Ghassan T. Kridli

Investigation of thickness variation and corner filling in tube hydroforming

Abstract Tube hydroforming is a near-net shape metal forming process in which a straight or a pre-bent tubular blank is placed in a closed die cavity and its cross-sectional shape is changed using internal hydraulic pressure that forces the tube to conform to the shape of the die cavity. This paper reports on the results of two-dimensional plane-strain finite element models of the tube hydroforming process, which were conducted using the commercial finite element code ABAQUS/Standard. The objective of the study is to examine the interaction of material properties and die geometry on the selection of hydroforming process parameters. The paper discusses the effects of the strain-hardening exponent, initial tube wall thickness, and die corner radii on corner filling and thickness distribution of the hydroformed tube.

Microstructural and mechanical investigation of aluminum tailor-welded blanks

The push to manufacture lighter-weight vehicles has forced the auto industry to look to alternative materials than steel for vehicle body structures. Aluminum is one such material that can greatly decrease the weight of vehicle body structures and is also consistent with existing manufacturing processes. As in steel structures, cost and weight can be saved in aluminum structures with the use of tailored blanks. These blanks consist of two or more sheets of dissimilar thicknesses and/or properties joined together through some type of welding process. This enables the design engineer to “tailor” the blank to meet the exact needs of a specific part. Cost savings can be gained by the elimination of reinforcement parts and the stamping dies used to manufacture them. Weight savings can be attained based on the fact that one thicker piece is more efficient than a welded structure and therefore can allow for down-gauging of parts.Although tailor-welded blanks (twbs) offer both potential weight and cost benefits, the continuous weldline and thickness differential in twbs can often result in difficulty in stamping. This problem is more severe in aluminum because of its limited formability as compared with typical drawing-quality steels. Additionally, welding of steel twbs tends to increase the strength of the weld material, which helps prevent failure in the weld during forming. Aluminum twbs do not experience this increase in strength and therefore may have a greater tendency to fail in the weld. In this study, several aspects of twbs manufactured from 6111-T4, 5754-O, and 5182-O aluminum alloys were analyzed and compared with those of a more conventional steel twb. The effect of gauge mismatch on the formability of these blanks is discussed as well as the overall potential of these blanks for automotive applications.

Formability improvement in aluminum tailor-welded blanks via material combinations

Abstract The use of tailor-welded blanks (TWBs) in automotive applications is increasing due to the potential of weight and cost savings. These blanks are manufactured by seam welding two or more sheets of dissimilar gauge, properties, or both, to form a lighter and stiffer blank. This allows engineers to “tailor” the properties of the blank to meet the design requirements of a particular part. TWBs are used in such places as door inner panels, lift gates, and floor pans. Initial applications of TWBs were for steel alloys, but investigating the potential of using aluminum TWBs is also of interest. One of the problems encountered with stamping TWBs is the difference in load-bearing capacities of the dissimilar sheets that make up the TWB. This can result in a reduction in the formability of the TWB and possibly a movement of the weld from its design-intended location. This paper presents the results of investigating the use of different material combinations to manipulate this type of preferential straining in the TWB in an effort to minimize the movement of the weld line.

FEM optimization of process parameters and in-process cooling in the friction stir processing of magnesium alloy AZ31B

Controlling the temperature in friction stir processing (FSP) of Magnesium alloy AZ31b is crucial given its low melting point and surface deformability. A numerical FEM study is presented in this paper where a thermo-mechanical-based model is used for optimizing the process parameters, including active in-process cooling, in FSP. This model is simulated using a solid mechanics FEM solver capable of analyzing the three dimensional flow and of estimating the state variables associated with materials processing. Such processing (input) parameters of the FSP as spindle rotational speed, travel speed, and cooling rate are optimized to minimize the heat affected zone, while maintaining reasonable travel speeds and producing uniformity of the desired grain size distribution of the microstructure in the stirred zone. The simulation results predict that such optimized parameters will result in submicron grain sized structure in the stirred zone and at the corresponding stirred surface. These simulation predictions were verified using published experimental data.Copyright

Prediction of flow stress and textures of AZ31 magnesium alloy at elevated temperature

The viscoplastic behaviour of magnesium alloys at high temperatures leads to highly temperature-dependent mechanical properties. While at high strain rates a notable strain hardening response is observed, at low strain rates the material shows a smooth plastic response with negligible amount of hardening. This complicated behaviour is due to different deformation mechanisms that are active at different strain rate regimes, resulting in different strain rate sensitivity parameters. In this study we show, by utilizing both numerical simulations and experiments, that this behaviour can be predicted by a model that combines two deformation mechanisms, grain boundary sliding mechanism and dislocation glide mechanism. We discuss the importance of each deformation mechanism at different strain rate regimes based on the findings of modelling and experimental results for AZ3 magnesium alloy. By developing a model that includes the above-mentioned two deformation mechanism, the prediction of flow properties is expanded to a wide range of strain rate regimes compared to previous study. The obtained numerical findings for the stress–strain behaviour as well as texture evolution show good agreement with the experimental results.

