J.T. Wood

Evolution of plastic strain during a flow forming process

The distribution of equivalent plastic strain through the thickness of several AISI 1020 steel plates formed under different conditions over a smooth cylindrical mandrel using a single-roller forward flow forming operation was studied by measuring the local micro-indentation hardness of the deformed material. The equivalent plastic strain was higher at the inner and outer surfaces and lowest at the center of the workpiece. Empirical expressions are presented which describe the contribution of the roller and mandrel to the total local equivalent plastic strain within the flow formed part. The dependence of these expressions upon the thickness reduction during flow forming is discussed.

Thermal relaxation of internal strain in two-phase Cu-Nb wire

Abstract Thermal relaxation of internal strain in high-strength two-phase Cu–18wt.%Nb wire was studied using neutron diffraction at temperatures from 100 to 450 °C. The average axial tensile strain in the Nb phase reduced significantly at temperatures above 300 °C. This was shown to correspond to the onset of spheroidization of the Nb phase.

Analytical solution of the tooling/workpiece contact interface shape during a flow forming operation

Flow forming involves complicated tooling/workpiece interactions. Purely analytical models of the tool contact area are difficult to formulate, resulting in numerical approaches that are case-specific. Provided are the details of an analytical model that describes the steady-state tooling/workpiece contact area allowing for easy modification of the dominant geometric variables. The assumptions made in formulating this analytical model are validated with experimental results attained from physical modelling. The analysis procedure can be extended to other rotary forming operations such as metal spinning, shear forming, thread rolling and crankshaft fillet rolling.

Orientation Effect of Clay Platelets in Transfer Molded Polymer Nanocomposites

Transfer molding has been increasingly used to process polymer composites with various shaped nanoparticles, including platelet type nanoparticles. Platelet nanoparticles exhibit high aspect ratios (length to thickness); therefore their distributions in polymer matrix can be greatly affected by the flow trajectories in the mold. In present study, the clay platelet-polyethylene nanocomposite was prepared by transfer molding. The orientation of clay platelets developed during molding process was analyzed and measured with wide angle X-ray diffraction. Due to large velocity and shear stress gradients in the mold, the platelets caught in surface regions were rotated towards the flow direction and formed the oriented morphologies. With weaker shear stresses at the central region, the platelets were mostly randomly distributed. The shear stresses may be amplified locally at regions near the mold walls, which can further lead to or accelerate the orientations of clay particles. The orientation distribution was found to depend upon the clay fraction and sample size. The oriented platelets can be re-randomized through the annealing process. The orientation of clay platelets enhanced the orientation of polyethylene lamellae and caused shear band formation in composites when deformed.

Process-Structure-Property Correlations for HPDC AM60B

In this paper, we characterize the local micro structure and mechanical properties of an AM60 magnesium alloy high-pressure die-casting in order to further develop predictive process-structure-property models. The casting process is simulated using the commercial code ProCAST™ in order to extract information regarding mold-filling and local solidification conditions including solidification rate and expected levels of shrinkage porosity. This data is used to predict the presence of knit lines, local porosity and the through-thickness variation of grain size (i.e. determination of skin and core thickness). Micro structural data is then used to predict upper and lower bounds on local mechanical properties including yield strength, tensile strength and ductility. A high degree of correlation between predicted and measured mechanical properties is found.

Overview of Key R&D Activities for the Development of High-Performance Magnesium Materials in Canada

Canadian researchers are actively engaged in the development of novel cast, wrought and composite materials that are based on Mg. An overview is provided of Canadian research projects for new applications of Mg alloys that are targeted to the growing needs of the automotive sector. The research work described is funded primarily through two federal programs: the Canadian Lightweight Materials Research Initiative, and the Materials and Manufacturing Theme of the AUTO21 Network of Centres of Excellence. It includes work on mechanical and corrosion performance of high-pressure die castings, gravity and low pressure castings using permanent and sand molds, sheet Mg development and magnesium matrix composites. The metallurgical research facilities at the CANMET Materials Technology Laboratory are featured.

Process-Structure-Property Relationships for Magnesium Alloys

Gravity step-casting experiments were performed to investigate process-structure-property relationships in three different die-cast magnesium alloys – AM60, AZ91 and AE44. The step-cast mold was instrumented to capture temperature profiles of the solidification of molten magnesium. This paper investigates the structure-property relationships of these magnesium alloys, specifically the dependence of the fracture properties upon the porosity that forms during the casting process. Sixteen tensile specimens were cut from the step-casting perpendicular to the solidification front, for each alloy examined. Correlations from X-ray tomography data were used to estimate the maximum area fraction of porosity from the average volumetric porosity in the specimens, assuming a typical size and spatial distribution of porosity. This relationship can be used in the absence of more accurate measure of porosity (i.e. serial sectioning, computed x-ray tomography). A failure model for die-cast alloys – which depends upon the strain-hardening coefficient and the maximum area fraction of porosity in the specimen – was used to predict fracture strains for each specimen. The experimental tensile elongation of each specimen was compared with predicted values. The resulting mechanical properties determined from these cast magnesium alloys will be used to develop process-structure-property relationships.

Mechanical Properties of AM60B Die Castings A Review of the AUTO21 Program on Magnesium Die-Casting

This paper provides an overview of the research efforts within the AUTO21 program on magnesium die-casting. The objective of the program is to better understand the mechanical properties of magnesium high pressure diecastings (HPDC) and to develop the capability to predict the local mechanical properties over a given component based on its shape, the design of the mold and the casting parameters used in its production. The paper highlights the research groups findings related to the microstructure and mechanical properties of a full-scale instrument panel beam casting and outlines the goals of the continuing research program.

Evaluation of Cooling Rate Effects on the Mechanical Properties of Die Cast Magnesium Alloy AM60

With the increased application of magnesium high-pressure die castings (HPDC), it is necessary to better understand process-structure-mechanical properties. In the case of HPDC, ductility and yield strength strongly depend on porosity, grain size, and the skin thickness. In this contribution, a new method is developed which employs knowledge of local cooling rates to predict the grain size and the skin thickness of HPDC magnesium components. The centreline cooling curve, together with the die temperature, and the thermodynamic properties of the alloy are then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting. The local cooling rate is used to calculate the resulting grain size and skin thickness via established relationships. The prediction of skin thickness and average grain size of skin region determined from this method compares quite well with the experimental results. Due to the presence of externally solidified grains, this method underestimates the grain size value in the core region, as compared to the experiment. Finally, we predict the locally varying yield strength using a modified Hall-Petch equation.

A Volume Averaged Finite-Volume Model for Solidification of Magnesium Alloys on a General Unstructured Collocated Grid

A formulation used to simulate the solidification process of magnesium alloys is developed based upon the volume-averaged finite-volume method on unstructured collocated grids. In casting the single equation formulation, a non-zero volume fraction gradient has been considered, and resulting additional terms are well reasoned. For discretization, the most modern approximations for gradients and hessians are used and novelties are outlined. The formulation is tested for a wedge-shaped magnesium alloy casting to predict the cooling curves and the grain size at six embedded thermocouples. The current predictions compare well against previously reported experimental results.