02-03-2013, 10:42 AM
Laser Surfacing
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INTRODUCTION
In today’s highly competitive global markets all industries are looking for cost savings, improved product performance and reliability, failure prevention, longer component life and better environmental protection. Surface engineering offers companies the possibility to improve the performance of engineering components through a range of technologies. Laser surfacing has become a profitable alternative to conventional surfacing technologies in many applications, and the laser has become a valuable and cost-effective tool. Laser surfacing offers a clean and reliable method of depositing coatings onto substrates to impart increased wear and corrosion resistance.
Laser surfacing has advantages over alternative surfacing processes that include: (Steen, 1998; Emmelmann & Lunding, 1995; Brandt & Scott, 1990; Li, 1989)
• Chemically clean, with no combustion or ion bombardment problems,
• Localized heating thus leading to minimum heat input to the substrate resulting in minimal thermal damage,
• Controlled heat penetration,
• Fast processing speed with good repeatability,
• Less post-machining procedures,
Transformation hardening
This is one of the oldest applications of lasers in the world and is now used in production environments in the automotive, power generation, process plant, electrical and electronics industries to improve wear properties of components (Mordike, 1994; Brenner, et al, 1994; Bach, et al, 1990; Shachrai and Secemski, 1992).
Laser melting
The metallurgical aim in laser surface melting is to refine or modify the microstructure of that surface through rapid solidification of the molten layer yielding very fine microstructures containing metastable and supersaturated phases. Three classes of materials can be melted (Mordike, 1995). The most common involves lamellar and spheroidal cast irons. The second class consists of hyper-eutectic and tool steels, especially those having a large carbon content. The third class contains all non-ferrous metals. Aluminium and its alloys are common laser melted materials. An example of the industrial application of this method is the remelting of grey cast camshafts for combustion engines (Tulloch, 1992). The main disadvantage of laser melting is that it is restricted in application because it offers limited property changes compared to laser alloying, cladding and particle injection methods.
Laser alloying
Laser alloying is similar to surface melting, except that alloying elements are added into the molten pool to change its chemical composition and hence surface properties. The high convection in the melt pool resulting from high temperature gradients created by the laser leads to almost complete mixing of alloying elements with the base material. The main characteristics of the process are:
• Many materials can be alloyed into different substrates,
• The thickness of the treated layer can be up to 2 mm,
• Minimal thermal penetration and hence component distortion, and
• High process flexibility.
The alloy can be placed in the melt zone using a number of techniques, however, the most common involves either melting a pre-placed powder layer on the surface of the material or injecting a powder alloy into the melt pool created by the laser on the surface of the material (Ricerby and Matthews, 1991). Illustrated in Figure 2.3 are the two alloying techniques. The pre-placed powder technique involves scanning a defocused laser beam over a powder bed, which is consequently melted and mixed with the substrate material. A disadvantage of the technique is that some form of binder is usually necessary to keep the powder in place in particular when processing complex surfaces. In the injected powder technique, the beam melts the surface creating a molten pool into which the alloying powder is delivered through a specially designed nozzle with the assistance of a carrier gas, typically argon. The advantages of this technique over the pre-placed powder technique are that it is a one-step process and can be used on components with complex geometry.
Summary
This chapter reviewed published papers on laser surfacing processes in general and laser cladding in particular. The review has examined the effects of laser parameters on both the quality of laser surfacing and laser cladding. The review has also examined papers on monitoring and control of the laser cladding process to date.
Utilization of sensors to monitor laser surfacing processes result in improved quality of the end-product. Parameters such as substrate temperature, substrate surface condition or substrate thickness may vary during operation. Therefore, pre-set fixed operating parameters are not appropriate. The main point of interest in process diagnosis of laser cladding is the measurement of radiation emitted from the melt pool. For this task, several types of sensors, such as photodiodes, pyrometers, CCD and COMS cameras have been used. The radiation was measured integrally over the entire melt as well as very locally.