Fusion Zone Size Research Articles

Effect of double-pulse frequency and post-weld heat treatment on microstructure and mechanical properties of metal-inert gas welded Al–Mg–Si alloy joints

The Al–Mg–Si alloy welded structure is widely used in aerospace, marine ship, automotive and other industries. However, the mechanical properties of welded joint were seriously weakened due to the microstructure coarsening and heat affected zone (HAZ) softening, making its service life decayed. In this paper, in order to enhance the mechanical properties of Al–Mg–Si alloy welded joint, a coupled method including double pulsed Metal Inert Gas (DP-MIG) welding technology and post-weld heat treatment (PWHT) were applied on the joining process of Al–Mg–Si alloy. With the increase of DP frequency from 2 Hz to 5 Hz, the average grain size of fusion zone decreased from 113.7 μm to 88.4 μm. And the tensile strength of welded joint was at the interval of 180∼190 MPa. After a suitable heat treatment (solution treatment at 530 °C for 2 h and aging treatment at 180 °C for 8 h), the tensile strength of welded joint increased to 318.3–333.8 MPa, which exceeds 90 % of the base metal (BM). Under the comprehensive effect of solid solution strengthening, fine grain strengthening, and precipitation strengthening, the mechanical properties of Al–Mg–Si alloy welded joint were significantly improved. And the research results in this paper can provide a research basis for high quality connection of Al–Mg–Si alloys.

Tailoring magnetic hysteresis of additive manufactured Fe-Ni permalloy via multiphysics-multiscale simulations of process-property relationships

Designing the microstructure of Fe-Ni permalloy produced by additive manufacturing (AM) opens new avenues to tailor its magnetic properties. Yet, AM-produced parts suffer from spatially inhomogeneous thermal-mechanical and magnetic responses, which are less investigated in terms of process modeling and simulations. We present a powder-resolved multiphysics-multiscale simulation scheme for describing magnetic hysteresis in AM-produced material, explicitly considering the coupled thermal-structural evolution with associated thermo-elasto-plastic behaviors and chemical order-disorder transitions. The residual stress is identified as the key thread in connecting the physical processes and phenomena across scales. By employing this scheme, we investigate the dependence of the fusion zone size, the residual stress and plastic strain, and the magnetic hysteresis of AM-produced Fe21.5Ni78.5 on beam power and scan speed. Simulation results also suggest a phenomenological relation between magnetic coercivity and average residual stress, which can guide the magnetic hysteresis design of soft magnetic materials by choosing appropriate processing parameters.

Transition in Interfacial Failure Mechanism of Resistance Spot Welds during Tensile–Shear Loading: Role of Fusion Zone Hardness

The failure of resistance spot welds through the fusion zone along the sheet/sheet interface (i.e., interfacial failure) is critical for automotive crashworthiness. This paper investigates the effect of fusion zone hardness on the interfacial failure behavior of resistance spot welds during the tensile–shear test. AISI 1040 medium carbon steel, producing a high level of hardness mismatch during resistance spot welding, was selected as the base metal. By ex situ tempering heat treatment, various levels of fusion zone hardness are achieved in the welds with constant fusion zone size. It is shown that the interfacial failure of the spot welds is a competition between ductile shear failure and rapid crack propagation. It is found that there is a critical fusion zone hardness beyond which the interfacial failure mechanism transitions from ductile shear failure to rapid crack propagation. In welds with high fusion zone hardness, the mechanism of interfacial failure is rapid crack growth, and fusion zone fracture toughness is the governing factor for the interfacial failure load. Conversely, in welds with low FZ hardness, the mechanism of interfacial failure is a ductile shear failure, and fusion zone hardness would be the governing factor for the interfacial failure load.

