Liliang Wang

Friction in Double Action Extrusion

A novel extrusion testing method, double action extrusion (DAE), to highlight the effect of friction at the die bearing in aluminum extrusion was developed. It was found that the lengths of the extrudates and extrusion force were indeed sensitive to the die bearing length and thus to the friction. FEM simulations of DAE were carried out to evaluate the shear and Coulomb friction models over a wide range of friction factors/coefficients from 0.2 to 1. The full sticking friction appeared to represent the interfacial contact between hot aluminum and die the best. The friction factor values in the shear friction model over a range of 0.3 to 0.6 commonly used to describe the contact at the billet-die interface in FEM simulation appeared to be too low. The comparison between the experimental and simulation results indicated that the shear friction model at m = 1 predicted the extrusion force the best, while the Coulomb friction model at µ = 1 predicted the extrudate lengths the best. Of the existing friction models and friction factors/coefficients, it is recommended to use the shear friction model at m = 1 to describe the friction at the billet-die interface in FEM simulation.

Free-standing, parylene-sealed copper interconnect for stretchable silicon electronics

In this paper, development and characterization of a freestanding electroplated copper interconnect for applications in flexible and stretchable electronics is presented. The copper layer with typical thickness of 5 mum is plated into a photoresist mould realizing meander and mesh-like patterns. These are subsequently released resulting in a free-standing electrical interconnect that is optionally conformally coated by a ~8 mum thick Parylene N layer. Parylene sealing provides electrical insulation and increases rigidity of the structures. Tensile tests on fabricated samples have shown the elongation capability up to 300% for the mesh design and more than 1000% for the meander design. The Parylene coated samples showed increased rigidity but about 50% reduced elongation. Furthermore, parameterized FEM simulations were performed in order to estimate stress levels for different geometries under tensile stress.

Mechanical Characterization Analysis of a Segmented Silicon Layer on Ultra-thin Polyimide Substrates by Experiment and FE Simulation

The acceptable flexibility for ultra-thin substrate would be reached by embedding the ultra-thin substrate into the flexible polyimide and patterning the poly-silicon or silicon into square segmentations. In this contribution, results of experiments and FE simulations on mechanical reliability issues of poly- and single crystalline silicon on ultra-thin polyimide substrates are presented. Generation of cracks within the silicon and dielectric layers was then studied under controlled bending (glass cylinders with diameters of 2-10 mm, compressive and tensile stress) using specially for this purpose designed bending tools. Specimen observation was done using an optical microscope with possibility of digital recording and evaluation by pattern recognition software. The results show that the cracks appear first in the dielectric layers in-between the silicon layer segments and only at higher loads propagate or are generated within the silicon itself. The development of first cracks depends significantly on the silicon layer segmentation size which affects both the crack density and the crack width. The crack density increases sharply with the strain for early stage and then increases slightly. The crack width increases steadily. The high flexibility result can be reached that no crack be detected under the bending tests with 2 mm diameters. The maximum strain failure criterion of the ultra-thin thermal silicon dioxide layer could be reached by specific bending and tensile tests and the results of tensile and bending tests match very well. Multilevel FEM simulations were performed in order to increase understanding of the major failure processes. Results of simulations and experiments compare quite well

Knowledge based cloud FE simulation of sheet metal forming processes

The use of Finite Element (FE) simulation software to adequately predict the outcome of sheet metal forming processes is crucial to enhancing the efficiency and lowering the development time of such processes, whilst reducing costs involved in trial-and-error prototyping. Recent focus on the substitution of steel components with aluminum alloy alternatives in the automotive and aerospace sectors has increased the need to simulate the forming behavior of such alloys for ever more complex component geometries. However these alloys, and in particular their high strength variants, exhibit limited formability at room temperature, and high temperature manufacturing technologies have been developed to form them. Consequently, advanced constitutive models are required to reflect the associated temperature and strain rate effects. Simulating such behavior is computationally very expensive using conventional FE simulation techniques. This paper presents a novel Knowledge Based Cloud FE (KBC-FE) simulation technique that combines advanced material and friction models with conventional FE simulations in an efficient manner thus enhancing the capability of commercial simulation software packages. The application of these methods is demonstrated through two example case studies, namely: the prediction of a materials forming limit under hot stamping conditions, and the tool life prediction under multi-cycle loading conditions.

