Showing papers on "Spot welding published in 2007"

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.

Nanorobotic spot welding: controlled metal deposition with attogram precision from copper-filled carbon nanotubes.

Abstract: With the continuing development of bottom-up nanotechnology fabrication processes, spot welding can play a role similar to its macro counterpart for the interconnection of nano building blocks for the assembly of nanoelectronics and nanoelectromechanical systems (NEMS). Spot welding using single-crystalline copper-filled carbon nanotubes (CNTs) is investigated experimentally using nanorobotic manipulation inside a transmission electron microscope (TEM). Controlled melting and flowing of copper inside nanotube shells are realized by applying bias voltages between 1.5 and 2.5 V. The average mass flow rate of the copper was found to be 120 ag/s according to TEM video imaging (measured visually at approximately 11.6 nm/s through the CNT). Successful soldering of a copper-filled CNT onto another CNT shows promise for nano spot welding and thermal dip-pen lithography.

Effect of tool geometry on microstructure and static strength in friction stir spot welded aluminium alloys

Abstract: The effect of tool geometry on microstructure and static strength in friction stir spot welds of 6061 aluminium alloy sheets was studied. Tools with three different probe lengths were used to join the aluminium sheet with different tool rotational speeds and tool holding times. The weld microstructures varied significantly depending on probe length, tool rotational speed and tool holding time. Two particular aspects were identified: the thickness of the upper sheet under the shoulder indentation and the nugget size. The former decreased with increasing probe length at the shortest tool holding time and the slowest tool rotational speed, but there were no discernible differences in other welding conditions, while the latter increased with increasing probe length, tool rotational speed and tool holding time. The tensile shear strength increased with increasing probe length, while the cross-tension strength was not affected significantly by probe length. Two fracture modes were observed: shear fracture of the nugget and mixed mode fracture under tensile shear loading, and nugget debonding and pull-out under cross-tension loading. Based on experimental observation of the microstructures, the effect of probe length on static strength and the fracture mechanisms were discussed.

Effects of fusion zone size and failure mode on peak load and energy absorption of advanced high-strength steel spot welds

Abstract: This paper examines the effects of fusion zone size on failure modes, static strength and energy absorption of resistance spot welds (RSW) of advanced high strength steels (AHSS). DP800 and TRIP800 spot welds are considered. The main failure modes for spot welds are nugget pullout and interfacial fracture. Partial interfacial fracture is also observed. The critical fusion zone sizes to ensure nugget pull-out failure mode are developed for both DP800 and TRIP800 using limit load based analytical model and micro-hardness measurements of the weld cross sections. Static weld strength tests using cross tension samples were performed on the joint populations with controlled fusion zone sizes. The resulted peak load and energy absorption levels associated with each failure mode were studied for all the weld populations using statistical data analysis tools. The results in this study show that AHSS spot welds with fusion zone size of can not produce nugget pullout mode for both the DP800 and TRIP800 materials examined. The critical fusion zone size for nugget pullout shall be derived for individual materials based on different base metal properties as well as different heat affected zone (HAZ) and weld properties resulted from different welding parameters.

Intermixing in Dissimilar Friction Stir Spot Welds

Abstract: Dissimilar intermixing during friction stir spot welding of Al 5754 and Al 6111 sheets is investigated using a combination of experimentation and numerical modeling. Dissimilar intermixing is not observed in (a) spot welds made using a tool having a smooth pin with or without a dwell period and (b) spot welds made using a threaded tool without the application of a dwell period. It is proposed that dissimilar intermixing during the dwell period in spot welding results from the incorporation of upper (Al 5754) and lower (Al 6111) sheet materials at the top of the thread on the rotating pin. A ribbon of contiguous Al 5754 and Al 6111 lamella is moved downward via the pin thread as the tool rotates during the dwell period in spot welding. A helical vertical rotational flow is created during the dwell period in spot welding because the ribbon of contiguous Al 5754 and Al 6111 lamellae discharged from the bottom of the pin thread moves outward and upward before moving back toward the tool periphery and downward again.