Microstructural modeling and measurements of anisotropic plasticity in large scale additively manufactured 316L stainless steel

In this paper, the wire + arc additive manufacturing process-induced plastic anisotropy of 316L stainless steel is analyzed by means of detailed 3D microstructural modeling and compared to experimental tensile tests. A spatially varying representative grain texture and morphology are incorporated in a representative volume element having the size of a single fusion zone and which is generated using a 3D anisotropic Voronoi algorithm. The constitutive behavior is modeled at the grain scale by a finite element crystal plasticity framework, of which the corresponding parameters are obtained from experimental tensile tests in one of the processing directions. As a result of the spatially correlated grain orientations inside the fusion zone, distinct deformation patterns and strain localizations have been observed during experimental tensile tests. The strain fields obtained from numerical simulations are compared to the experimental deformation patterns and a remarkable correspondence is observed. Numerical simulations are also performed in various uniaxial loading directions to predict the 3D yield behavior. A strongly anisotropic plastic response is obtained and a convincing match between the numerical model and experimental tensile tests is found in various loading directions.

High-Power Fiber Laser Welding of High-Strength AA7075-T6 Aluminum Alloy Welds for Mechanical Properties Research.

The results disclosed that both the microstructure and mechanical properties of AA7075-T6 laser welds are considerably influenced by the heat input. In comparison with high heat input (arc welding), a smaller weld fusion zone with a finer dendrite arm spacing, limited loss of alloying elements, less intergranular segregation, and reduced residual tensile stress was obtained using low heat input. This resulted in a lower tendency of porosity and hot cracking, which improved the welded metal’s soundness. Subsequently, higher hardness as well as higher tensile strength for the welded joint was obtained with lower heat input. A welded joint with better mechanical properties and less mechanical discrepancy is important for better productivity. The implemented high-power fiber laser has enabled the production of a low heat input welded joint using a high welding speed, which is of considerable importance for minimizing not only the fusion zone size but also the deterioration of its properties. In other words, high-power fiber laser welding is a viable solution for recovering the mechanical properties of the high-strength AA 7075-T6 welds. These results are encouraging to build upon for further improvement of the mechanical properties to be comparable with the base metal.

Analytical estimation of fusion zone dimensions and cooling rates in part scale laser powder bed fusion

The laser powder bed fusion process is increasingly used for the building of metallic parts by melting and solidification of alloy powders under a fast-moving finely focussed laser beam. A quick estimation of the resulting temperature field, fusion zone dimensions, and cooling rates is needed to ensure the manufacture of dimensionally accurate parts with minimum defects. A novel three-dimensional analytical heat transfer model with a volumetric heat source that can simulate the laser powder bed fusion process in part scale quickly and reliably is proposed here. The volumetric heat source term is constructed to analytically simulate the evolution of melt pools with a fair range of depth to width ratio. The proposed analytical model can simulate the building of multiple tracks and layers in part scale dimensions significantly faster than all the numerical models reported in the literature. The computed results of fusion zone shapes and sizes and cooling rates are found to be in good agreement with the experimentally reported results in builds of three commonly used alloys with diverse materials properties, SS316L, Ti6Al4V, and AlSi10Mg. Based on the analytically computed results, a set of easy-to-use process maps is presented to estimate multiple process conditions to obtain a set of target fusion zone dimensions without trial-and-error testing.

Addressing the abnormal grain coarsening during post-weld heat treatment of TIG repair welded joint of sand-cast Mg-Y-RE-Zr alloy

Tungsten inert gas (TIG) repair welding is one of the key methods to reduce the rejection rate of sand-cast Mg-Y-RE-Zr alloys containing casting defects, and welding heat treatment can further improve the joint efficiency of the repaired joints. In this study, the microstructural evolution and mechanical properties of the repaired joints of sand-cast Mg-4Y-2Nd-1Gd-0.5Zr alloy during different pre- and post-weld heat treatments have been investigated. The results indicated that abnormal grain coarsening (AGC) was formed in the fusion zone (FZ) during the post-weld solution treatment of the repaired joint. As the solution temperature increased from 480 °C to 520 °C, the average grain size of FZ of the T6-treated joint increased from 35 μm to 657 μm. AGC was first generated in the lower-middle part of FZ where the residual welding strain was the highest. Moreover, fine grains with high-misorientation grain boundaries in FZ were also found to facilitate the grain boundary migration and the development of AGC. The detailed formation mechanism of AGC, as well as a physical model revealing the failure procedure of the repaired joints, were proposed. The repaired joint with neither residual eutectics nor AGC was successfully prepared by a combined solution treatment of 520 °C × 8 h before welding and 520 °C × 2 h after welding, and the T6-treated joint exhibited excellent comprehensive properties with the tensile strength of 314 MPa and elongation of 4.4%.

