M. Goodarzi

Effect of weld nugget size on overload failure mode of resistance spot welds

Abstract In the present paper, effects of welding current, welding time, electrode pressure and holding time on the weld nugget size were studied. A failure mechanism was proposed to describe both interfacial and pullout failure modes. This mechanism was confirmed by SEM investigations. In the light of this mechanism, the effect of welding parameters on static weld strength and failure mode was studied. Then, an analytical model was proposed to predict failure mode and to estimate minimum nugget diameter (critical diameter) to ensure pullout failure mode in shear tensile test. On the contrary to existing industrial standards, in this model, critical nugget diameter is attributed to metallurgical characterisation of material (weld nugget hardness to failure location hardness ratio), in addition to sheet thickness. For a given sheet thickness, decreasing HWN/HFL increases interfacial failure mode tendency. The results of this model were compared with experimental data and also with the literature.

Effect of expulsion on peak load and energy absorption of low carbon steel resistance spot welds

Abstract The effects of weld nugget size and expulsion on the performance of low carbon steel resistance spot weld have been investigated in the present paper. Failure mode, peak load and failure energy obtained in tensile–shear test have been used to describe the performance of spot weld. The influence of voids and porosity as well as electrode indentation associated with expulsion on peak load and failure energy is discussed. The results showed that although expulsion does not reduce the load carrying capacity of spot welds, it decreases their energy absorption capability which was attributed to the change of failure location due to excessive electrode indentation associated with expulsion.

Microstructure and failure behaviour of resistance spot welded DP980 dual phase steel

Abstract In this research, microstructure and overload failure behaviour of resistance spot welded DP980 were investigated. Microstructural characterisation, microhardness test and static tensile shear test were conducted. Fusion zone size proved to be the most important controlling factor of spot weld peak load and energy absorption. The results of this study demonstrated that the conventional weld size recommendation of d=4t1/2 is not sufficient to ensure the pullout failure mode for DP980 steel resistance spot welds during the tensile shear test. In pullout mode, generally, failure was initiated at heat affected zone/base metal interface, where softening occurs due to the tempering of martensite. However, when heavy expulsion occurs, pullout failure tends to be initiated at fusion zone/heat affected zone interface. It was shown that heavy expulsion and associated large electrode indentation can reduce load carrying capacity and energy absorption capability of DP980 spot welds.

Relationship between failure behaviour and weld fusion zone attributes of austenitic stainless steel resistance spot welds

Abstract Resistance spot welding was used to join austenitic stainless steel sheets. Mechanical properties of the spot welds were evaluated using tensile shear test. Mechanical behaviour was described by peak load, failure energy and failure mode. The relationship between weld fusion zone attributes and failure behaviour was studied. Generally, it was observed that increasing fusion zone size is accompanied by an increase in load carrying capacity and energy absorption capability. However, when expulsion occurs, despite almost constant weld fusion zone size, energy absorption capability reduces significantly due to increase in electrode indentation depth. Considering the failure location and failure mechanism in the tensile shear test, minimum required fusion zone size to ensure the pull-out failure mode was estimated using an analytical model. According to this model, in addition to sheet thickness, ratio of fusion zone hardness to failure location hardness is the key metallurgical factor governing failure mode of spot welds during the tensile shear test.

Mechanochemical synthesis of alumina–zirconia nanocomposite powder

Abstract The alumina–zirconia nanocomposite powder has been synthesised by mechanical activation of a dry powder mixture of AlCl3, ZrCl4 and CaO. Mechanical milling of the above raw materials with the conditions adopted in this study resulted in the formation of a mixture consisting of crystalline CaO and amorphous aluminium and zirconium chlorides phases. There was no sign of chemical reaction occurring during milling stage as evidenced by X-ray diffraction (XRD) studies. Subsequent heat treatment of the milled powder at 350°C resulted in the occurrence of displacement reaction and the formation of ZrO2 and Al2O3 particles within a water soluble CaCl2 matrix. The effect of higher temperature calcination on the phase development in this powder mixture was followed by XRD analysis. Scanning electron microscopy and differential thermal analysis were also used in the characterisation of the powders. Perhaps the most important observation in this study was the formation of α-Al2O3 phase at a very low temperature of 400°C.

Effects of welding parameters on weld pool characteristics and shape in hybrid laser-TIG welding of AA6082 aluminum alloy: numerical and experimental studies

The effects of three important welding parameters including laser power, welding current and welding speed on the weld pool characteristics, shape and dimensions in hybrid laser-TIG welding of AA6082 aluminum alloy are studied by numerical, experimental, and statistical approaches. For this aim, first, a 3D numerical model is used to simulate heat transfer and fluid flow in the weld pool and then resultant weld shape for various welding conditions. Besides, a set of experiments are performed to validate and calibrate the model. Finally, analysis of variance (ANOVA) method is applied to investigate more precisely how welding parameters affect weld dimensions. The simulation results show with increasing the laser power and welding current and decreasing the welding speed, the Marangoni and buoyancy forces increase. With increasing the laser power, the weld depth increases more significantly than the weld width. The weld half width increases with increasing the welding current, whereas the weld pool depth is relatively unchanged. Furthermore, with increasing the welding speed, both weld pool depth and half width decrease with similar slope. Generally, the presented model showed a good capability to predict the weld geometry and characteristics under various applied welding conditions which can reduce number of needed experiments.

