Multivariate modelling of variability in sheet metal forming

The inherent variability in incoming material and process conditions in sheet metal forming makes quality control and the maintenance of consistency extremely difficult. A single FEM simulation is successful at predicting the formability for a given system, however lacks the ability to capture the variability in an actual production process due to the numerical deterministic nature. This paper investigates a probabilistic analytical model where the variation of five input parameters and their relationship to the sensitivity of springback in a stamping process is examined. A range of sheet tensions are investigated, simulating different operating windows in an attempt to highlight robust regions where the distribution of springback is small. A series of FEM simulations were also performed, to compare with the findings from the analytical model using AutoForm Sigma v4.04 and to validate the analytical model assumptions.

Results show that an increase in sheet tension not only decreases springback, but more importantly reduces the sensitivity of the process to variation. A relative sensitivity analysis has been performed where the most influential parameters and the changes in sensitivity at various sheet tensions have been investigated. Variation in the material parameters, yield stress and n-value were the most influential causes of springback variation, when compared to process input parameters such as friction, which had a small effect. The probabilistic model presented allows manufacturers to develop a more comprehensive assessment of the success of their forming processes by capturing the effects of inherent variation.

Numerical investigation of ramp and constant pressurization system during tube crushing process

The requirement for the automotive industry at present and even more so in the future is to simultaneously develop materials, economic forming processes and techniques for weight reduction of the component. To fulfil this need steel manufacturers have developed Advanced High Strength Steels which have high strength and good formability. Due to high strength, material thickness can be reduced without compromising the function of the component. High pressure hydro forming is one process that can be used to produce complex components from these materials. However, reduction in material thickness of these steels does not result in a large decrease of internal fluid pressure and die closing force during tube hydro forming and hence the higher strengths of these steels will require higher pressures. Tube crushing is a process in which the component can be formed with low pressures. In this paper numerical comparison of ramp and constant pressurization system during tube crushing for a TRIP steel is studied. It is proposed that ramp pressure is the best option to obtain a part with accurate geometrical shape from tube crushing with less die closing force. The stress and thickness distribution of the part during tube crushing were critically analysed.

Experimental and numerical evaluation of forming and fracture behaviour of high strength steel

Car manufacturers are under pressure to reduce vehicle mass while maintaining comfort and passenger safety for current and future vehicles. To meet this demand the steel industry has developed Advanced High Strength Steels (AHSS) that promise higher strength and improved formability compared to conventional steel grades. Even though significant research has already been performed to evaluate the material properties and forming behaviour of most AHSS types, only a limited literature is available on their necking and fracture behaviour and the effect on formability. This paper examines and compares the thinning, necking and fracture behaviour of two AHSS and one conventional steel type, namely TRIP, DP and HSLA. Uniaxial, plane and biaxial strain conditions are investigated by tensile, cup drawing and stretch forming tests and by using numerical methods. The test results indicate that significant differences exist in necking and fracture behaviour between all three steel types.

Some issues relating to the ductility of wrought magnesium alloys

The present work is concerned with gaining a better understanding of the factors that control the ductility of wrought magnesium alloys. The ultimate aim is to develop alloys with vastly improved room temperature formability. It is shown that 3D tomography of fractured tensile specimens reveals disk shaped voids aligned more or less at 45 deg. to the tensile axis. These voids are consistent with twin induced void formation. It is also shown that the double twins that produce such voids form in contradiction to Schmid predictions. Finally, it is demonstrated that low levels of rare-earth additions leads to vastly improved texture and ductility in extrusions, as they do in rolled sheet.

FEA comparison of high and low pressure tube hydroforming of TRIP steel

The increasing application of hydroforming for the production of automotive lightweight components is mainly due to the attainable advantages regarding part properties and improving technology of the forming equipment. However, the high pressure requirements during hydroforming decreases the costs benefit and make the part expensive. Another requirement of automotive industries is weight reduction and better crash performance. Thereby steel industries developed advanced high strength steels which have high strength, good formability and better crash performance. Even though the thickness of the sheet to form the component is reduced, the pressure requirement to form the part during expansion is still high during high pressure hydroforming. This paper details the comparison between high and low pressure tube hydroforming for the square cross-section geometry. It is determined that the internal pressure and die closing force required for low pressure tube hydroforming process is much less than that of high pressure tube hydroforming process. The stress and thickness distribution of the part during tube crushing were critically analysed. Further, the stress distribution and forming mode were studied in this paper. Also friction effect on both processes was discussed.