Minimum fusion zone size required to ensure pullout failure mode of resistance spot welds during tensile-shear test

Minimum fusion zone size required to ensure pullout failure mode of resistance spot welds during tensile-shear test

Study on the microstructure and mechanical performance for integrated resistance element welded aluminum alloy/press hardened steel joints

Resistance element welding (REW) is a recently developed hybrid joining process for aluminum (Al)/steel dissimilar materials. In this study, the microstructure and mechanical performance of the Al/press hardened steel (PHS) joints generated using an integrated REW process were investigated. The microstructure of fusion zone (FZ) between rivet and PHS was lath martensite, which was transformed from the fast cooling of austenite. The Al could be divided into four zones according to the microstructure and microhardness distribution: re-solidified zone (RZ), softening zone (SZ), transition zone (TZ) and base metal (BM). The dissolution and coarsening of the precipitates are responsible for the hardness reduction of Al sheet. The variation of mechanical performance was explained in light of the increasing FZ size and the softening Al sheet as heat input rising. Moreover, four block shear models were introduced to predict the peak load of REW joints using the average hardness of SZ in Al sheet, among which the models from Architectural Institute of Japan (AIJ) provided a relatively good prediction. Considering both the average hardness of SZ and sheet thickness, an analytical model was established to predict the variation of critical nugget sizes and explain the failure mode transition.

The microstructure and weldability in welded joints for AA 5356 aluminum alloy after adding modified trace amounts of Sc and Zr

Abstract Different AA5356 alloy filler metals containing various amounts of Sc and Zr are obtained and applied to join the 5083 aluminium plate with 2.8 mm thickness. A significant grain refinement can be observed in the fusion zone (FZ) of welded joints after the additions of Sc and Zr. The filler metal modified with 0.2 wt.% Zr+0.1 wt.% Sc obtained the smallest grain size with 29 μm, and the filler metal with 0.2 wt.% Sc+0.1 wt.% Zr additions got the similar grain size with 30 μm. The 0.2 wt.% Sc addition can effectively modify the microstructure of filler metal and improve the mechanical properties. The grain size of fusion zone for AA5356 + 0.2 wt.% Sc+0.1 wt.% Zr filler metal decreased by 75.8 % and the highest ultimate tensile strength (UTS) increases by 15 % compared with the unmodified alloy joints. However, individual 0.2 wt.% Zr addition cannot effectively modify the filler metal. The improvement of mechanical properties can attribute to the presence of refined grains.

Laser pressure welding of Al-Li alloy 2198: effect of welding parameters on fusion zone characteristics associated with mechanical properties

AbstractAl-Li alloy 2198 exhibits good combination of toughness and strength but its application is strongly limited by the poor weldability due to the formation of porosities during fusion-welding. This is the first study proposing and verifying a new approach to produce defect-free laser welds of poorly fusion-weldable Al-Li alloy 2198 with applied external pressure, i.e., feasibility of laser pressure welding to Al-Li alloy 2198 was examined. The microstructures associated with tensile shear behavior of laser pressure welded Al-Li alloy 2198 obtained at various welding parameters were analyzed. The results showed that formation of the welding defect in the weld could be successfully suppressed by applying laser pressure welding, even without shielding gas. Three microstructural zones, including the chill zone, the columnar zone and the equiaxed zone were observed in the fusion zone. Size of fusion zone and area fraction of porosities generally increased with increasing roller pressure and welding heat-input, and they dominantly affected the tensile shear behavior, including the peak load and the failure mode, of the weld.