Resistance and friction stir spot welding of DP600: a comparative study

Abstract: Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Friction stir spot welding (FSSW) was invented as a novel method to spot welding sheet metal and has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn coated DP600 AHSS (1 . 2 mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process. Results show a correlation found among microstructure, failure loads, energy requirements and bonded area for both spot welding processes.

The Effect of Anvil Geometry and Welding Energy on Microstructures in Ultrasonic Spot Welds of AA6111-T4

Abstract: The formation of ultrasonic spot welds of AA6111-T4 has been investigated using a single-transducer unidirectional wedge-reed welder. The evolution of weld microstructures and weld strength due to anvil cap geometry and welding energy was studied. The variations in lap-shear failure load and weld microstructures as a function of welding energy were only slightly influenced by the changes in the anvil cap geometry. Weld failure in lap-shear tensile tests occurs by interface fracture for low energy welds and by button formation for high energy welds. Initially, microwelds or weld islands several microns in diameter are generated presumably at asperities of the two pieces being joined. As the welding energy increases, the weld interface can change from a planar to a wavy morphology and the weld strength increases. Deformation wakes and bifurcation are ubiquitous in strong welds. Microporosity is observed at the periphery of growing weld islands and along the flow lines associated with the wavy deformation microstructures. Grain growth occurs inside the weld zone after isothermal annealing. However, the porous microstructure at the weld interface is not affected by isothermal annealing. Ultrasonic spot welding of AA6111-T4 aluminum was found to be insensitive to variations in anvil cap size and the knurl patterns investigated in this research.

Effect of processing parameters on static strength of dissimilar friction stir spot welds between different aluminium alloys

Abstract: The effect of processing parameters on static strength and fracture mechanisms of dissimilar friction stir spot welds between different aluminium alloys was studied. Tensile shear strength increased with decreasing tool rotational speed and increasing tool holding time, while cross tension strength decreased with increasing both parameters. Two fracture modes were observed: nugget shear fracture and mixed mode fracture under tensile shear loading, and debonding and pull-out of the nugget under cross tension loading. Based on experimental observation, the dependence of static strength on processing parameters and the occurrence of different fracture modes were discussed.

Strain rates and grain growth in Al 5754 and Al 6061 friction stir spot welds

Abstract: The stir zone temperature and microstructures are compared in friction stir spot welds produced in Al 5754 and Al 6061 alloys Electron backscattered diffraction was used to determine the relationship between tool rotation speed during welding and final stir zone grain size Comparison of the grain sizes in rapidly quenched welds with those in air-cooled joints confirmed that grain growth occurred only in Al 6061 spot welds There was no evidence of abnormal grain growth in the stir zones of Al 6061 welds; the final grain size could be represented using an Arrhenius equation The strain rates during welding were determined by incorporating the stir zone temperature and average subgrain sizes in quenched spot welds in the Zener–Hollomon relation When the tool rotation speed increased from 750 to 3000 RPM, the strain rate values ranged from 180 to 497 s−1 in Al 5754 spot welds and from 55 to 395 s−1 in Al 6061 spot welds It is suggested that a no-slip boundary condition may be appropriate during numerical modeling of Al 5754 and 6061 friction stir spot welding This is not the case during Al 7075, Al 2024, and Mg-alloy AZ91 spot welding because spontaneous melting facilitates slippage at the tool contact interface

Effect of welding parameters on the strain rate and microstructure of friction stir spot welded 2024 aluminum alloy

