Rudolf Vallant

Friction Stir Spot Welds between Aluminium and Steel Automotive Sheets: Influence of Welding Parameters on Mechanical Properties and Microstructure

Hybrid configurations between aluminium and steel are needed to meet today’s requirements for lightweight construction in the automotive industry. Different studies showed that Friction Stir Welding (FSW) as well as Friction Stir Spot Welding (FSSW) processes are suitable for joining aluminium to steel. In this work, dissimilar FSSW of aluminium AA5754 and galvanised steel HX 340LAD were examined. In particular the influence of different spindle speeds and dwell times on microstructure and the mechanical properties of the weld were analysed. In doing so, the cross-section microstructure of the weld interface was observed by light optical microscope (LOM) and scanning electron microscope (SEM). The strength of the welds was evaluated both by tensile shear and vibration fatigue tests. The influences of the individual parameters on the weld are presented in detail. The appearance of intermetallic phases (IMPs), a severe problem for conventional fusion welding processes between aluminium and steel, were investigated for the welded samples and a link to the mechanical properties is given.

Influence of the Soft Zone on The Strength of Welded Modern Hsla Steels

The objective of this study was to investigate the influence of softening in the heat-affected zone of welds and the constraint effect on the transverse tensile strength of a microalloyed, thermomechanically controlled processed (TMCP) high-strength steel grade. The welding was performed with three different levels of energy input to establish soft zones with varied extensions to investigate the dimension of softening and to determine the constraint effect on various widths of the soft zone. The results showed that the tensile strength was not significantly compromised by softening in the HAZ. The reason for this is the constraint effect of the base metal and the high strength of the weld metal. In conclusion low heat input welding processes keep the soft zone small and the strength high.

Evaluation of the factors influencing the strength of HSLA steel weld joint with softened HAZ

Softening in the heat-affected zone (HAZ) may occur at welding of high-strength low-alloy steels and might have a negative effect on the static strength of welded joints. This study investigates the influence of HAZ softening in combination with different filler metal strength classes, weld seams, and hot rolled strips on static performance of a GMA-welded joints. The results of these welding experiments were compared and discussed with a numerical investigation on factors influencing the static strength of HSLA steel welds. At the numerical investigation, a systematical study on potential strength influencing factors and a Pareto analysis based on a multiple regression was done and the potential strength influencing factors were ranked according their degree of influence. The comparison of the results of this experimental and numerical study showed a good accordance.

Vibration Stress Relief Treatment of welded high-strength martensitic steel

The influence of a vibration stress relief (VSR) treatment on the residual stresses for welded specimens of ultra high-strength steel (ARMOX 500T®) was investigated. For the residual stress measurements X-ray diffraction and magnetic Barkhausen noise analysis (BNA) were utilized. Before and after VSR treatment, a 3D measurement technique was used to control possible deformations. Only small changes in macroscopic residual stresses could be found at the surface of the specimens close to the weld. The BNA measurements showed changes in macroscopic residual stresses; however, no clear tendencies could be seen. Nevertheless, fatigue damage due to the VSR treatment could be found. The changes in residual stresses did not cause macroscopic deformations of the specimens.

Electron Beam Welding Of Atmcp Steel With 700 Mpa Yield Strength

This study investigates the influence of the soft zone on the static tensile strength of welded joints. A micro-alloyed, thermo-mechanically controlled processed (TMCP) high strength low alloy steel grade (HSLA) was joined with electron beam welding at different energy input levels. The welded joints were investigated by means of light optical microscopy, FE-SEM (field emission scanning electron microscopy) analyse, hardness measurement and quasi- static tensile test. The results of this investigation were compared with the mechanical properties of GMAW welded joints of same base metal.

Welding of S960MC with undermatching filler material

High strength structural steels are in high demand thanks to their favorable mechanical properties. They offer high strength with sufficient toughness and good forming capabilities. Applications range from shipbuilding, to offshore constructions, cranes, and pipelines. A lot of current research focuses on weldability of high strength low alloy (HSLA) steels, especially improving the toughness in the weld zone, i.e., weld metal (WM) and heat affected zone (HAZ). In the present work, four different fusion welding processes using undermatching filler metal are compared on 8-mm thick sheets of S960MC structural steel. The welding processes include electron beam, laser hybrid, plasma, and gas metal arc welding. The welded joints are characterized by means of mechanical testing, tensile, impact, and hardness testing, and microstructural investigaton, light optical, and scanning electron microscopy. Furthermore, microprobe analysis of the weld metal was used to investigate the chemical composition of the weld metal.

