No 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.

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.

The peak temperatures during friction stir spot welding of similar and dissimilar aluminium and magnesium alloys are investigated. The peak temperatures attained during friction stir spot welding of Al 6111, Al 2024, and AZ91 are within 6% of their solidus temperatures. In dissimilar AZ91/Al 6111 spot welds the peak temperature corresponds with the α-Mg solid solution and Mg17Al12 eutectic temperature of 437°C. An a-Mg plus Mg17Al12 eutectic microstructure is produced in dissimilar friction stir spot welds when material displaced during pin penetration into the lower sheet material contacts the upper sheet material at the eutectic temperature.

Energy utilisation during spot welding is investigated using a combination of calorimetry, peak temperature measurement and plunge testing. When a steel tool, clamp and anvil support is used, only 12 . 6% of the energy generated during the spot welding is transferred into the welded Al 6111 sheets. In contrast, when a mica clamp and anvil support are used, 50% of the energy generated during spot welding transfers into the welded Al 6111 sheets. Only a small percentage of the energy generated during the friction stir spot welding operation is required for stir zone formation. During plunge testing of 6 . 3 mm thick Al 6061-T6 material, less than 4 . 03% of the energy which is generated during friction stir spot welding is required for stir zone formation. The remainder of the energy generated dissipates into the tool assembly, clamp, anvil support and the aluminium sheets which are being welded. The rotating pin produces around 70% of the energy generated during spot welding of 6 . 3 mm thick Al-6061 material, with the remainder being contributed by the tool shoulder.

Material flow and intermixing during dissimilar friction stir spot welding and friction stir seam welding are investigated. During friction stir spot welding, a ribbon of contiguous dissimilar lamellae is produced during each rotation of the tool and the number of intermingled lamellae contained in the intermixed region is determined by the tool rotational speed setting and the dwell time applied. When the rotating tool moves across the component, the ribbon of dissimilar contiguous lamellae continues to be produced and the linear distance in the traversing direction between dissimilar lamellae corresponds with the pitch distance [the travel speed (mm s 21 ) divided by the tool rotational speed (Hz)]. The material flow pattern produced when a threaded tool moves across a component is therefore a variant of that produced during the touch down period when the rotating tool is held stationary. It is suggested that the onion ring structures observed in similar and dissimilar friction stir seam welds made using threaded tools are produced by material incorporation from the locations beneath the tool shoulder and the bottom of the rotating pin and the creation of a helical vertical rotational flow within the intermixed region formed beside the periphery of the rotating pin.

No abstract

Material flow during friction stir spot welding is investigated. An examination of dissimilar Al 5754/ Al 6111 spot welds was conducted to allow visualisation of material flow based on their differing etching characteristics. In addition, Al 6061-T6 spot welds containing Al 2 O 3 tracer particles were examined to highlight the movement of material in different joint regions. It has been confirmed that upper sheet material is moved downwards into the lower sheet when a layer of upper sheet material is pushed ahead at the tip of the rotating pin, when upper sheet material becomes trapped at the root of the pin thread, and when an adhering layer of upper sheet material forms at the pin periphery during spot welding using a smooth pin. Lower sheet material is displaced upwards and outwards in a spiral motion when the rotating pin forms the keyhole. Two distinct zones of material flow are produced during friction stir spot welding: an inner flow zone close to the pin periphery where upper sheet material moves downwards in an anticlockwise direction with the rotating pin; and an outer flow zone where lower sheet material moves upwards and outwards in a spiral motion.