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An innovative overlap joint concept was tested to evaluate the quality improvement of welds between aluminum alloy AA5754-H22 (2 mm) and steel DX54 (1.5 mm). The innovation is a wave-shaped interface produced on the steel being directly processed by the tip of the probe, generating localized heat, extensive chemically active surfaces, and additional mechanical interlocking. Welds with different parameters were evaluated by metallographic analysis and mechanical tests. The best set of parameters was then implemented in a conventional overlap joint, plus in two-and three-passes welding, with the innovative overlap joint concept, to evaluate the effect on microstructure and mechanical efficiency. With a single-pass weld, the new concept presented lower strength in tensile shear tests, but higher strength in peeling tests. The main mechanism governing this behavior was the reduction of effective thickness in the aluminum alloy sheet, due to the flow of steel into the aluminum alloy. The characterization and distribution of the intermetallic compounds were evaluated via SEM-EDX. The two-passes weld resulted in the best strength values in tensile shear tests, reaching about 50% of the ultimate tensile strength of the aluminum alloy base material.

The weldability of high-strength steels (HSSs) is limited by their loss of strength, toughness and fatigue properties. In demanding applications, the fatigue properties of welds are among the strictest requirements. This paper presents a weldability analysis focusing on the microstructure and fatigue properties of 6 mm thick S690 and S355 HSS plates joined by friction stir welding. Their fatigue properties are compared to design recommendations and to fatigue properties obtained with other welding techniques. Results show that the high-quality friction stir welded steel joints outperform high-quality arc welds and FAT80 design recommendations. The fatigue strength of the friction stir welded joints is increased with material yield strength. The mechanisms governing crack initiation at different maximum stress levels are discussed. ARTICLE HISTORY

The need for weight reduction without compromising load-bearing capacity has driven the development of modern high-strength steels (HSS). The ability to take full advantage of these modern HSS is limited by their weldability. Friction stir welding (FSW) can join HSS at low peak temperatures, better preserving the properties of the base material. This work evaluates the influence of different FSW peak temperatures and cooling rates on the properties of a modern HSS. A HSS produced by a thermomechanically controlled process was welded by FSW with peak temperatures, measured within the processed zone, ranging from about 900 to 650°C. Temperatures were measured using thermocouples positioned at the mid-thickness of the workpiece, at 5 to 10 mm from the joint line. The effect of the different peak temperatures was evaluated by mechanical testing, including bending, tensile testing with digital imaging correlation, Charpy impact test, and hardness measurements. Microscopic analyses, including optical microscopy and electron backscatter diffraction, were used to evaluate the microstructure. Results show that FSW enables welding HSS within the intercritical temperature domain. The impact toughness of the best weld condition overmatched the base material by over 37%, at both − 40 and − 60°C. The yield strength efficiency was about 70%.

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