Cracks on the Roots of Turbine Blades of the Low-Pressure Turbine in a Steam Power Plant

Abstract Cracks were found in the roots of steam turbine blades of the third blade row (LA-2) of the low-pressure turbine during revision works in a coal-fired power plant. Metallographic examinations reveal pitting corrosion sites, branched cracks, and local plastic deformation at the root of the first load bearing flank radius. The fracture surface was analyzed using light microscopy and scanning electron microscopy (SEM). Finite element (FE) simulations were performed in order to qualitatively represent the stress peak positions on the blade root.

Surface Modification of Pure Magnesium and Magnesium Alloy AZ91 by Friction Stir Processing

In recent years an interest in magnesium and magnesium alloys not only for the automotive industry but also for medical applications was increasing due to the low density and good specific strength. Magnesium alloys show good castability but lower ductility and strength than wrought materials. For this reason, refinement of grains and homogenous distribution of intermetallic phases are needed to improve formability and mechanical properties. On the other hand, the degradation of the material by corrosion is influenced by the grain size and phase distribution. This work investigates the microstructure evolution of pure Mg and magnesium alloy AZ91 by friction stir processing (FSP) technique. FSP experiments are carried out by constant force, optimizing the rotation and feed rate to obtain a homogenous microstructure, free of defects stir zone, good surface finishing and stable conditions during the process. The results show that the grain size is affected by the spindle speed. Increasing the number of passes reduces also the size of the grains and the intermetallic phases in the AZ91 alloy. The overlapping of passes between overlapping ratio 0.5 to 1 determines an uniform depth of the stir zone over a larger surface area.Hardness measurements are performed to evaluate the influence of FSP parameters on the mechanical properties. The degradation rate of the studied FSP Mg alloys is determined by hydrogen evolution in corrosion immersion tests, which depend strongly on the phase distribution and grain size.

The effects of friction stir processing on the wear behavior of cast AZ91C magnesium alloy

Abstract The microstructure and wear properties of AZ91C alloy were studied by performing solution annealing and then aging heat treatment, friction stir processing, and friction stir processing followed by solution annealing and aging. The heat-treated microstructure included fine dendritic grains (50 ± 14.2 μm) with a considerable dissolution and dispersion of continuous network-like β-Mg17Al12 precipitates at grain boundaries. Friction stir processing significantly refined the microstructure and grains (9 ± 2.3 μm); followed by the breaking-up and dissolution of dendrites and continuous β-Mg17Al12 precipitates. Heat treatment of the friction stir processed area resulted in excessive grain growth (175 ± 71.4 μm) and dispersion of fine β precipitates. Under a range of applied stresses (0.78 MPa, 2.44 MPa and 3.66 MPa) for the wear tests, only friction stir processed samples showed improvement in wear behavior at low stress, while at the higher stresses, along with the effectiveness of other processes it was the most effective process on improving the wear resistance. Heat treatment had the most effect on improving the wear resistance at the intermediate stress; moreover, applying it to the friction stir processed area remarkably enhanced the wear resistance at all stresses. Heat treatment of the friction stir processed area also resulted in the lowest friction coefficient values during the wear test, indicating the convenience of this process for wear performance of cast AZ91C alloy. Abrasion was shown as the dominant wear mechanism.

Friction stir welding of API X70 steel incorporating Ti-dioxide

ABSTRACT In the present work, the microstructural evolution and hardness of HSLA X70 joined by friction stir welding were investigated. The FSW was applied to HSLA X70 with and without the addition of titanium dioxide (TiO2) powders. To evaluate the microstructural features and hardness of different weld zones, optical microscopy and Vickers microhardness measurements were applied. The results show that the distribution of TiO2 powders is strongly dependent on the applied friction stir processing, which in turn changed significantly the microstructure and hardness profile. In this regard, the optimum stirring action resulted in a homogeneous and fine dispersion of particles leading to the domination of an acicular ferrite phase with a hardness of 370 HV. On the other hand, the lower stirring action resulted in coarse particles as well as the development of the polygonal ferrite structure with a hardness of ∼185 HV.