are quantitatively interpreted based on a previously proposed processing model. The model is used to demonstrate that, in terms of welding parameters, the dilution level can be correlated exclusively to the ratio of the volumetric filler-metal feed rate (V _fm) to arc power (VI), i.e., the individual values of V _fm and VI are not important in controlling the dilution and resultant weld-metal composition. Good agreement is obtained between experimental and calculated dilution values using the model. It is also demonstrated that the melting enthalpies of the filler metal and substrate have only a minor influence on dilution at dilution levels in the range from 40 to 100 pct. This knowledge facilitates estimates of dilution levels in this range when the substrate and fillermetal thermal properties are not accurately known. The results presented from this study provide guidelines for controlling the weld-metal composition in these fusion-zone combinations.     

Effects of Friction Stir Processing Parameters and In Situ Passes on Microstructure and Tensile Properties of Al-Si-Mg Casting

Friction stir processing (FSP) was applied to modify the microstructure of an as-cast A356 alloy. The effects of rotation rate, travel speed, in situ FSP pass, FSP direction, and artificial aging on microstructures and tensile properties were investigated. FSP broke up the coarse eutectic Si phase into 2.5 to 3.5 μm particles and distributed them homogeneously, and resulted in the dissolution of the coarse Mg₂Si particles and the elimination of porosity, thereby improving both the strength and the ductility of the casting. Increasing the rotation rate was beneficial to breaking up and dissolving the particles, but it contributed little to eliminating the porosity. The travel speed did not affect the size of the particles apparently, but lower speed was beneficial to eliminating the porosity. 2-pass FSP showed an obvious advantage in the microstructure modification and tensile properties compared with the single-pass. However, a further increase of FSP passes only resulted in slight improvement. The FSP direction of the following pass did not show distinct effect on the microstructure and tensile properties. After post-FSP artificial aging, the strengthening phase (β″-Mg₂Si) precipitated, which increased the strength and decreased the ductility of the FSP samples.     

Enhanced Relative Slip Distance in Gas-Tungsten-Arc-Welded Al<Subscript>0.5</Subscript>CoCrFeNi High-Entropy Alloy

Gas-tungsten-arc-welded (GTAW) Al_0.5CoCrFeNi high-entropy alloy (HEA) was analyzed using scanning electron microscopy (SEM), microhardness, and tensile testing. The weld metal having refined equiaxed and elongated columnar dendritic microstructure experienced 6.38 pct reduction in strength and marginally reduced hardness compared to the base metal (BM). Lower work hardening with enhanced relative slip distance, which was observed through the Kocks–Mecking plot and slip distance–true strain plots, was attributed to the reduced bcc fraction in the weld.     

Corrosion-Fatigue Behavior of Aluminum Alloy 5083-H131 Sensitized at 448?K (175?°C)

The fatigue crack growth behavior of aluminum alloy 5083-H131 has been systematically studied as a function of degree of sensitization for aging at 448?K (175?°C). Fatigue crack growth rates were measured at load ratios of 0.1 and 0.85, in vacuum, air, and a corrosive aqueous environment containing 1?pct NaCl with dilute inhibitor. Sensitization does not affect the fatigue crack growth behavior of Al 5083-H131 significantly in vacuum or air, at low- or high-load ratio. For high-load ratio, in the 1?pct NaCl+inhibitor solution, the threshold drops by nearly 50?pct during the first 200?hours of aging, then it degrades more slowly for longer aging times up to 2000?hours. The change in aging behavior at approximately 200?hours seems to be correlated with the transition from partial coverage of the grain boundaries by ?? Al₃Mg₂ phase, to continuous full ?? coverage. ASTM G-67 mass loss levels below approximately 30?mg/cm² do not exhibit degraded corrosion-fatigue properties for R?=?0.85, but degradation of the threshold is rapid for higher mass loss levels.     

