Finite element analysis of resistance spot welding in aluminium

Abstract

In the present work, an integrated simulation model has been developed for analysis of the resistance spot welding process in aluminium alloys via the finite element method. A coupled electrothermal analysis for an axisymmetric sheet-electrode geometry has been carried out that can predict the expected nugget diameter, penetration, and electrode face heating at any instant throughout the welding time. Several calculations have been carried out for different welding currents, welding times, and electrode forces and for different surface conditions of the aluminium sheets. Non-linear, temperature dependent, thermophysical material properties have been considered. An observation of interest is that in most instances the maximum nugget diameter is reached well within 0·02-0·04 s and further flow of welding current simply increases the electrode face heating. Also, the initial surface condition influences the growth of the fusion zone to a great extent. Various other conclusions have also been drawn.     

Selection of parameters for μE-beam welding

Abstract

Electron beam welding is a well known process used where high precision, high reliability welds are needed, often in exotic materials. Recently, it has been proposed to apply the electron beam produced in a standard scanning electron microscope (SEM), with reversible modifications to increase beam current, for microscale welding. In addition to providing the clean environment associated with the column vacuum, the SEM in imaging mode provides exceptional capabilities in visualising extremely small parts. Furthermore, the standard stage and beam motion controls offer the possibility of flexible programming of beam path with relatively minor software additions. In order to better evaluate the requirements for and effects of μE-beam welding (μEBW) on typical microtechnologically important materials, a clear understanding of the characteristics of the SEM's beam and its interaction with possible target materials is needed. The penetration ability of electrons depends strongly upon their accelerating voltage and the target they are being directed at. Hence, in some circumstances the beam may interact as a surface heat source, while in others it may act as a volume heat source, with important consequences on weld schedule development for the parts and geometry being welded. In this work, the authors explore some of the factors involved and propose simple models for the electron beam heat source which depend on the parameters being used.     

Effects of transition from lamellar to compacted graphite on thermal conductivity of cast iron

Abstract

Different levels of magnesium were added to a standard grey iron alloy in order to obtain a range of graphite morphologies from lamellar to compacted graphite. The thermal conductivity/diffusivity of samples, solidified at different cooling rates, was investigated by means of the laser flash technique. There is a significant decrease in the thermal conductivity as the morphology transits from lamellar to compacted graphite. The thermal conductivity of grey iron decreases considerably at elevated temperatures, whereas the thermal conductivity of compacted graphite iron is less sensitive to changes in temperature. At increased nodularities, compacted graphite irons exhibit a maximum thermal conductivity at ～400°C. The influence from the cooling conditions on the thermal conductivity decreases as the morphology alters from lamellar graphite to compacted graphite. The effective thermal conductivity of cast iron is modelled by means of existing models for composites.     

Homogenisation of heat treatment process in high pressure gas quenching

Abstract

High pressure gas quenching has the advantages of pure convective heat transfer, high levels of control, avoidance of cleaning the quenched parts and low environmental impact. However, typical gas quenching facilities exhibit inhomogeneous flow conditions through the quench load and the parts, resulting in scatter in final properties. The upstream flow profile of the load has been identified as a key factor determining local flow conditions and heat transfer. The intensity of the quenching process is determined by the pressure drop that results from the flow resistance of the quench load, although a significant part of the flow passes between the load and the chamber walls and does not contribute to the quenching process. A simulation of the flow inside a commercial high pressure gas quenching chamber was carried out, using a multiscale model to give faster convergence. An experimental analysis of the flow inside a model quenching chamber through velocity measurements and flow visualisation was also performed. Finally, a quenching run with cylindrical parts in a double-chamber vacuum furnace was used to validate the model results. Various upstream velocity profiles were applied to demonstrate their influence on the quenching result. The multiscale simulation approach and the results of the flow process investigation are reported. Guidelines for gas quenching process optimisation are outlined.     

