Paul T. Wang

Cradle-to-grave simulation-based design incorporating multiscale microstructure-property modeling: Reinvigorating design with science

We propose a new paradigm for design that incorporates scientifically oriented research directly and feasibly into engineering design practice. The goal is to use this simulation-based tool earlier in design to achieve more optimized components and systems. The method to accomplish this bridge of science and engineering is by using thermodynamically constrained internal state variables that are physically based upon microstructure-property relations. When the microstructure-property relations are included in the internal state variable rate equations, history effects can be captured. Hence, the cradle-to-grave notion arises. The method to help determine the appropriate microstructure-property relations for the internal state variables is through a multiscale modeling methodology which includes experimentation. As such, scientifically oriented research occurs in the multiscale methodology, and the engineering design practice employs the cradle-to-grave internal state variable model. An example of the multiscale methodology is presented in terms of a cast A356 aluminum alloy used in automotive design, and an example of the cradle-to-grave simulation based design is presented in terms of a stamped product used in a crash scenario.

Torsion test of aluminum in the large strain range

Abstract A series of experiments was conducted on cast and extruded high purity aluminum material under monotonic large strain torsion condition. Both free-end and fixed-end torsions were studied using tubular specimens of different gage lengths (long, medium and short). The experiments used an axial–torsional extensometer. A procedure of calibration for elevated temperature test was determined. The torque versus angle of twist curves were recorded and converted into true shear stress–strain curves by use of the modified Nadai method developed previously by the authors. The axial extension for free-end torsion and the axial stress developed during fixed-end torsion were recorded. The hoop strain was also measured and was found to be approximately 0.8–0.9 times the axial strain when the shear strain is 150%. The effect of specimen geometry was studied. It was found that the long, thick-walled tubular specimen is suitable for torsion test in the large strain range.

Cyclic stress-strain response of porous aluminum

Abstract A model of endochronic constitutive theory is applied to the analysis of stress-strain behavior of sintered high purity aluminum powder-based material (P/M) under uniaxial strain-controlled cyclic test conditions. Different strain amplitudes were used on specimens which during fabrication were subjected to cold isostatic pressure of 25 ksi. It was found that mean strain does not significantly affect the fatigue life. It was also discovered that Poissons ratio at the peak tensile strain decreases gradually with the number of loading cycles, while it reaches a constant value at the peak compressive strain as the number of cycles increases. Results provide a comparison between theory and experiment for both hydrostatic and deviatoric stress-strain behavior for several cases of peak strains in the strain-controlled test. Reasonable agreement has been achieved. The changing trends of the peak volumetric strain, deviatoric stress, hydrostatic stress and Poissons ratio versus loading cycles are also reported for each case.

A comparative study of design optimisation methodologies for side-impact crashworthiness, using injury-based versus energy-based criterion

The tension between occupant safety during a crash and lightweight designs continues to be an important driver of modern vehicle designs. While occupant safety may be defined and evaluated in various ways, maximising energy absorption of structural components during impact has been adopted for vehicle designs by many manufacturers. An alternative method to evaluate safety but often not directly used in the design of structural components is the use of a dummy model in the finite element (FE) simulation to estimate the forces and accelerations that would be experienced by a human in a crash environment. This paper investigates the similarities/differences between designing vehicular structural components experiencing side impacts based upon two different criteria: (1) the energy absorption of collapsed components in the absence of a dummy and (2) an injury metric–based approach with the responses of the dummy as the bases. Multi-objective optimisation methods are used with finite element analysis (FEA) in the lightweight design for side-impact crashworthiness, considering the two different criterion. The results show that the optimum designs based on the two criteria are significantly different and that the injury-based approach should be incorporated into vehicular lightweight design process when considering crashworthiness.

Evolution of porosity during thin plate rolling of powder-based porous aluminum

Abstract The evolution of porosity during hot rolling of powder-based porous aluminum plates is analyzed by using a pressure dependent constitutive model and a finite element based rolling process model. The constitutive model, which is capable of describing both matrix and densification hardening, consists of two state variables: matrix strength and volume fraction of porosity. The matrix strength is represented by an internal state variable constitutive relationship whose parameters were evaluated by a uniaxial stress compression test at various strain rates and elevated temperatures. The rolling process model uses a Eulerian, velocity-based, finite element formulation. A quasi-three-dimensional nature arises because normal strain rates are independent of through-thickness position. Only one element is taken through a half-thickness rolling plate. Variations are permitted across the width of the plate. High purity aluminum powders were used to fabricate porous thin plates for rolling experiments. Plates with uniform initial porosity underwent various reductions on a bench scale rolling mill at 400°C. Experimental results were compared with numerical solutions generated from the rolling model and the agreement was very good. Experimental results consistently showed that a variation of porosity exists from the center to the edge of plate during the void healing process. The pressure also varies along the width which contributes to the porosity variation. However, the internal state variable which represents the hardness of the matrix does not vary much across the width.