Microstructure and mechanical properties of dissimilar nickel-based superalloys resistance spot welds

This paper addresses the factors affecting the mechanical properties of dissimilar resistance spot welds between Nimonic 263 and Hastelloy X nickel-based superalloys. The fusion zone (FZ) of the dissimilar weld displayed higher hardness values compared to the base metals which can be attributed to (i) the formation of Mo-rich carbide in the interdendritic zone due to positive segregation behavior of Mo and C during non-equilibrium solidification of the weld, (ii) the enhanced solid solution strengthening due to mixing of the base metals, and (iii) the formation of ultra-fine dendritic structure in the FZ due to ultra-fast cooling rate of the resistance spot welding. Formation of interdendritic carbides did not induce any brittleness in the FZ. The variation of weld heat input did not significantly influence the weld metallurgical attributes (e.g., chemical composition, dendrite arm spacing) and the hardness values of the FZ and heat affected zone. It is identified that controlling the weld physical attributes (i.e., fusion zone size and electrode indentation) is the key to achieve a dissimilar Nimonic 263/Hastelloy X resistance spot weld with adequate peak load and energy absorption and desirable failure mode (i.e., pullout mode).

Evaluation of Electrode Degradation and Projection Weld Strength in the Joining of Steel Nuts to Galvanized Advanced High Strength Steel

Abstract Projection welding of steel weld nuts to advanced high strength steel (AHSS) in automotive applications allows for the reliable mounting of critical components with different thicknesses to the vehicle body. However, the galvanized coatings commonly used on AHSS result in electrode surface degradation during welding. In this study, the electrode degradation and its effect on the mechanical properties of welded steel nuts and AHSS sheets are investigated. Two common electrode materials are tungsten/copper and beryllium-free class III copper—both display the formation of an oxidized alloy surface layer and pitting as weld number increases. Unlike resistance spot welding, where electrodes grow in the contact area diameter as they degrade, projection welding electrodes do not experience this type of mechanical degradation. Instead, increased resistance at the electrode interface with increasing weld number results in higher temperatures at the weld interface and a larger fusion zone size, which is responsible for an observed 30% increase in weld strength over the span of 10,000 welds.

Residual stresses and distortion in the patterned printing of titanium and nickel alloys

Since the deposition patterns affect the stresses and distortions, we examined their effects on multi-layer wire arc additive manufacturing (WAAM) of Ti-6Al-4V and Inconel 718 components experimentally and theoretically. We measured residual stresses by hole drilling method in three identical components printed using different deposition patterns. In order to understand the origin and the temporal evolution of residual stresses and distortion, we used a well-tested thermo-mechanical model after validating the computed results with experimental data for different deposition patterns. Distortions were also examined based on non-dimensional analysis.We show that printing with short track lengths can minimize residual stresses and distortion among the three patterns investigated for both alloys. Both Ti-6Al-4V and Inconel 718 had similar fusion zone shape and size and were equally susceptible to deformation and warping, although Ti-6Al-4V was relatively less vulnerable to delamination due to its higher yield strength. A dimensionless strain parameter accurately predicted the effects of WAAM parameters on distortion and this approach is especially useful when the detailed thermo-mechanical calculations cannot be undertaken.

Resistance spot welding of Nimonic 263 nickel-based superalloy: microstructure and mechanical properties

ABSTRACTThis paper addresses the metallurgical and mechanical response of Nimonic 263 nickel-based superalloy to resistance spot welding. Solidification structure of the fusion zone is described in terms of dendrite arm spacing, segregation behaviour and secondary carbide formation in the inter-dendritic area. The heat affected zone is featured by negligible grain growth and constitutional liquation of primary (Ti, Mo)C carbide present in the initial microstructure of the base metal. Mechanical behaviour of the welds was characterised by interfacial to pullout failure mode transition, peak load and energy absorption of the joints during the tensile-shear loading. The fusion zone size and the electrode indentation depth are the key factors controlling peak load and energy absorption of the Nimonic 263 resistance spot welds.

Different Ratios of Electron Beam Welding of Dissimilar Titanium Alloys: Ti-10V-2Fe-3Al and Ti-6Al-4V

A welding of dissimilar alloy gives an optimum combination of high strength of Ti-10V-2Fe3Al (Ti1023) and low cost of Ti-6Al-4V (Ti64). Three different fusion ratios of Ti1023 and Ti64 were resulting in a similar width size of the fusion zone (FZ) and heat affected zones (HAZ) whereas a thicker sample with the same fusion ratio generated a narrower size of FZ and HAZ. It resulted chemical composition in the FZ within a low β phase stability than in Ti1023, which allowed the precipitation of high hardness α” martensitic in the as weld condition. In the HAZ of Ti1023, αs transformed into β phase, which reduced the hardness and strength. Vickers hardness measurements confirmed the HAZ of Ti1023 was the softest area whereas the FZ was the hardest area. Consequently, in the as weld condition, the fracture occurred in the HAZ of TI1023 after the tensile test.