High performance mg alloy by grain refinement

Mg alloys are one of promising eco-materials. The present paper describes the importance of grain refinement to develop high performance Mg alloys. The fine-grained Mg alloys exhibit not only a good combination of high strength and high ductility at room temperature, but also high formability (superplasticity) at elevated temperatures.

Extrusion properties of a Zr-based bulk metallic glass

The extrusion behavior of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 metallic glasses in the supercooled liquid region was investigated. Good extrusion formability was observed under low strain rates at temperatures higher than 395 °C. The metallic glasses were fully extruded without crystallization and failure within the range of T=395–415 °C under strain rates from 5×10⁻³ s⁻¹ to 5×10⁻² s⁻¹, and the deformation behavior of the metallic glasses during the extrusion was found to be in a Newtonian viscous flow mode by a strain rate sensitivity of 1.0.

Compression and extrusion of metals using rotating dies

Experimental and theoretical investigations of compression and extrusion of metals with steadily or cyclically rotating dies were carried out. Reasonably simple models were produced by classical plasticity theory and analytical equations were developed to establish a theoretical basis for the associated phenomena. Analytical solutions agreed well with the experimental results.

Low strain processing of LC and ULC steels

Temper rolling and tension levelling are commonly used to manufacture flat rolled steel. Both processes lengthen the steel at strains up to 3% by applying a load and stretching the strip. By latering the balance between the load and the tension the formability of the low carbon and ultra low carbon steel may be optimised.

Springback behaviour of TRIP steels in sheet metal forming

Examines the methods for numerical modelling of the springback effect in TRansformation Induced Plasticity (TRIP) steels under conditions of various bending processes. It represents a largely unexplored part of the TRIP steel literature and therefore makes a valuable contribution toward a practical approach to predicting springback in TRIP steels.

Advanced modelling of forming high strength steels

The thesis presents a novel approach in the multiscale modelling of Advanced High Strength Steels for prediction of the microstructural effects in forming processes. The results are compared with that of experiments and finite element method. The method is proved to be suitable for complexities in the multiphase AHSS.

Effects of aging bake hardenable steel in sheet metal forming

Bake hardening steels are being used as body panels in modern cars. These steels are stronger through an ageing process after baking in the paint ovens, which allows the thickness of the steel to be reduced without reducing the denting performance. The current study examined whether the steel ages at room temperature prior to forming which would deteriorate the formability. The work developed a new test and provided insight into acceptable levels of natural ageing which could be tolerated in the manufacturing process.

Press forming performance of galvanneal coated steel

This project quantified the press formability of galvanneal coated steel. A coil of galvanneal coated steel was produced containing various coating structures. Using different lubricants, the powdering, friction and forming properties were determined. It was found that optimum formability was obtained by effective lubrication while galvanneal coating structure controlled powdering resistance.

Mechanical properties and blow forming of rolled AZ31 Mg alloy sheet

Rolling was conducted at 373-673 K for AZ31 Mg alloy; mechanical properties of the rolled Mg alloy were investigated by tensile and blow forming tests. The grain sizes of all the rolled specimens were smaller than that of the specimen prior to rolling. At tensile temperatures under 373 K, the rolled specimens showed much higher 0.2% proof stresses than the non-rolled specimens due to their fine-grained microstructure. However, the strength of the rolled specimens decreased significantly at 473 K. Superplastic behavior was obtained at 573-723 K for the specimens rolled at 498 K. Blow forming tests demonstrated that specimens rolled at 498 K exhibited a high degree of formability at 723 K.

Cyclic deformation behaviour of ultrfine-grained aluminium

Ultrafine-grained aluminium was produced by cryo-rolling and their deformation response under cyclic loading was investigated. Shear banding and grain coarsening were recognized as the main damage mechanism reducing their performance under cyclic loading. However presence of precipitates in ultrafine-grained A1 can actively hinder the operation of cyclic softening mechanisms and increase microstructural stability under cyclic loading.