Abstract: The stir zone microstructure, crystallographic texture, temperature and strain rate in the stir zones produced during Al 2024 spot welding using different tool rotational speed settings are investigated. The calculated strain rate during spot welding decreases from 1600 to 0.6 s−1 when the tool rotational speed increases from 750 to 3000 rpm. The low strain rate values are associated with tool slippage resulting from spontaneous melting of S phase particles at temperatures ≥490 °C. However, the calculated strain rate is 1600 s−1 in Al 2024 spot welds made using tool rotational speed of 750 rpm since the temperature never reaches 490 °C. Material transfers downwards via that pin thread during the dwell period in Al 2024 spot welding. It is proposed that this downward transfer of material provides a continuous supply of undissolved S phase particles, which melt spontaneously when the welding parameter settings produce stir zone temperatures ≥490 °C. A weak crystallographic texture where the {100} planes are oriented at about 45° to the θ-direction exists in the stir zones of spot welds made using different tool rotational speeds (from 750 to 3000 rpm). Another crystallographic texture where the {100} planes are parallel to the Z-direction (to the tool axis) is stronger in spot welds made using higher tool rotational speed settings. Also, material located at the root of the pin thread has a quite different crystallographic texture from that in the bulk of the stir zone.

Numerical analysis of magnetic fluid dynamics behaviors during resistance spot welding

Abstract: Magnetic fluid dynamic behaviors during resistance spot welding have important influences on weld nugget formation. In this paper, a coupled multiphysics finite element model, which realizes the coupling of mass conservation, momentum conservation, energy conservation, and the Maxwell system of equations, is proposed to systematically investigate the fluid flow and heat transfer behaviors in a spot weld nugget. It is found that the flow field in the spot weld nugget evolves through five consecutive stages: incubation, fluctuation, rapid growth, rapid attenuation, and termination. Molten metal in the nugget makes regular rotational flow in four symmetrical loops, which changes the temperature gradient in weld nugget during heating and holding, and also yields significant effects on the shape and thickness of weld nugget.

Joint product between steel product and aluminum material, spot welding method for the joint product, and electrode chip for use in the joint product

Abstract: This invention provides a joint product between a steel product and an aluminum material, which can realize spot welding with high bonding strength, and a spot welding method for the joint product. In one embodiment, a steel product (1) having a sheet thickness t1 of 0.3 to 3.0 mm is joined to an aluminum material (2) having a sheet thickness t2 of 0.5 to 4.0 mm by spot welding to form a joint product between the steel product and the aluminum material. The joint product is characterized in that the nugget area in the joint is 20 × t20.5 to 100 × t20.5 mm2, the area of a part where the thickness of an interface reaction layer is 0.5 to 3 µm, is not less than 10 × t20.5 mm2, and the difference in thickness of the interface reaction layer between the joint center and a point distant from the joint center by a distance of one-fourth of the joint diameter DC is not more than 5 μm. The above constitution can provide a high-bonding strength joint product, between dissimilar materials, which can be formed without newly using other materials such as clad materials and without newly adding a separate step using an existing spot welding apparatus at low cost, and a spot welding method.

Resistance Spot Weldability of High Strength Steel (HSS) Sheets for Automobiles

Abstract: Resistance spot welding technologies of high strength steel (HSS) sheets were investigated in order to get high reliability in welded-joints of automobile components. Suitable welding current range shifted to lower currant side by the effect of electric resistance increase of steel sheet and the width of this range was affected by electrode force. Vickers hardness at welded zone increased with increase of C content, mainly. Tensile shear strength (TSS) of welded-joints increased with increase of nugget diameter, sheet thickness and base steel strength. Cross tension strength (CTS) increased with increase of nugget diameter, sheet thickness, however, showed peak for the base steel strength and carbon equivalent. Fatigue strength of welded-joints increased with increase of sheet thickness, however, it didn’t increase with increase of base steel strength. Suitable welding current range of galvannealed HSS sheets shifted to higher current side by the effect of melting of coated material. Electrode tip life of these coated HSS sheets were over 3,000 points and these were practical use level.

