Anders Ivarson

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.

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.

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.

The role of oxygen pressure in laser cutting mild steels

This paper presents the results of an experimental program investigating the effects of using high pressure oxygen as assist gas in combination with a pulsed laser while cutting medium thick mild steel plates. It was discovered that if the pulse conditions are optimized, the maximum cutting speed for a set average laser power could be increased by up to 10% compared to low oxygen pressure continuous wave (CW) laser cutting. The assist gas was found to have two optimum pressure ranges between which the material suffered from burning on the cut edge. The paper presents a phenomenological model to explain the changes in cut front dynamics as the oxygen pressure is increased and the role of pulsing in suppressing edge burning.

LASER CUTTING OF STEELS: A physical and chemical analysis of the particles ejected during cutting. Part II

This paper is a continuation of work published in an earlier edition of this journal[1]. Both papers deal with the results of a detailed analysis of the particles ejected from the cut zone during laser cutting of mild and stainless steels. Reference 1 presented the thermochemical data and discussion generated by the experimental investigation. This paper concentrates on the physical characteristics and metallography of the particles. Analysis techniques used included: Scanning Electron Microscopy, optical microscopy, metallography and particle sizing. The results from this extensive analysis have enabled the authors to postulate oxidation histories for the particles. In addition to this it has been possible to explain the variation in particle size for various material thicknesses.

Optimisation of the Piercing or Drilling Mechanism of Abrasive Water Jets

Although Abrasive Water Jet drilling is an industrial application in its own right its most common use is the production of a start-up hole for a subsequent cutting operation. This paper presents the results of an experimental analysis of the piercing process concentrating on the improvement in penetration time possible if the abrasive water jet is moving rather than stationary. Linear and circular movements of the jet have been investigated and it is shown that penetration times can be reduced by an order of magnitude. From the results the authors have developed a phenomenalogical model which explains the generally superior performance of moving jet piercing.

Comparison between Abrasive Water Jet Cutting and Laser Cutting

This paper is intended to demonstrate the advantages and disadvantages of laser profiling techniques as compared with the Abrasive Water Jet (AWJ). The growth of AWJ as a cutting tool has provided engineers with a new profiling technique which often offers great technical and commercial advantages over more traditional methods. However, AWJ cutting is not the best solution to all profiling problems. There are a number of techniques which compete with or complement the process and the optimum profiling method can be difficult to identify[1]. The following paper serves as a general guide‐line comparing two competitive cutting methods (CO2 laser cutting and Nd: YAG laser cutting) with AWJ cutting. The subject of cutting covers a great many more processes than can be reviewed in one article but the techniques to be discussed were chosen because they all involve profiling using an axially symmetric energy beam of some sort.

Effects of different shielding gas compositions on the process of cw CO2 laser welding in the hyperbaric range.

A continuous carbon-dioxide laser of 1.35 kW has been used to study the welding of 5 mm thick stainless steel for pressures ranging from 0.1 to 0.8 MPa in increments of 0.1 MPa. Experimental data, including penetration depths, weld widths, and in some cases weld pool profiles, has been obtained for each value of the pressure using different mixtures of argon and helium shielding gases. In a previous paper it has been reported that keyhole welding could not be carried out for pressures significantly in excess of atmospheric pressure using pure argon and nitrogen shielding gases, but that the process was possible at pressures up to 0.8 MPa using helium. In the present paper the critical pressure for keyhole welding is determined as a function of the mixed shielding gas composition. The laser material interaction is analyzed by solving the heat conduction equation with line and point heat sources representing the keyhole and plume respectively. The line source strength is itself calculated from consideration of the inverse bremsstrahlung and Fresnel absorption processes in the keyhole. It is concluded that successful laser welding in the hyperbaric range crucially hinges on good plume control through the effective delivery of an appropriate shielding gas mixture.