Microstructures and properties of single-pass laser-arc hybrid welded stainless clad steel plate

The effects of wire Cr content on the microstructures and properties of single-pass laser-arc hybrid welded stainless clad steel plate (SCSP) were studied. Increasing Cr content strengthened the weld corrosion resistance, but it increased the average grain size of fusion zone (FZ), deteriorated the weld microstructure uniformity, and weakened the weld mechanical property. The variations of the weld properties were closely related to the microstructure changes. The solidification mode and microstructure transformation mechanism of the SCSP hybrid weld were clarified based on the quantitative calculation of the Cr and Ni equivalent contents (Creq and Nieq) after the mixing of the wire and base metal. The calculated ratio of the Creq and Nieq (θ) could effectively predict the solidification mode and microstructure of the SCSP weld, and then indicate its properties. The skeletal and acicular ferrites formed in the FZ when the θ was larger than 1.35, which benefited to strengthen the SCSP weld.

Study of Fusion Thickness of Tin Solder Heating by Self-Propagating Exothermic Reaction

Heat released from aluminum-nickel (Al/Ni) self-propagating exothermic reaction has been developed as a rapid heating source and is called a localized heating source due to its short duration and narrow heat affected zone (HAZ). It is suitable for the interconnection of thermal mismatch material couples, temperature-sensitive devices, microbial systems and microanalysis systems. However, the fusion thickness of the solder layer has not been deeply studied to minimize the HAZ. In this study, Sn was electroplated on the AlNi nanofoil. Experimental and numerical methods were used to evaluate the heating capability of AlNi nanofoil through analyzing the size of fusion zone and HAZ within the Sn solder–AlNi nanofoil–Sn solder sandwich structure. Finite element analysis (FEA) was used for simulating the temperature field and temperature history of the structure. Different thicknesses of tin layer were considered to analyze the effect of the change of structures. The temperature of the surface of the tin layer and fusion zone was measured to validate the FEA model. Furthermore, the analysis showed that the maximum thickness for melting the Sn layer within the substrate–Sn solder–Al/Ni nanofoil–Sn solder–substrate sandwich structure changes with the thickness of substrates and preheating temperature. FEA predictions indicated that a preheating temperature of 125°C is effective to bond Cu substrates at the thickness of 0.3 mm with tin solder.

Failure Study of Two Dissimilar Steels Joined by Spot Welding Technique

Resistance spot welding (RSW) process is of paramount importance in automotive industry for the fabrication of metallic components. Several dissimilar alloys could easily be joined by resistance spot welding. However, the joining of the stainless steel and galvanized carbon steel is challenging task since weld fusion zone properties are affected significantly. Indeed, the reliability of the component lies in the sound quality of spot weld. The overload failure mode of the weld zone was determined by preparing lap-shear specimens and then carrying out tensile-shear test. Microstructures and hardness of the weld nuggets were also brought under considerations. It was found that weld nugget size and strength of that sheet material which has lower electrical resistance are the controlling factors of the failure mode. The aim of this study was to find out the causes of spot welds failure in terms of parameters favoring the pull-out failure mode, role of fusion zone size (FZS), nugget and base metal by controlling the process parameters.

Mechanical properties of martensitic stainless steel weld/adhesive hybrid bonds

ABSTRACTHybrid weld-adhesive bonding (AB) was used as an approach to improve the mechanical properties of martensitic stainless steel resistance spot welds. The weld fusion zone in both techniques was composed of predominantly martensite and δ-ferrite. It was found that the presence of adhesive does not induce excess hardening effect in the fusion zone. It was shown that hybrid AB/resistance spot welding) i.e. weld-bonding) technique can double the peak load and energy absorption of the joints compared to those of the resistance spot welds. To obtain high performance weld-bonded joints, heat generation during welding should be controlled to produce weld bonds with large fusion zone size, but without detrimental effect on the adhesive strength.