Showing papers on "Heat-affected zone published in 2015"

Dissimilar laser welding of AISI 316L stainless steel to Ti6–Al4–6V alloy via pure vanadium interlayer

Abstract: Successful continuous laser joining of AISI 316L stainless steel with Ti6Al4V titanium alloy through pure vanadium interlayer has been performed. Three welding configurations were tested: one-pass welding involving all three materials and two pass and double spot welding involving creation of two melted zones separated by remaining solid vanadium. For the most relevant welds, the investigation of microstructure, phase content and mechanical properties has been carried out. In case of formation of a single melted zone, the insertion of steel elements into V-based solid solution embrittles the weld. In case of creation of two separated melted zones, the mechanical resistance of the junction is determined by annealing of remaining vanadium interlayer, which can be witnessed by observing the increase of grain size and decrease of UTS. The two pass configuration allows attain highest mechanical resistance: 367 MPa or 92% of UTS of annealed vanadium. Double spot configuration produces excessive heat supply to vanadium interlayer, which results in important decrease of tensile strength down to 72% of UTS of annealed vanadium. It was found that undesirable σ phase which forms between Fe and V is not created during the laser welding process because of high cooling rates. However, the zones whose composition corresponds to σ homogeneity range are crack-susceptible, so the best choice is to reduce the V content in steel/vanadium melted zone below σ phase formation limit. In the same time, the proportion between V and Ti in Ti6Al4V/vanadium melted zones does not influence mechanical properties as these elements form ideal solid solution.

The evolution of precipitation and microstructure in friction stir welded 2195-T8 Al–Li alloy

Abstract: Precipitate and microstructure evolution in friction stir welding of 2195-T8 aluminum alloy was characterized by transmission electron microscopy. The results show that precipitations in the base metal primarily consist of T1 (Al2CuLi) platelets and small amounts of θ′ (Al2Cu) and τ2 (Al7Cu2Fe) phase. In the heat affected zone (HAZ), these precipitations dissolve during welding, allowing the re-precipitation of δ′ (Al3Li) and β′ (Al3Zr) during cooling. δ′ and β′ phase are the major strengthening phase in the weld nugget zone (WNZ), which results in the observed lower microhardness of the nugget region. Differential scanning calorimetry (DSC) curve is used to confirm and interpret the results provided by the microscopy.

Correlation of microstructure and mechanical properties in friction stir welded 2198-T8 Al–Li alloy

Abstract: In this study, the 1.8 mm thick cold-rolled sheets of 2198-T8 Al–Li alloy were manufactured by friction stir welding (FSW) at a rotation rate of 800 rpm and a travel speed of 300 mm/min. The microstructure and mechanical properties of different regions of the produced joint were evaluated by means of optical microscopy (OM), transmission electron microscopy (TEM), hardness testing and tensile testing. Results show that the original “pancake” grains became coarser in the heat affected zone (HAZ), transformed into equiaxed grains in the stir shoulder zone (SsZ) and stir pin zone (SpZ), and formed mixed grains with both “pancake” and equiaxed shapes in the thermo-mechanical affected zone (TMAZ). The hardness distribution in the cross-section of the FSW joint exhibited a “basin” shape. When approaching the weld centre, the hardness gradually decreased compared to the base metal (BM). The BM exhibited the highest strength due to the presence of fine T1 phase. In the HAZ, the strength decreased as T1 phase was partially dissolved. In the SsZ and SpZ, in spite of strength contribution from grain refinement, the strength further decreased as T1 phase was fully dissolved. The minimum strength in the TMAZ was related to the reduced amount of T1 phase and the presence of transition layer with sharp gradient of grain size.

