Claes Magnusson

An analytical thermodynamic model of laser welding

An earlier model of deep-penetration laser welding has been simplified in order to provide a useful model of process analysis. This work involves the modelling of the various energy-absorption mechanisms which determine the keyhole shape and thus the dimensions of the melt pool. The penetration depth and weld width (top and bottom) predicted by the model are shown to be in close agreement with experimental results. The widening of the top of the weld seam as a result of Marangoni flow is accurately modelled by introducing an artificially enhanced value for the workpieces thermal conductivity towards the top of the weld. The model allows analysis of the dependence of the weld profile on the process parameters.

The oxidation dynamics of laser cutting of mild steel and the generation of striations on the cut edge

Over the past two decades CO2 laser cutting has grown from an obscure laboratory technique into an important branch of manufacturing engineering. The most commonly cut materials are steels and a great deal of industrial and scientific research has been carried out on the laser-material interactions that generate a cut. This paper concentrates on the phenomena which give rise to a cyclic cutting event when a CO2 laser in conjunction with an oxygen jet is used to cut mild steel. The nature of the cut edge striation produced by the cyclic oxidation reaction is explained thoroughly and a possible oxidation cycle is postulated. It is demonstrated that the key to understanding the cyclic nature of the cutting event is the self-limiting nature of the oxidation of the steel in the cut zone.

Energy redistribution during CO2 laser cladding

This article examines the factors that effect the efficiency of the CO2-laser powder cladding process. By theoretical calculation and experimental work it has been possible to identify how much of the original laser energy contributes to the cladding process and how much is lost to the surrounding environment by reflection, radiation, convection, etc. Every aspect of energy redistribution has been analyzed and quantified and this has led to a deeper understanding of the process. The article concludes with a number of suggestions for improving the efficiency of blown powder laser cladding.

The Bauschinger effect in compression-tension of sheet metals

It is important to know how sheet metals behave under complicated loading conditions, since yielding and hardening of a metal are actually dependent upon the stress state and deformation history. A ...

The Role of Oxidation in Laser Cutting Stainless and Mild Steel

This paper gives the results of a detailed examination of the particles ejected from the cut zone during CO2 laser cutting of mild and stainless steels. Cuts were carried out over a range of material thickness at the optimum speed for each at a laser power of 900 Watts. Particles ejected from the cut zone were collected and analyzed to establish their chemical and physical characteristics. Analysis techniques included Scanning Electron Microscopy, wet chemical analysis, optical microscopy, metallography and particle sizing. The results from this extensive analysis have enabled the authors to estimate the heat generated by the oxidation process during cutting of both mild and stainless steels.

Plastic bending of anisotropic sheet metals

Abstract The effect of anisotropy on pure bending of sheet metals has been studied. Anisotropy is known as the variation of mechanical properties with respect to orientation in sheet metals due to preferred crystallographic orientation or “fibring”. Anisotropy is an important parameter to be considered in simulating the bending process. A constant specifying the plane bending of anisotropic sheets has be defined and incorporated in the two models presented. It was found that the effect of anisotropy on material thinning of a bend is small but has a relatively large effect on the bending moment. The Bauschinger effect is also considered in the study, and the model incorporating the Bauschinger effect predicts greater thinning of the thickness than that predicted without considering the effect.

Application of an image processing technique in strain measurement in sheet metal forming

Abstract In sheet metal forming, strains or strain increments are evaluated usually from the shape change of grids marked previously on the surface of the workpiece, the present understanding of formability in a forming process being based largely on the knowledge gained from the strain measurement. The theory of square-grid analysis is a great help in gaining knowledge of sheet metal forming. In the application of this theory, nodal points on a sheet metal surface etched with square-grids have to be measured. Conventional grid measurements are performed manually, which is either time-consuming or of low accuracy. It is known that the image processing technique is very powerful in digitizing the image of an object. A newly developed image processing device which consists of a video camera, a monitor and a personal computer, has therefore been introduced into strain measurement. The computer is equipped with an AD card and with data processing software. With such a device, the measurement of a gridded surface can be performed conveniently, with accuracy and efficiency. The combination of the image processing technique and the theory of square-grid strain analysis permits rapid measurement and strain analysis over the surface of a workpiece marked with a large number of nodal points.

An empiric model for controlling springback in V-die bending of sheet metals

Abstract A process model is presented for v-die bending of sheet metals. This model is especially valuable to control a press brake equipped with a computer integrated controlling and planning system. A bent part recovers elastically on release of load, and the springback varies with both material and process parameters. A desired bend angle is obtained by controlling the punch insertion depth so that the springback can be compensated by right amount of overbending. The model combines analytical method with trial and error work to establish the relationship of punch insertion depths and corresponding unloaded bend angles. To apply this model, three trial bendings are required for sheet metal and tool combination, so that three coefficients in the half-empiric model can be obtained in a regressive way. Experiments have shown that process parameters affect springback more obviously than material properties do, and the model predicts accurate punch insertion depths for desired angles in air bending. The model is simple, so it can easily be used for real time control.

The role of oxygen purity in laser cutting of mild steel

This paper begins by describing the thermal inputs to the cut zone when cutting mild steel with oxygen as the assist gas. The investigation concentrates on the chemical thermo-dynamics in the cut zone and pays particular attention to the influence of oxygen dilution. A detailed experimental program has allowed the authors to theoretically clarify why the process is highly sensitive to small levels of contamination of the oxygen jet.This paper begins by describing the thermal inputs to the cut zone when cutting mild steel with oxygen as the assist gas. The investigation concentrates on the chemical thermo-dynamics in the cut zone and pays particular attention to the influence of oxygen dilution. A detailed experimental program has allowed the authors to theoretically clarify why the process is highly sensitive to small levels of contamination of the oxygen jet.

Finite-element simulation of V-die bending: a comparison with experimental results

A major problem in sheet bending is to compensate for springback. Analytical descriptions are not sufficiently general to accommodate influences from the material and the geometry due to the simpli ...