Yuri Hovanski

Friction stir spot welding of DP780 carbon steel

Abstract Friction stir spot welds were made in uncoated and galvannealed DP780 sheets using polycrystalline boron nitride stir tools. The tools were plunged at either a single continuous rate or in two segments consisting of a relatively high rate followed by a slower rate of shorter depth. Welding times ranged from 1 to 10 s. Increasing tool rotation speed from 800 to 1600 rev min–1 increased strength values. The 2-segment welding procedures also produced higher strength joints. Average lap shear strengths exceeding 10·3 kN were consistently obtained in 4 s on both the uncoated and the galvannealed DP780. The likelihood of diffusion and mechanical interlocking contributing to bond formation was supported by metallographic examinations. A cost analysis based on spot welding in automobile assembly showed that for friction stir spot welding to be economically competitive with resistance spot welding the cost of stir tools must approach that of resistance spot welding electrode tips.

Fatigue behaviour of magnesium to steel dissimilar friction stir lap joints

Abstract A short study has been conducted to assess the performance of friction stir welded Mg/steel lap joints under dynamic loads. The major mode of failure was found to be top Mg sheet fracture. Crack initiation is noted to have taken place at the Mg/steel interface. The fatigue life of the joints is found to be significantly different than the fatigue data of the Mg alloy obtained from the literature. The reasons behind such a difference have been examined in this work.

Microstructural Characterization of Friction Stir Welded Aluminum-Steel Joints

This work focuses on the microstructural characterization of aluminum to steel friction stir welded joints. Lap weld configuration coupled with scribe technology used for the weld tool have produced joints of adequate quality, despite the significant differences in hardness and melting temperatures of the alloys. Common to friction stir processes, especially those of dissimilar alloys, are microstructural gradients including grain size, crystallographic texture, and precipitation of intermetallic compounds. Because of the significant influence that intermetallic compound formation has on mechanical and ballistic behavior, the characterization of the specific intermetallic phases and the degree to which they are formed in the weld microstructure is critical to predicting weld performance. This study used electron backscatter diffraction, energy dispersive spectroscopy, scanning electron microscopy, and Vickers micro-hardness indentation to explore and characterize the microstructures of lap friction stir welds between an applique 6061-T6 aluminum armor plate alloy and a RHA homogeneous armor plate steel alloy. Macroscopic defects such as micro-cracks were observed in the cross-sectional samples, and binary intermetallic compound layers were found to exist at the aluminum-steel interfaces of the steel particles stirred into the aluminum weld matrix and across the interfaces of the weld joints. Energy dispersive spectroscopy chemical analysis identified the intermetallic layer as monoclinic Al3Fe. Dramatic decreases in grain size in the thermo-mechanically affected zones and weld zones that evidenced grain refinement through plastic deformation and recrystallization. Crystallographic grain orientation and texture were examined using electron backscatter diffraction. Striated regions in the orientations of the aluminum alloy were determined to be the result of the severe deformation induced by the complex weld tool geometry. Many of the textures observed in the weld zone and thermo-mechanically affected zones exhibited shear texture components; however, there were many textures that deviated from ideal simple shear. Factors affecting the microstructure which are characteristic of the friction stir welding process, such as post-recrystallization deformation and complex deformation induced by tool geometry were discussed as causes for deviation from simple shear textures.

Applications of Abrasive-Waterjets for Machining Fatigue-Critical Aerospace Aluminum Parts

Current specifications require AWJ-cut aluminum parts for fatigue critical aerospace structures to go through subsequent processing due to concerns of degradation in fatigue performance. The requirement of secondary process for AWJ-machined parts greatly negates the cost effectiveness of waterjet technology. Some cost savings are envisioned if it can be shown that AWJ net cut parts have comparable durability properties as those conventionally machined. To revisit and upgrade the specifications for AWJ machining of aircraft aluminum, “Dog-bone” specimens, with and without secondary processes, were prepared for independent fatigue tests at Boeing and Pacific Northwest National Laboratory (PNNL). Test results show that the fatigue life is proportional to quality levels of machined edges or inversely proportional to the surface roughness Ra . Even at highest quality level, the average fatigue life of AWJ-machined parts is about 30% shorter than those of conventionally machined counterparts. Between two secondary processes, dry-grit blasting with aluminum oxide abrasives until the striation is removed visually yields excellent result. It actually prolongs the fatigue life of parts at least three times higher than that achievable with conventional machining. Dry-grit blasting is relatively simple and inexpensive to administrate and, equally important, alleviates the concerns of garnet embedment.Copyright

Friction Stir Scribe Welding of Dissimilar Aluminum to Steel Lap Joints

The use of dissimilar material combinations such as aluminum to steel has been increasing in automobile and aerospace industries due to its potential for energy savings. Achieving an acceptable joint quality with fusion welding can be problematic due to the significant differences in physical and thermal properties between materials. One alternative to conventional fusion welding is friction stir scribe welding and because of the nature of the process, it can be a viable option for joining dissimilar metal combinations. The primary emphasis of this work is to investigate the feasibility of using FSS welding as a possible option for joining 1.0 mm thick 6022 aluminum to 0.7mm electro galvanized steel sheets in a dissimilar lap weld configuration. An H13 steel pin tool featuring a tungsten carbide scribe insert was used. An investigation on the optimum size of the scribe insert was conducted to evaluate the effects of microstructure and mechanical properties.