Mesh Interconnects for Silicone Embedded Stretchable Silicon Electronics

A process development for stretchable silicon electronics encapsulated in a layer of polydimethylsiloxane is presented. Stretchability is achieved by segmenting the normally rigid silicon substrate into small islands (<2×2 mm2) and connecting these by flexible metal interconnects. The metal interconnects have a mesh shape providing stretchability and improved reliability. The mesh-shaped interconnects have been simulated and measured as free standing before being integrated in the final stretchable system design. PDMS (polydimethylsiloxane) encapsulation using either Elastosil or Sylgard, having good chemical and mechanical resistance, provides protection of the final system but it also helps during fabrication. The fabricated samples (ID array silicon islands with mesh interconnects embedded in Elastosil) have been mechanically and electrically tested and provide reversible stretchability to around 100%.

Compressive formability of 7075 aluminum alloy rings under hydrostatic pressure

In order to investigate the influence of hydrostatic pressure on compression limit of the ring, numerical simulation and experimental research were carried out. The effect of hydrostatic pressure on the deformation of aluminum alloy 7075 ring was obtained by numerical simulation. The die set for compressing ring under high hydrostatic pressure was designed and manufactured. Experimental results show that the compression limit increases linearly as the hydrostatic pressure increases in a certain range. At 100 MPa the maximum compressive strain is increased by 32.42%. At strain limit, the cracks initiate from the corner of the outer wall to the middle of the inner wall along the direction of the maximum shear stress.

Determination of Processing Windows for the Hot Stamping of AA7075

Hot stamping of aluminium alloys is a tailored forming process, with the assigned processing windows determining the quality of each hot stamped component in terms of its post-form strength. In this work, a processing window calculator, ‘Tailor’, was developed in order to define the optimal processing parameters that should be used in a production line to successfully produce a component with the desired post-form strength using hot stamping. ‘Tailor’ was developed using the results of forming tests, air-cooling tests and multi-stage artificial ageing tests, which provided guidance on the values for the die closing force, transfer time and artificial ageing time to be used in the hot stamping process. The effectiveness of ‘Tailor’ was demonstrated in two case studies.

Formability of AA6082-T6 at Warm and Hot Stamping Conditions

Forming limit diagrams (FLDs) of AA6082 at warm/hot stamping conditions were determined by using a specially designed test rig. The tests were carried out at various temperatures from 300 to 450°C and forming speeds ranging from 75 to 400 mm/s. The strain was visualized and measured using ARGUS software provided by GOM. The results clearly show that the formability of AA6082-T6 sheet metal, in terms of the limit major strain, increased by 38.9 % when the forming temperature was increased from 300°C to 450°C at a speed of 250 mm/s, and increased by 42.4 % when the forming speed was decreased from 400 to 75 mm/s at a temperature of 400°C. It was verified that hot stamping is a promising technology for manufacturing complex-shaped components.

Melt Conditioned Twin Roll Casting (MC-TRC) of Thin Mg-Alloy Strips for Direct Stamping of Mg Components

In this paper we introduce a novel process for the production of thin-walled magnesium components by direct stamping of twin roll cast thin Mg strips. In this process, the melt conditioned twin roll casting (MC-TRC) process is used to produce thin Mg strips (thickness <2 mm) which have a fine equiaxed grain structure and little basal texture and, more importantly, are free from centreline segregation. Such thin Mg strips can be used for thin-walled component production by direct stamping without any rolling. A major advantage of this process is that it circumvents the low formability problem inherently associated with Mg based alloys. In this paper, AZ31 alloy is used to demonstrate this new process. For both TRC and MC-TRC strips, we will analyze the microstructures, assess the mechanical performance at elevated temperatures and conduct hot stamping in the as-cast condition without any prior rolling.

Numerical Investigation on the Hot Forming and Cold-Die Quenching of an Aluminium-Magnesium Alloy into a Complex Component

An FE model for the hot forming and cold-die quenching (HFQ) process was developed. This model was verified by HFQ experiments through a comparison of the thickness distribution between the simulated and experimental results; good correlation with a deviation of less than 5% was achieved. In addition, this FE model was used to study the effects of forming speed on the thickness distribution of a HFQ formed part, and it was found that a higher forming speed is beneficial for HFQ forming, as it led to improved thickness homogeneity and less thinning.