Pore Formation in Laser-Assisted Powder Deposition Process

Pore formation remains a concern in the area of rapid manufacturing by the laser engineered net shaping process. Results usually conflict on the origin of these pores; whether it should stem from an effect due to the physical/mechanical properties of the material or from an effect purely related to the processing parameters. We investigated this problem spanning a range of process parameters for deposition and using three different material powders, namely, an AISI 410 grade stainless steel, AISI 316L grade stainless steel, and AISI 4140 grade medium-carbon low alloy steel. The volume fraction, number density, and size distribution of pores were quantified using X-ray computed tomography and optical microscopy. Pores formed both at the interface between the adjacent layers and within the bulk of the layer. They were systematically sensitive to both the powder material composition and the process parameters.

Thermomechanical deformation of powder-based porous aluminium: Part II. Constitutive model including densification hardening

Abstract A constitutive model describing the effect of thermomechanical deformation histories on powder-based porous aluminium is evaluated. A modified form of Gursons yield function in conjunction with the hyperbolic sine stress dependence for the matrix strain rate is used. Two state variables involved in the model are the matrix effective stress and the void fraction, which are related to the matrix microhardness and mean pore intercept length in the microstructure, respectively. The activating energy, the steady state flow stress of the matrix and a nonlinear equation describing evolution of porosity can be derived from this model. In addition, the results from the quantitative morphology analysis established in Part I of this work are employed to link the macroscopic parameters of the constitutive theory to the microstructural features of deformed porous aluminium. Briefly, the constitutive model is used to describe two basic fabricating processes for powder-based materials: closed-die compaction and hot isostatic pressing (HIP). In general, this model is applicable to forging, extrusion, rolling and consolidation of powder-based materials containing voids.

Time Dependent Springback of a Magnesium Alloy

Abstract A time dependent springback was observed in a Mg–Al–Zn (AZ31) rolled sheet after three-point bending at room temperature (RT). Two types of test were performed: (1) baseline springback – specimens were bent and immediately released and the springback was measured every month; (2) springback after holding – after bending, specimens were held in the bent state for up to five months, and the springback was measured after release. It was found that the springback increased nonlinearly with time in all the specimens. The springback dramatically decreased after being held for one month compared with specimens that were not held. The decline in springback increased as the holding time increased. These results indicate that creep and creep recovery occurred. Microstructure examinations revealed high density { 1 0 1 ‾ 2 } 〈 1 0 1 ‾ 1 ‾ 〉 twins in the compression zone of the bent specimens in the form of localized twin bands. After the specimens were unloaded, detwinning occurred and continued spontaneously over the five month time period, contributing to the observed time dependent springback.

Effect of additives on sintering response of titanium by powder injection moulding

Abstract Powder injection moulding is a maturing technology that has proven most useful for the production of complex metallic and ceramic components of modest sizes. Considering the inevitable demand for cost effectiveness in automotive applications, components manufactured from low cost sponge titanium (Ti) powder currently reflect the most advantageous economics among the available Ti powders. This paper describes the net shape fabrication of Ti components and considers the role of iron and zirconium powder additions. Sintering cycle optimisation relied on differential scanning calorimetry to identify a cycle in the 1275–1300°C range for 1–2 h. The sintered material was characterised using tensile and hardness testing and microscopic examinations. The influence of test conditions on densification, microstructure and mechanical properties was analysed.

Investigating thermal effects on morphological evolution during crystallisation of hcp metals: three-dimensional phase field study

Abstract Computational models simulating the crystallisation of hexagonal close packed (hcp) metals have traditionally used simple solid/fluid interface anisotropy relations. However, this method does not provide a proper representation of the crystallisation anisotropy, particularly for non-isothermal crystallisation. In this work, we used a phase field (PF) model to investigate the effects of thermal properties (thermal diffusivity and latent heat) on the morphological evolution of hcp metals during crystallisation. The governing equation of the PF model was coupled to the temperature evolution equation, and a three-dimensional model was developed in a finite element framework using COMSOL software. We applied the model to study the crystal formation and evolution of magnesium and yttrium. The unique aspect of this work is that we used Qin and Bhadeshia’s three-dimensional interface anisotropic model to accurately simulate solid/fluid interface anisotropy. The results showed that the thermal effects significantly influenced the shape evolution of the crystals and can control the formation of potential sites for void nucleation inside the crystal structures.