Comparing Laser Welding Technologies with Friction Stir Welding for Production of Aluminum Tailor-Welded Blanks

A comparison of welding techniques was performed to determine the most effective method for producing aluminum tailor-welded blanks for high volume automotive applications. Aluminum sheet was joined with an emphasis on post weld formability, surface quality and weld speed. Comparative results from several laser based welding techniques along with friction stir welding are presented. The results of this study demonstrate a quantitative comparison of weld methodologies in preparing tailor-welded aluminum stampings for high volume production in the automotive industry. Evaluation of nearly a dozen welding variations ultimately led to down selecting a single process based on post-weld quality and performance.

Friction stir scribe welding technique for dissimilar joining of aluminium and galvanised steel

ABSTRACT Friction stir scribe technology, a derivative of friction stir welding, was applied for the dissimilar lap welding of an aluminium alloy and galvanised mild steel sheets. During the process, the rotating tool with a cobalt steel scribe first penetrated the top material – aluminium – and then the scribe cuts the bottom material – steel. The steel was displaced into the upper material to produce a characteristic hook feature. Lap welds were shear tested, and their fracture paths were studied. Welding parameters affected the welding features, including hook height, which turned out to be highly related to fracture position. Therefore, in this paper, the relationships among welding parameters, hook height, joint strength and fracture position are presented. In addition, the influence of zinc coating on joint strength was also studied.

Solid-State Joining of Thick-Section Dissimilar Materials Using a New Friction Stir Dovetailing (FSD) Process

Solid-state joining of thick section aluminum to steel plate has been achieved using a new process called friction stir dovetailing (FSD). In FSD, a custom designed pin tool is used to flow a lower melting point material (AA6061) into dovetail grooves machined into the surface of an underlying material that has a higher melting point (rolled homogeneous armor [RHA]). Repeating dovetails form a mechanical interlocking structure akin to metallic Velcro. In this study, 38.1 mm (1.5 in.) thick AA6061 was joined to 12.7 mm (0.5 in.) thick RHA plates. The effectiveness of FSD is demonstrated through tensile test data that shows specimens failing in the processed aluminum rather than at the joint interface. Numerical simulations that highlight the importance of optimizing dovetail geometry are presented. The effect of process parameters on joint strength and microstructure also are discussed.

Prediction of Joint Line Movement and Temperatures in Friction Stir Spot Welding of DP 980 Steel

The friction stir spot welding (FSSW) process is non steady state and therefore lends itself to modeling using a Lagrangian approach. However a 3 dimensional model of this process can be overly time-consuming, particularly when an implicit scheme is used to solve the equilibrium equations that arise from discretization of the continuum body into finite elements. In this paper a novel 2 dimensional, finite element approach was used to model the FSSW process. An updated Lagrangian scheme was employed to predict the flow of the sheet material, subjected to boundary conditions of a descending tool and a fixed backing plate. Material flow was calculated from a velocity field that is two dimensional, but heat generated by friction was computed by an approach where the rotational velocity component from the tool surface was included in the thermal boundary conditions. An isotropic, viscoplastic Norton-Hoff law was used to model the material flow stress as a function of strain, strain rate, and temperature, while a viscoplastic friction law was used to compute the shear stress at the tool/sheet interface. The model predicted welding temperatures to within a few percent of experiment, but welding loads were significantly overpredicted. Comparison with a 3D model of FSSW showed that frictional heating and proportion of total heat generated by friction were similar. It was also shown that the position of the joint interface was reasonably well-predicted by the 2D model, compared to experiment.

Aluminum Tailor-Welded Blanks for High Volume Automotive Applications

Design of Experiment based approach is used to systematically investigate relationships between 8 different welding factors and resulting weld properties including strength, elongation and formability in 1.2mm-2mm thick friction stir welding of AA5182-O for TWB application. The factors that result in most significant effects are elucidated. The interactions between several key factors like plunge depth, tool tilt, pin feature and pin length on the overall weld quality is discussed. Appropriate levels of factors that lead to excellent weld properties are also identified.