Intermetallic compound formation in Al/Mg friction stir welded (FSW) butt joints

In this work, friction stir welding (FSW) is used to produce butt joints of 3-mm-thick sheets of AZ31B magnesium alloy to two different aluminum alloys: AA1100 (minimum 99% aluminum) and AA6061 (97.9% Al). The paper reports on utilizing metallurgical techniques to determine the distribution profiles of elemental aluminum and magnesium within the joints were produced using energy dispersive x-ray spectroscopy (EDX). Furthermore, X-ray diffraction (XRD) was used to identify the intermetallic compounds that form in the joints as a result of the stirring action at processing temperatures. Measurements confirmed the presence of primary intermetallic compounds in the welded joints and were identified to be the α-phase (Al12Mg17) and the β-phase (Al3Mg2). Lastly, micro-hardness studies were conducted at the intermetallic-compounds-rich locations resulting in hardness profiles.Copyright

Experimentally Validated Thermo-Mechanically Coupled FE Simulations of Al/Mg Friction Stir Welded Joints

Numerical simulations of the friction stir welding of dissimilar metal joints is a daunting task given the complex issues involved such as the flow mixing action and the phase transformations. In this work, a 3D thermo-mechanical FE model is developed to simulate the dissimilar friction stir welding (DFSW) of aluminum-magnesium bi-metallic joints. The model is built using a manufacturing-processing-specific FEM software package (DEFORM 3D). Suitable constitutive laws are implemented to describe flow stress for both welded constituents: Al and Mg. The flow patterns of the stirring action from the simulations were verified against flow patterns of steel shots reported from experiments published in the literature. Also, the simulated interface patterns were found to be in agreement with microscopic images of welded sections taken from reported experiments. Furthermore, simulated temperature profiles favorably compare with temperature measurements previously published in the literature. The numerical model output includes relevant results such as material flow and volume fractions throughout the joint but most importantly in the recrystallized stir zone.Copyright

Modelling the rate and temperature-dependent behaviour and texture evolution of the Mg AZ31B alloy TRC sheets

Abstract In this work, the mechanical behaviour and texture evolution of AZ31B magnesium alloy under uniaxial tensile testing are investigated at different strain rates and temperatures. A crystal plasticity model is developed and calibrated to predict the mechanical response of the AZ31B at different temperatures and strain rates. The model results show that the relative activity of the pyramidal slip increases with increasing temperature, reaching a maximum activity at 200 °C. In order to achieve the continuous increase in the relative activity of the pyramidal slip as reported in the literature, a grain boundary sliding mechanism is implemented in the crystal plasticity framework. The incorporation of the grain boundary sliding at elevated temperatures results in considerable improvement in the model’s capabilities for prediction of yielding, hardening and texture evolution.

Mechanical Response and Evolution of Damage of Al6061‐T6 Under Different Strain Rates and Temperatures

The mechanical response and damage mechanisms of rolled Al 6061-T6 alloy subjected to tensile testing at different temperatures and various strain rates have been investigated in this paper. The evolution of the microstructure has been examined for the different testing conditions showing strain rate and temperature effects. The fracture surfaces of samples damaged at different uniaxial testing conditions were observed through Scanning Electron Microscope (SEM). Annealing tests at different temperatures have been performed and microstructure analyses for each condition have been achieved showing grain size evolution. Investigation of the fracture initiation sites has been achieved by conducting interrupted tests and observing the microstructure through SEM. Observations has pointed out that precipitates and iron rich phases are privilege cites for crack initiation.

The Effect of Tool Geometry on Material Mixing during Friction Stir Welding (FSW) of Magnesium AZ31B Welds

In friction stir welding (FSW) material flow determines to a great extent the weld feasibility and, ultimately, quality. This work reports on the investigation of the effect of tool geometry on the mixing and material flow during FSW of magnesium AZ31B to magnesium AZ31B. The study involves both FEM simulations and experiments. Nondestructive X-ray imaging is used to pinpoint the location of several steel shots (beads) which were pre-placed within the joint prior to FSW. The experimental results were also augmented with finite element analyses using the commercial engineering FEM software Deform the results of which compare favorably with those of the experiments. Resulting mixing behavior was contrasted for several tools with different configurations and geometries including tool with round shoulder and straight pin and others with concave-shoulder with tapered pin.