Cracking in the stir zones of Mg-alloy friction stir spot welds

Abstract: Liquid penetration induced (LPI) cracking is investigated during friction stir spot weld of AZ91, AZ31 and AM60 magnesium alloys. A combination of stir zone temperature measurement and detailed metallography has revealed differences in the cracking tendencies of different magnesium alloys when the dwell time during spot welding is varied. LPI cracking in AZ91 spot welds involves the following sequence of events: the formation of \(\alpha-\hbox{Mg}\,+\,\hbox{Mg}_{17}\hbox{Al}_{12}\) eutectic films in the thermo-mechanically affected zone (TMAZ) region immediately adjacent to the stir zone extremity, engulfment of melted eutectic films as the stir zone width increases during the dwell period, penetration of α−Mg grain boundaries and crack propagation when torque is applied by the rotating tool. Cracking occurs early in the dwell period during AZ91 spot welding and almost the entire stir zone is removed when the rotating tool is withdrawn. However, crack-free AZ31 and AM60 spot welds are produced when a dwell time of 4 s is used since the stir zone temperatures are much higher than the α-Mg + Mg17Al12 eutectic temperature (437 °C) and melted eutectic films dissolve rapidly following their engulfment by the growing stir zone. In contrast, the temperature during the dwell period in AZ91 spot welding is close to 437 °C and melted eutectic films are not completely dissolved so that spot welds produced using a dwell time of 4 s exhibit LPI cracking.

Local melting and cracking in Al 7075-T6 and Al 2024-T3 friction stir spot welds

Abstract: Local melting of eutectic films and cracking is found in Al 2024 and Al 7075 alloy friction stir spot welds. Dissolution of melted films removes all evidence melted film formation in spot welds made using typical welding parameter settings. For this reason friction stir spot welding is carried out at a rapid plunge rate of 10 mm s−1 and an extremely short dwell time of 0·05 s and after tool retraction, the welded samples are rapidly quenched using a mixture of methanol and liquid nitrogen at a temperature of −80°C. Eutectic films rich in Zn and Cu are formed in Al 7075 spot welds while melted Al2CuMg particles promote the formation of α-Al+Al2CuMg eutectic films in Al 2024 spot welds. Melted eutectic formation and cracking is also observed beneath the tip of the rotating pin during Al 7075 friction stir spot welding and is consistent with the occurrence of melt wear in this location.

Quality evaluation in resistance spot welding by analysing the weld fingerprint on metal bands by computer vision

Abstract: This paper introduces a novel method for the quality evaluation of resistance spot welds. The evaluation is based on computer vision methods, which allow non-destructive on-line real-time processing. The input of the system is the image of a weld imprint on a metal band which covers the electrodes against wear and soiling. The shape and size of the structures within the imprint correlate with the nugget area and, therefore, allow an accurate estimation of the quality of the spot weld. The system segments the electrode imprint and computes the nugget area from the minimum and maximum axis of a fitted ellipse. This method does not need training samples to perform reliable quality estimation. Additionally, the used algorithms are easy to implement and efficient, which guarantees real-time ability. Since there is only a single low-cost camera needed, the hardware can be placed directly on the gun arm, which makes a fast evaluation possible.

Resistance Spot Weldability of High Strength Steel Sheets for Automobiles and the Quality Assurance of Joints

Abstract: Resistance spot weldability of high strength steel (HSS) sheets for automobiles and the quality assurance technique of joints were investigated. The suitable welding current range in spot welding of HSS sheets shifted to the lower current side compared with that of mild steel sheets and it was affected by the electrode force. Vickers hardness at the welded zone increased with an increase of base steel carbon content. Tensile shear strength (TSS) of joints increased with an increase of nugget diameter, sheet thickness, and base steel strength. The cross tension strength (CTS) of joints increased with an increase of nugget diameter and sheet thickness. However, it showed a peak for base steel strength and carbon equivalent. Fatigue strength didn’t increase with base steel strength. The suitable welding current range in spot welding of galvannealed HSS sheets shifted to the higher current side compared with that of bare HSS sheets. Electrode tip life for galvannealed HSS sheets was over 3 000 points and it was a practical use level. It’s possible to assure the joint strength by estimating the nugget diameter using current and voltage during spot welding.