Comparison of energy consumption and environmental impact of friction stir welding and gas metal arc welding for aluminum

Abstract: One of the advantages of friction stir welding (FSW) is reduced energy consumption as compared to arc welding processes. This advantage has been predicted and qualitatively established. However, a quantitative analysis based on energy measurements during the processes and how to equitably compare them is missing. The objective of this work is to quantitatively compare the energy consumption associated with the creation of full-penetration welds in aluminum 6061-T6 workpieces by FSW and gas metal arc welding (GMAW) processes. The workpiece thicknesses for the two processes (5-mm-thick for FSW and 7.1-mm-thick for GMAW) are chosen such that the maximum tensile force sustained by the joints during tensile testing is similar. This accounts for material saving due to the higher ultimate tensile strength resulting from FSW. The energy consumed for any pre-processes, the welding processes, and post-processes was measured. Finally, a life cycle assessment (LCA) approach was used to determine and compare the environmental impact of FSW and GMAW. For the welding parameters used in this study joining by FSW consumes 42% less energy as compared to GMAW and utilizes approximately 10% less material for the design criteria of similar maximum tensile force. This leads to approximately 31% less greenhouse gas emissions for FSW as compared to GMAW. Both, the lower energy consumption during FSW, and involved pre and post processes contributed in the overall energy reduction.

Effect of power distribution on the weld quality during hybrid laser welding of an Al-Mg alloy

Abstract: This paper treats of the analysis of the effect of arc and laser powers on the quality of the arc assisted fiber laser welding of an Al–Mg alloy in the butt configuration. Grain size, weld geometry defects, porosity, and magnesium loss were measured. Magnesium content of the fused zone decreased as the laser power increased while the porosity increased with laser power. Microhardness profiles and tensile properties were explained on the basis of the joint microstructure and defects and related to the power distribution. The porosity level and Mg content in the fused zone affected both tensile strength and ductility. The power distribution that stabilized the welding process and minimize the weld porosity was defined.

Improved weld macrosection, microstructure and mechanical properties of 2024Al-T4 butt joints in ultrasonic vibration enhanced friction stir welding

Abstract: Ultrasonic vibration enhanced friction stir welding (UVeFSW) is a new variant of friction stir welding (FSW) in which a sonotrode transmits ultrasonic energy directly into the localised area of the workpiece near and ahead of the rotating tool. This study investigated the influence of ultrasonic vibration on the formation, microstructure and mechanical properties of butt welded 2024Al-T4 joints, and attempted to unveil the underlying mechanism of UVeFSW by experimental methods. Morphology inspection, X-ray detection and metallographic inspection of the welds revealed that ultrasonic vibration can improve the weld formation at higher welding speeds. The stir zone in the UVeFSW broadened, while the grains in the heat affected zone had no obvious growth contrary to that in the base metal. Results of the mechanical tests indicated that the tensile strength and elongation of joints, and the microhardness value in the stir zone increased at the same welding parameters.

Explanation of penetration depth variation during laser welding under variable ambient pressure

Abstract: It has been observed that the penetration depth during laser welding (LW) under vacuum or reduced ambient pressure could be significantly greater than that during welding under atmospheric pressure. Previous explanations of this phenomenon usually limit to specific wavelength laser welding and have difficulties in explaining why the variation will disappear, as the welding speed increases. Here, we propose that this variation is caused by the temperature difference of keyhole wall under variable ambient pressure based on a correct physical description of related processes. A new surface pressure model, dependent on ambient pressure, is proposed for describing the evaporation process during laser material interaction under variable ambient pressure. For laser welding of a 304 stainless steel with 2.0 kW laser power and 3 m/min welding speed, it is shown that the average keyhole wall temperature is around 2900 K under atmospheric pressure, and only around 2300 K under vacuum, which results in significant penetration depth variations. Interestingly, it is also shown that as the welding speed increases, the average temperature of the front keyhole wall gradually rises due to the reduction of the mean incident angle of laser, and the magnitude of this increase is larger in welding under vacuum than under atmospheric pressure. It allows us to explain why the penetration depth improvement decreases to zero with the increase of welding speed.

Characteristics of aluminum-to-steel joint made by friction stir welding: A review

Abstract: Friction stir welding exploits its solid state process behaviour to join aluminum to steel, which differs in thermal and mechanical properties, and where combination of these metallic alloys by fusion welding prompts a deleterious reaction as a result of the melting and resolidification phases. This review investigates the distinction and characterization of aluminum to steel joints made by this welding method. An attempt to address sub-techniques into three categories i.e., diffusion, plunging, and annealing, is made. Steel fragments spattered at the weld zone, weld defects, sharp difference in grain size distribution, and the formed phases of the intermetallic layer and its thickness are discussed, these factors and the process welding parameters are significantly influenced the joint’s strength of this welding method.