Comparative study on dissimilar welding of Inconel 718 and Udimet 500 precipitation strengthened nickel based superalloys

Abstract Microstructural, mechanical and weldability aspects in the similar and dissimilar welds of alloy 718 and alloy 500 nickel based superalloys have been investigated. Alloy 500 weld metal showed high tendency of titanium to the segregation. Coalescence of the microvoids led to propagation of hot solidification microfissures. The alloy 718 weld metal displayed the formation of Nb rich low melting eutectic type morphologies, which can reduce the weldability. The microstructure of dissimilar weld metal with dilution of 65 wt-% displayed semideveloped dendritic boundaries. The less segregation and decrease in the low melting eutectics caused less susceptibility of dissimilar weld to solidification cracking. The segregation elimination phenomenon has occurred in the heat affected zone of alloy 500. In the partially melted zone, remelted and resolidified regions have been observed. These locations provided sites for nucleation of liquation cracks. For the alloy 718 heat affected zone, dissolution of γ″-Ni3Nb needle-like precipitations has taken place. It was the chief reason for sharp decline of the microhardness. The heat affected zone of alloy 500 revealed intense liquation cracking, in which the crack is initiated at the partially melted zone. The hot liquation cracking in the heat affected zone of Alloy 718 was observed as a result of γ″-Ni3Nb dissolution.

Nanocrystalline growth activation energy of alumina polymorphs synthesised by mechanochemical technique

Abstract Al2O3 nanopowders were synthesised via mechanochemical method using AlCl3 and CaO as raw materials. The effect of thermal treatment on the structural evolutions and morphological characteristics of the nanopowders was investigated using X-ray diffractometry, transmission electron microscopy, scanning electron microscopy, differential thermal analysis and Rietveld refinement. The results showed that the average crystallite size of Al2O3 was <100 nm up to ∼1200°C. The activation energy for Al2O3 nanocrystallite growth during calcinations was calculated to be ∼22 598 and 30 195 J mol−1 for η- and κ-alumina respectively, while for α-Al2O3, it was 8373 and 34 131 J mol−1 at temperatures up to 1200°C and >1200°C respectively. The mechanism of nanocrystalline growth of Al2O3 polymorphs during annealing is also discussed.

Data Fusion of Non Destructive Testing for Detection of Defects in Welding

In this paper the specimens of Aluminum 2024 with 5 millimeter in thickness are joined together by friction stir welding with travel speed of 100 mm/min and tool rotational speeds of 450, 900 and 1800 rpm and a tool were made of hot working steel, H13, firstly. Thus three kinds of welds are produced. Radiography and ultrasonic (UT) non-destructive testing (NDT) procedure were applied to characterize the presence and geometry of possible weld defects prior to mechanical destructive testing. A Echograph Model 1090 digital UT instrumentation and a 4 MHz angle beam probe (refraction angle α = 70°) was used for C-scan of UT contact testing of welded samples (transverse UT velocity 2850 m/s and signal amplification 40 dB). The detection accuracy of defects can be improved by image fusion of ultrasonic and radiography data. For this reason, the data of the two sensors are transformed into a same scale images (length, width and also depth). Pixel by pixel image fusion is used for fusion and analysis. Comparing these results with the destructed part shows that the fusion of two tests improves the results.Copyright

Influence of shielding gas on the mechanical and metallurgical properties of DP-GMA-welded 5083-H321 aluminum alloy

In the present research, 6-mm-thick 5083-H321 aluminum alloy was joined by the double-pulsed gas metal arc welding (DP-GMAW) process. The objective was to investigate the influence of the shielding gas composition on the microstructure and properties of GMA welds. A macrostructural study indicated that the addition of nitrogen and oxygen to the argon shielding gas resulted in better weld penetration. Furthermore, the tensile strength and bending strength of the welds were improved when oxygen and nitrogen (at concentrations as high as approximately 0.1vol%) were added to the shielding gas; however, these properties were adversely affected when the oxygen and nitrogen contents were increased further. This behavior was attributed to the formation of excessive brown and black oxide films on the bead surface, the formation of intermetallic compounds in the weld metal, and the formation of thicker oxide layers on the bead surface with increasing nitrogen and oxygen contents in the argon-based shielding gas. Analysis by energy-dispersive X-ray spectroscopy revealed that most of these compounds are nitrides or oxides.