William M. Steen

Plasma charge sensor for in-process, non-contact monitoring of the laser welding process

Laser keyhole welding is an important modern manufacturing technology. During such welding a plasma cloud is generated and the behaviour of this plasma is closely coupled to the behaviour of the weld. The plasma behaviour has been observed during welding through measurement of the space charge voltage induced on an electrically insulated welding nozzle, the plasma charge sensor (PCS). It is shown both theoretically and experimentally that the induced voltage is a measure of plasma temperature and thus of the welding performance. In particular, it is shown under laboratory conditions that the PCS signal can measure weld penetration and detect a wide range of weld defects. Results have also been obtained from the PCS installed on an industrial laser can welder. An expert system has been used to analyse the PCS signal automatically and classify weld defects. It is reported that a greater than 90% success rate has been achieved in detecting and classifying defects in high-speed industrial laser can welding. The PCS is shown to be a reliable and robust non-contact method of on-line weld monitoring well suited to installation on existing laser welding stations.

Design characteristics and development of a nozzle for coaxial laser cladding

Laser cladding by blowing powder into a laser generated melt pool is finding greater use in industry particularly with the growing interest in direct build and repair techniques currently being investigated. The process is traditionally done with a side blown powder feed. This has the limitation of not being omnidirectional. To overcome this, a coaxial powder feed system has been developed. This article describes a particular system by both mathematical models and experiment. The system has so far achieved a catchment efficiency of up to 40% with a 1 kW CO2 laser beam. The parameters controlling this important commercial parameter are described. They include the flow structure of the powder in the beam. The control of this structure is discussed with reference to this particular coaxial flow system.Laser cladding by blowing powder into a laser generated melt pool is finding greater use in industry particularly with the growing interest in direct build and repair techniques currently being investigated. The process is traditionally done with a side blown powder feed. This has the limitation of not being omnidirectional. To overcome this, a coaxial powder feed system has been developed. This article describes a particular system by both mathematical models and experiment. The system has so far achieved a catchment efficiency of up to 40% with a 1 kW CO2 laser beam. The parameters controlling this important commercial parameter are described. They include the flow structure of the powder in the beam. The control of this structure is discussed with reference to this particular coaxial flow system.

Laser removal of oxides from a copper substrate using Q-switched Nd:YAG radiation at 1064 nm, 532 nm and 266 nm

The objective of the work reported is to study the effect of wavelength and the material removal mechanisms of a laser process for removing copper oxide from copper. The removal of copper oxide is necessary in electronics device fabrication in order to improve the surface wetability and so achieve a good quality solder joint. Such a laser cleaning process could be incorporated into an existing laser soldering process. The effect of Q-switched Nd:YAG radiation at 1064 nm, 532 nm and . 266 nm the first, second and fourth Nd:YAG harmonics on the oxidised surface of a copper alloy foil was studied with increasing fluence. The laser-treated surfaces were characterised using optical microscopy, scanning electron microscopy . . SEM , and static secondary ion mass spectroscopy SSIMS . Successful oxide removal was achieved at all wavelengths above certain thresholds that defined the lower end of the process operating window for single-pulse operation. The ablation mechanisms involved in the removal of copper oxide from copper at 1064 nm, 532 nm and 266 nm were found to combine both thermal and mechanical effects, the surface melting first occurring at fluences lower than those at which cleaning is initialised. q 1998 Elsevier Science B.V.

Laser generating metallic components

Recent developments in rapid prototyping have led to the concept of laser generating, the first additive manufacturing technology. This paper presents an innovative process of depositing multi-layer tracks, by fusing successive powder tracks, to generate three dimensional components, thereby offering an alternative to casting for small metal component manufacture. A coaxial nozzle assembly has been designed and manufactured enabling consistent omni-directional multi-layer deposition. In conjunction with this the software route from a CAD drawing to machine code generation has been established. The part is manufactured on a six axes machining center incorporating a 1.8 kW carbon-dioxide laser, providing an integrated opto-mechanical workstation. The part build-up program is controlled by a P150 host computer, linked directly to the DNC machining center. The direct manufacturing route is shown, including initial examples of simple objects (primitives -- cube, cylinder, cone) leading to more complex turbine blade generation, incorporating build-up techniques and the associated mechanical properties.

An in-process method for the inverse estimation of the powder catchment efficiency during laser cladding

An estimation of the heat loss by conduction can be obtained from measurements of the surface temperature and an overall heat balance at the clad laser interaction zone. Through an inverse calculation of the boundary temperature from observed surface temperatures the powder catchment efficiency can be estimated along with the variation in the clad height expected during laser cladding. This method shows a possible way to monitor and control the clad height and profile as required by near net shape manufacturing methods based on laser cladding.

