Norbert Enzinger

Characterisation of interface of steel/magnesium FSW

Abstract Mild steel with/without zinc coating and magnesium alloy AZ31B were lap joined by friction stir welding during the tool plunging through the magnesium into the steel. With increasing welding speed, the fracture strength increases. At the interface of zinc coated steel and magnesium, a liquid eutectic layer was detected for higher but not for lower welding speeds. At the interface between the steel without zinc coating and magnesium, no melting could be detected.

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.

Microstructural and mechanical characterisation of friction stir welded 15-5PH steel

Abstract Martensitic precipitation hardening steels are characterised by high strength which is achieved by a martensitic matrix and precipitates. The material also shows a good ductility and toughness if properly heat treated. But welding of these steel types is often problematic and requires a special procedure (e.g. post-weld heat treatment) in order to achieve satisfactory results. In this contribution, the solid state welding process – friction stir welding was used to weld 15-5PH and the results of the investigations are shown. The butt welds for 2·6 mm thick steel sheets have been carried out at Institute for Materials Science and Welding at Graz University of Technology using tungsten based tools, different welding speeds and tool rotational rates. Temperature measurements using thermocouples have been performed on the advancing and retreating sides of the weld. Detailed microstructural observations were performed for base material, heat affected zone, thermomechanically affected zone and stir zone. The appearance of retained austenite, which reduces the strength of the material, has been studied for the distinct regions of the friction stir weld. A quantitative spot analysis by energy dispersive spectroscopy was performed to identify tool remanents in the stir zone of the weld. For further characterisation, hardness profiles of the weld have been created. Tensile tests and surface fracture analysis using scanning electron microscopy have been performed. Welds with low energy input have shown better results than welds with high energy input. Additionally, effects of post-weld heat treatment on microstructure and properties of the joint have been analysed.

Mechanical Testing of Flow Drill Screw Joints Between Fibre-Reinforced Plastics and Metals

Abstract The paper describes a novel testing method for mechanical joints between fibre-reinforced plastics and metals. As an example, the paper discusses a connection between a continuous carbon-fibre-reinforced plastic and aluminium via a flow drill screw. The failure load of the joints under different loading angles was tested with the help of a modified KS-II testing facility. A detailed analysis of the damage and failure behaviour of the connection was performed by means of optical recordings and micrographs. The results indicate that the tested material combination has nearly the same failure load at different loading angles with various failure mechanisms. The experimental results can be used to devise new failure criteria for these connections and to detect limit values for the design process.

FE modelling of microstructure evolution during friction stir spot welding in AA6082-T6

Friction stir spot welding (FSSW) is a solid-state joining method, which is a variant of friction-stir welding. Microstructure analysis shows that the FSSW joint contains four different zones, namely the stir zone, thermo-mechanical affected zone, heat-affected zone and base metal, respectively. In this paper, the results of a FE analysis of the FSSW process of AA6082-T6 considering geometric dynamic recrystallization are presented. A physically based model taking into account three internal state variables was implemented into the commercial FE package DEFORM-3D to describe the microstructure evolution during FSSW. This model allows predicting the dislocation density, grain size, temperature, effective strain, and strain rate during FSSW. The microstructure in stir zone was analysed by electron backscattered diffraction. Experimental and simulation results have been compared to validate the model.

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.

Friction stir welding of multilayered steel

Abstract This study investigates the mechanical properties and microstructure of friction stir butt welded high strength/ductility multilayered steel consisting of 15 alternating layers of SUS 301 austenitic stainless steel (eight layers) and SUS 420J2 martensitic stainless steel (seven layers) with a total thickness of 1·2 mm. With optimised welding parameters, defect free welds with an ultimate tensile strength (UTS) of 1240 MPa and a fracture elongation of 13% were accomplished. This corresponds to a joint efficiency of 90%. In this case, fracture occurred in the heat affected zone as a result of a very pronounced hardness drop in the martensitic layers resulting from the formation of a large amount of grain boundary precipitates, which were formed at temperatures ∼750°C slightly below Ac1. By applying post-weld heat treatment, the hardness drop in the martensitic layers was removed and the tensile properties were enhanced to UTS of 1310 MPa (95% joint efficiency) and a fracture elongation of 22%.

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.

Thermo-Mechanical Investigations during Friction Stir Spot Welding (FSSW) of AA6082-T6

Friction Stir Spot Welding (FSSW) is a variant of the Friction Stir Welding (FSW) process and has been successfully used in industrial applications. During the FSSW process, thermal inputs due to friction and deformation are commenced simultaneously, as the non-consumable rotating tool plunges into the workpiece to be welded. Various assumptions and hypotheses for mechanisms of heat generation and material deformation during FSW/FSSW process are reported, but a consensus is still to be reached. The joining quality is mainly dependent upon the material flow in this solid state joining technique. The material flow and deformations in the near and far fields of the weld are directly affected by the temperature-sensitive mechanical properties. Therefore, a comprehension of thermo-mechanical responses are of high importance from the viewpoints of parameter optimization and understanding of the mechanisms. The FSSW process is experimentally and theoretically studied to address these issues of the mechanism of heat generation and coupled thermo-mechanical response of the workpiece, as well as the effects of tool rotation and plunge speeds. For theoretical studies, a 3-dimensional, physical-based FEM (Finite Element Method) model is developed using commercial code. For heat generation, friction and deformation-based formulations are used. For material responses, thermal and strain rate-sensitive, elastic-plastic data are employed by a constitutive Johnson Cook material model and thermo-mechanical behaviour is analyzed with respect to experimental observations. To cope with high calculation time and distortion of the mesh, built-in features of the code, mass scaling, ALE (Arbitrary Lagrangian Eulerian) and mesh re-mapping were used. As a result of this work, a basic platform in the form of a physical-based, coupled, thermo-mechanical model is developed. With the help of this model, effects of process parameters on the temperature — displacement behaviour of the workpiece are studied. The role of interaction conditions at the tool-workpiece interface is emphasized and a simplified conceptual mechanism for effects of process variables on the physical phenomena is presented.