Mohamadreza Nourani

On the Microstructural Evolution of 4130 Steel During Hot Compression

This article first gives a brief patent review of recent trends of steel alloys used in the manufacturing of high- pressure gas containers. 4130 steel is among such alloys and has been extensively used in natural gas cylinders that are manufactured through the hot deformation processes. Due to the importance of process parameters such as temperature and strain rate on the ensuing microstructure, primarily grain size, and mechanical properties of the cylinder, the dynamic recrystallization characteristics of 4130 steel are investigated in the second part of this article. Hot compression tests on 4130 steel specimens have been performed at a temperature range of 900-1100°C, strain rate range of 0.001-0.1s - � and the strain of 0.9. The resulting flow stress curves show the occurrence of dynamic recrystallization with single or multiple peaks, before reaching the steady state flow at different temperatures and strain rates. The effect of various processing pa- rameters on the microstructure of the alloy is identified by microstructural examination focusing on revealing primary austenite grains using special etchant solutions. It is found that the average grains size of the deformed 4130 steel in- creases with an increase of the forming temperature and a decrease in the strain rate. The grain size is decreased with an increase of the steady state stress.

An integrated multiphysics model for friction stir welding of 6061 Aluminum alloy

This article presents a new, combined ‘integrated’- ‘multiphysics’ model of friction stir welding (FSW) where a set of governing equations from non-Newtonian incompressible fluid dynamics, conductive and convective heat transfer, and plain stress solid mechanics have been coupled for calculating the process variables and material behaviour both during and after welding. More specifically, regarding the multiphysics feature, the model is capable of simultaneously predicting the local distribution, location and magnitude of maximum temperature, strain, and strain rate fields around the tool pin during the process; while for the integrated (post-analysis) part, the above predictions have been used to study the microstructure and residual stress field of welded parts within the same developed code. A slip/stick condition between the tool and workpiece, friction and deformation heat source, convection and conduction heat transfer in the workpiece, a solid mechanics-based viscosity definition, and the Zener-Hollomon- based rigid-viscoplastic material properties with solidus cut-off temperature and empirical softening regime have been employed. In order to validate all the predicted variables collectively, the model has been compared to a series of published case studies on individual/limited set of variables, as well as in-house experiments on FSW of aluminum 6061.

Predicting Residual Stresses in Friction Stir Welding of Aluminum Alloy 6061 Using an Integrated Multiphysics Model

Experimental results in the literature show that there are two flow areas of material during the friction stir welding (FSW) process [1]; namely the “pin-driven flow” and the “shoulder-driven flow”. These areas should completely join together to create a weld with no defect. First, in order to numerically predict the local distribution of flow stress around the pin as well as the temperature, strain, and strain rate fields during FSW, a two-dimensional steady-state Eulerian multiphysics finite element model has been employed in this work for aluminum alloy 6061using the COMSOL software. In this model, the non-Newtonian flow mode of computational fluid dynamics (CFD) module, general heat transfer mode of the heat transfer module, and the plain stress mode of the structural mechanics module of the software have been coupled. Slip/stick condition between the tool and workpiece, frictional and deformation heat sources, the convectional heat transfer in the workpiece, the solid mechanics-based viscosity definition, the temperature-dependent physical properties and the Zener-Hollomon- based thermo-visco-plastic mechanical properties with a cut-off temperature of 582oC were considered. Next, the thermal history during the process predicted by the model was used as input for an elasto-visco-plastic analysis to estimate the local residual stresses distribution due to the workpiece thermal expansion effect. Finally, the predicted longitudinal and transverse residual stresses were verified by comparing to experimental data.

A New Approach to Measure Strain During Friction Stir Welding Using Visioplasticity

During modeling of the friction stir welding (FSW) process, the prediction of strain range experienced by the material is important as it affects the microstructure and mechanical properties of the final weld [1–7]. For aluminum alloys, this range has been reported very scarcely and/or scattered widely in the literature (the range of the maximum equivalent plastic strain has been reported to be from 2.4 to 184 [8–24]). A new approach is proposed in this article for measuring strain during friction stir welding using visioplasticity. In this approach, strains are calculated from changes in the boundaries of a small cylindrical Al-30% SiC composite marker mounted in the advancing side of mid-plane of adjacent plates during welding. The marker shape change is observed by a “stop action” (freeze-in) technique midway the process. COMSOL numerical modeling is then used to compute the strain distribution using the observed boundary changes compared to the initial marker boundaries. As an illustrative example, the method is applied to the results reported by London et al. [25] for the friction stir welding of 6.35 mm thick 7050 aluminum plates, welded with tool RPM of 350, welding speed of 1.69 mm/sec, tool pin diameter of 8 mm, tool shoulder diameter of 24 mm, and tool tilt angle of 3 degrees. A lower and upper bound of cumulative equivalent plastic strain of 14.1 and 20.3, respectively, were found to be in the neighborhood where the marker enters the severe deformation zone at mid-plane of plates in front of the leading edge of the pin.© 2011 ASME

A Review on Thermomechanical Models of Friction Stir Welding

There are several reported thermomechanical models that can be used to predict friction stir welding (FSW) properties of different alloys. A major application of these models is the computation of material temperature, flow stress, strain rate and strain during the process and/or the resulting residual stress after the process. The models are normally applied to solve energy, mass and force equilibrium equations simultaneously using different numerical approaches. All of the validated models can be reliably used to optimize the FSW process parameters such as tool RPM and transverse speed. The brief review in this article is indented to summarize some of the most commonly used thermomechanical models of FSW along with their main characteristics namely; the Solid Mechanics-based models, Fluid Dynamics-based models, and hybrid/ multiphysics models.

A Study on the Formability of IF and Plain Carbon Mild Steels

Steel sheet metals are widely used in different industries due to their high strength, good weldability, availability, moderate cost, and the ability to form to complex 3D parts. The study of the formability of sheet metals is often done by means of Forming Limit Diagram (FLD) which presents the major and minor engineering strain thresholds under different deformation states. In this article, the formability parameters of three different steel sheet metals with the same thickness have been determined by uniaxial tension test and their FLDs have been produced by Hecker method: RRSt14O3, Zinc coated IF (Interstitial Free) steel and uncoated IF steel. Also the materials’ formability during the stamping process of a car door inner panel has been investigated as a case study to substitute the original design of raw material, coated IF steel, with a cheaper alternative. Among the tested materials to form the part, the uniaxial tension results showed that the formability parameters of uncoated IF steel was higher than the coated IF steel and the parameters of RRSt14O3 sheet metal was the lowest. The FLD of coated IF steel sheet was the highest (best formability). Differences among the formability parameters in uniaxial tension, the FLDs, and the stamping behavior of the part with different steel sheet metals have been explained by their surface roughnesses and the friction coefficients that affect the material flow during the FLD test as well as the stamping process.Copyright