In-process clad quality monitoring using optical method

Although laser cladding has been accepted by industry as one of the methods for the improvement of material surface properties, it is always difficult to produce consistent products over long periods. The reason for these could be the instability of laser parameters, powder feeding parameters (when powder feeder is used) and the rise of substrate surface temperature during processing. This paper investigates the possibility of using an opto-elecironic sensor for in-process monitoring of clad quality. The experiments have revealed that clad bonding condition, clad porosity, clad uniformity (i.e. clad roughness, thickness variation and overlap consistency), substrate faults and system failure (e.g. nozzle clipping, lens cracking, powder feed failure ) etc. could be recognized by the optical signals from the melt pool radiation picked up by the sensor during processing. The use of the sensor for on-line laser cladding fault diagnostic and quality control is also discussed.

Laser removal of surface and embedded contaminations on/in building structures

This paper demonstrates the technical feasibility and basic phenomena of using laser techniques for the non-contact removal of embedded contamination down to depths of 0.1 - 4 mm thick in construction materials such a concrete, brick, plaster/mortar, stones and stainless/mild steels. In this study a high power CO2 laser and a YAG laser were used. The techniques investigated include laser vaporization removal, laser combustion/decomposition removal, laser melt ejection removal, laser thermal fracture removal and laser HAZ delamination removal. The work showed that melt ejection removal can be applied to metal objects with removal depth up to 1.5 mm/pulse while the other four methods are more effective for nonmetallic materials with removal depth up to 3 mm for each pass. Particularly when hydraulic bond materials such as concrete, cement, mortar/plaster, rendering and stones are involved the thermal fracture and HAZ delamination methods were found very effective. Paint, epoxy, and plants such as moss and lichen on the construction bodies can be removed effectively by laser generated combustion/combustion. One of the advantages of laser contamination removal is the energy controllability, remote operation capability (convenient for nuclear decontamination), low waste and high efficiency. In addition amorphous glazing can be generated on the surfaces of construction materials such as bricks and concrete during vaporization removal providing a means of sealing the remaining surface. Optical microscopy, SEM, EDAX, and x-ray diffraction were used to study the affects of laser treatment under various conditions. Mathematical representation of the processes were discussed. Comparison was made between the methods and optimum operating condition provided.

Powder flow and catchment during coaxial laser cladding

The blown powder laser cladding process has recently been greatly enhanced by the development of a coaxial powder feed system. This system is described in this paper together with some of the new applications arising from it. The powder feeder depends on various gas flow streams. The effect of these streams on the focus of the powder stream is discussed from experiments using image analysis. The thermal behavior of the powder particles as they fall in a laser heated stream is also analyzed theoretically and experimentally. The effect of the powder focus and preheat on the catchment efficient of the resulting clads is mathematically modelled and compared to the experimental results for a stainless steel clad on a mild steel substrate.

Moldless casting by laser

The principle of laser cladding involves the use of high power carbon-dioxide lasers and powder deposition technology to provide wear and corrosion resistant surface coatings to engineering components. By injecting metal powder into a laser generated melt pool on a moving substrate a solidified metal track can be produced. Deposition of successive tracks produces a multi-layer build. Laser direct casting (LDC) utilizes a coaxial nozzle enabling consistent omnidirectional deposition to produce 3D components from a selection of metal powders. The influence of the principal process parameters over the process features namely, powder catchment efficiency, beam shape and build rates are presented with several successfully generated 3D components. Nickel, stainless steel and satellite powders were deposited at laser powders of 0.4 to 1.4 kW and speeds of 500 to 1000 mm/min achieving build rates of 3 to 9 mm3/s. Fully dense metallurgical structures have been produced with no cracking or porosity and powder catchment efficiencies up to 85% have been achieved.

Multi-frequency fibre optic sensors for in-process laser welding quality monitoring

Abstract Laser welding is a high temperature machining process in which there are two principal methods of beam absorption: absorption by the plasma and Fresnel absorption by the surface. The plasma absorption and reradiation generates shorter wavelength light waves whereas the heated surface reradiates in the longer wavelengths. This paper describes the experimental relationships between weld quality and the multi-frequency fibre optic sensor signals during laser welding. An analysis of the likely meaning of the signals is presented together with an appraisal of the technique as an on-line weld monitor.