A model for the silicon-manganese deoxidation of steel weld metals

From an analytical and theoretical study of flat and out-of-position gas metal arc (GMA) C-Mn steel welds containing varying additions of silicon and manganese, we conclude that the buoyancy effect (flotation obeying Stokes’ law) does not play a significant role in the separation of oxide inclusions during weld metal deoxidation. Consequently, the separation rate of the particles is controlled solely by the fluid flow pattern in the weld pool. A proposed two-step model for the weld metal deoxidation reactions suggests that inclusions formed in the hot, turbulent-flow region of the weld pool are rapidly brought to the upper surface behind the arc because of the high-velocity flow fields set up within the liquid metal. In contrast, those formed in the cooler, less-turbulent flow regions of the weld pool are to a large extent trapped in the weld metal as finely dispersed particles as a result of inadequate melt stirring. The boundary between “hot” and “cold” parts for possible inclusion removal is not well defined, but depends on the applied welding parameters, flux, and shielding gas composition. As a result of the intricate mechanism of inclusion separation, the final weld metal oxygen content depends on complex interactions among the following three main factors: (1) the operational conditions applied, (2) the total amount of silicon and manganese present, and (3) the resulting manganeseto-silicon ratio. The combined effect of the latter two contributions has been included in a new deoxidation parameter, ([pct Si][pct Mn])^−0.25. The small, negative exponent in the deoxidation parameter indicates that control of the weld metal oxygen concentrations through additions of silicon and manganese is limited and that choice of operational conditions in many instances is the primary factor in determining the final degree of deoxidation to be achieved.     

Effect of Processing Parameters on Microstructure and Mechanical Properties of an Al-Al11Ce3-Al2O3In-Situ Composite Produced by Friction Stir Processing

Friction stir processing (FSP) was applied to produce aluminum-based in-situ composites from powder mixtures of Al-5 mol pct CeO₂. A billet of powder mixtures was prepared using the conventional pressing and sintering route. The sintered billet was then subjected to multiple passages of FSP. This technique has combined the hot-working nature of FSP and the exothermic reaction between Al and CeO₂. The reinforcing phases were identified as Al₁₁Ce₃ and δ ^* -Al₂O₃. The Al₂O₃ particles with an average size of ~10 nm are uniformly distributed in the aluminum matrix, which has an average grain size of approximately 390 to 500 nm. Both the sintering temperature and the tool traversing speed used in FSP have significant influence on the microstructure and mechanical properties of the composite. The composite produced by sintering at 833 K followed by FSP with a tool traversing speed of 30 mm/min possesses an enhanced modulus (E = 109 GPa) and strength (ultimate tensile strength (UTS) = 488 MPa) as well as a tensile ductility of ~3 pct.     

Crossed-Wire Laser Microwelding of Pt-10 Pct Ir to 316 LVM Stainless Steel: Part II. Effect of Orientation on Joining Mechanism

With the increasing complexity of medical devices and with efforts to reduce manufacturing costs, challenges arise in joining dissimilar materials. In this study, the laser weldability of dissimilar joints between Pt-10 pct Ir and 316 low-carbon vacuum melted (LVM) stainless steel (SS) crossed wires was investigated by characterizing the weld geometry, joint strength, morphology of weld cross sections, and differences in joining behavior, depending on which material is subject to the incident laser beam. With the Pt-Ir alloy on top, a significant amount of porosity was observed on the surface of the welds as well as throughout the weld cross sections. This unique form of porosity is believed to be a result of preferential vaporization of 316 LVM SS alloying elements that become mixed with the molten Pt-10 pct Ir during welding. The joining mechanism documented in micrographs of cross-sectioned welds was found to transition from laser brazing to fusion welding. It is inferred that the orientation of the two dissimilar metals (i.e., which material is subject to the incident laser beam) plays an important role in weld quality of crossed-wire laser welds.     

Submerged Friction-Stir Welding (SFSW) Underwater and Under Liquid Nitrogen: An Improved Method to Join Al Alloys to Mg Alloys

Submerged friction-stir welding (SFSW) underwater and under liquid nitrogen is demonstrated as an alternative and improved method for creating fine-grained welds in dissimilar metals. Plates of AZ31 (Mg alloy) and AA5083 H34 were joined by friction-stir welding in three different environments, i.e., in air, water, and liquid nitrogen at 400?rpm and 50?mm/min. The temperature profile, microstructure, scanning electron microscopy (SEM)-energy-dispersive spectroscopy (EDS) analysis, X-ray diffraction (XRD), hardness, and tensile testing results were evaluated. In the stir zone of an air-welded specimen, formation of brittle intermetallic compounds of Al₃Mg₂, Al₁₂Mg₁₇, and Al₂Mg₃ contributed to cracking in the weld nugget. These phases were formed because of constitutional liquation. Friction-stir welding underwater and under liquid nitrogen significantly suppresses the formation of intermetallic compounds because of the lower peak temperature. Furthermore, the temperature profiles plotted during this investigation indicate that the largest amount of ?T is generated by the weld under liquid nitrogen, which is performed at the lowest temperature. It is shown that in low-temperature FSW, the flow stress is higher, plastic contribution increases, and so adiabatic heating, a result of high strain and high strain-rate deformation, drives the recrystallization process beside frictional heat.