Microstructure and strength modelling of Al–Cu–Mg alloys during non-isothermal treatments: Part 1 – Controlled heating and cooling

Abstract

A model is developed to predict the precipitation kinetics and strengthening in Al–Cu–Mg alloys during non-isothermal treatments consisting of controlled heating and cooling. The prediction of the precipitation kinetics is based on the Kampmann and Wagner model. The precipitation strengthening by the shearable Cu–Mg co-clusters is modelled on the basis of the modulus strengthening mechanism and the strengthening by the non-shearable S phase precipitates is based on the Orowan looping mechanism. The model predictions are verified by comparing with hardness, transmission electron microscopy and differential scanning calorimetry data on 2024-T351 aluminium alloys. The microstructural development and strength predictions of the model are generally in close agreement with the experimental data.     

Numerical analysis of factors influencing Charpy impact properties of TMCR structural steels using fuzzy modelling

Abstract

The influence of chemical composition and thermomechanical processing parameters on the Charpy impact energy of grade 420/460 TMCR (thermomechanically controlled rolled) steels was investigated using fuzzy modelling technology. Fuzzy modelling was applied to develop generic models for the prediction of Charpy impact properties. The fuzzy models obtained, which involved chemical compositions, processing conditions, and Charpy impact energy, were used to reveal correlations between individual influence factors and Charpy impact toughness. Numeric analysis shows that low carbon equivalent value (CEV), low sulphur residual, and low processing temperature are all beneficial to impact toughness, predictions which fit with known metallurgical knowledge thus demonstrating the integrity of the model.     

Comparison of sintering of titanium and titanium hydride powders

Abstract

The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ～1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol^?. The activation energy for Ti hydride also appeared to be about 160 kJ mol^?, but the model did not fit the experimental data well.     

Simulation of tuyere–raceway system in blast furnace

Abstract

A uniform distribution of the blast is an important prerequisite of a balanced blast furnace operation, because the blast is the main source of the hot gases that are needed to preheat, reduce and melt iron ores. The supply of hot gas from the raceways is not necessarily uniform along the furnace periphery, but depends on flow resistances encountered on the individual bustle main tuyere–raceway–raceway boundary routes. A model for this system has been developed in order to study and analyse the effects of changes in tuyere parameters and boundary conditions. Variables such as the total blast volume, blast pressure, tuyere diameter and the combustion degree of injected reductants in the tuyeres can be studied. An online version of the model has also been developed to track how the conditions on the tuyere level change with time in operating blast furnaces.     

Bath mixing behaviour in top–bottom–side blown converter

Abstract

The influence of blowing process parameters on bath stirring was investigated in a model of a top–bottom–side blown converter using physical modelling experiments. It was shown that the side blowing gas flowrate has an important influence on bath mixing time which decreases as side tuyere gas flowrate increases up to a critical flowrate and then plateaus. Bottom gas injection is favourable for bath mixing for top–bottom–side blown converters; however, top lance height, top gas flowrate and bath level have little influence.     

Mild steel corrosion modelling in presence of hydrogen sulphide in aqueous solutions

Abstract

Corrosion of mild steel in aqueous solutions containing hydrogen sulphide was modelled under the condition that an iron sulphide film was formed on the steel surface. In the present model, the iron sulphide forms on the steel surface as a result of a solid state reaction between iron and hydrogen sulphide which has several steps. First a very thin film of iron sulphide nucleates on the steel surface. Then, due to further growth of the initial thin layer, a more porous layer of iron sulphide forms on the initial film. In the present model, it is assumed that mass transfer through the thin iron sulphide layer (i.e. adjacent to the steel substrate) controls the corrosion rate of steel in H₂S aqueous solutions, and as a result electrochemical reactions were not considered. The model was verified against the published experimental data and effects of some parameters such as hydrogen sulphide partial pressure were investigated. The results show that increase in partial pressure of hydrogen sulphide leads to an increase in the corrosion rate of mild steel at the primary stages of the reaction, but as a consequence of formation of iron sulphide scales on the steel surface, it drops with respect to time.