HEAT TREATING WITH LASERS

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1 HEAT TREATING WITH LASERS Dr. Mohammed Naeem GSI Lumonics Rugby, England T he material used in a manufactured component often must exhibit properties that are a compromise between those necessary for the bulk of the component and those required at its surface. The modification of the surface by heat treatment or similar surfacing technique provides a solution to that compromise. It allows the specification of the bulk material to be dictated by critical constraints such as tensile strength or economic justification, while the surface is tailored to other properties required for its service In some applications, specific properties desired at the surface of a component differ from those required of the component s bulk. In some of these situations, lasers have proven ideal in achieving localized material property modifications. use or life, such as wear or corrosion resistance. There is currently a variety of surface modification techniques available. These include: thermal treatments such as induction, flame, and laser hardening; thermochemical diffusion treatments such as carburizing, carbonitriding, and nitrocarburizing; and mechanical treatments, such as peening and fillet rolling. The high-power heat source produced by a laser beam is ideal for surface modification. Laser heating produces rapid, local changes at the HEAT TREATING PROGRESS MAY/JUNE

2 Power density, W/mm 2 Fig. 1 Range of laser processes mapped against power density and interaction time. Power density, W/mm Shock hardening Drilling Glazing HEATING Laser glazing Fig. 2 Laser heat treating regimes. Cutting MELTING Welding Melting Cladding Transformation hardening Interaction time, sec Interaction time, sec VAPORIZATION Laser cladding and alloying Transformation Hardening surface of the material, leaving the properties of the rest of the component unaffected. In so doing, it enables localized microstructural modifications to a range of materials. Surface Engineering Categories Major laser surface engineering applications can be divided into three broad categories: those relying on heating without melting (heat treating, annealing, transformation hardening); those requiring metallurgical changes in the surface of the bulk material (transformation hardening, annealing, grain refining, glazing, shock hardening); and those involving a chemical modification to the surface by the addition of a new material (alloying and cladding). Heating without melting, commonly known as heat treating, involves solid-state transformation so that the surface of the metal is not melted. The fraction of the beam power absorbed by the material is controlled by the absorptivity of the material surface. Both mechanical and chemical properties can often be greatly enhanced through the metallurgical reactions produced during these heating and cooling cycles. Heating with melting, such as utilized in laser glazing, surface homogenization, and re-melting, requires very rapid heating, melting and cooling cycles to alter surface properties. Modifications such as cladding and alloying impregnation require the melting of the surface with the addition of other material to the surface to form a new modified surface layer. There are a number of mechanisms by which these changes can be brought about, but all depend on the high power density of the laser beam and the ability to manipulate the beam accurately. Advantages of Laser Heat Treating Laser heating principles are similar to those of conventional heating processes. The time scales involved in the former are, however, typically an order of magnitude shorter. Whereas heating is traditionally induced by a furnace, flame, arc, or induction coil, the laser beam is focused or shaped into a suitable pattern, which is scanned over the component. The high energy density of the laser beam heats the surface much more rapidly, reducing the time for conduction into the bulk of the component. Because laser heat treatment and surfacing techniques must compete directly with other comparatively low cost conventional processes, laser must therefore offer significant advantages to be a competitive option. These specific advantages include: Chemical cleanliness. Minimal heat input Since the source temperature is quite high, surface transformation occurs so quickly that the heat input to the part is very low, with a very small heat-affected zone (HAZ), thus reducing distortion to the part. Non-contact process. Elimination of post-machining processes. 48 HEAT TREATING PROGRESS MAY/JUNE 2005

3 Ease of integration. Cosmetically-favorable appearance. The laser emits a beam of energy in a continuous wave or pulsed form. The power and the diameter of the focused laser beam can be combined to give one laser parameter called power density. The second parameter of laser treatment is the rate, or speed, at which the power density is moved across a part s surface. This is often expressed as the interaction time the length of time the laser beam is focused on any one point on the surface. Figure 1 shows a range of laser material processes that can occur at different power densities and interaction times. Figure 2 shows a modified version representing only the heat treatment processes. Materials of high hardenability may be processed with lower power density and a higher interaction time, in order to achieve a homogeneous case with significant depth. Materials with low hardenability are processed with higher power density and lower interaction times in order to generate the rapid cooling rates required for martensite formation. Fig. 3 Beam distributions (top) at end of fiber; and (bottom) at focus. Laser Sources and Research Trials There are currently four different types of laser sources being used for laser heat-treating applications. These are: CO 2 lasers, lamp and diodepumped Nd:YAG lasers, and high power diode lasers. Until about 10 years ago, only CO 2 lasers were able to deliver the combination of power density and interaction time necessary for laser heat treatment. The development of multi-kilowatt Nd:YAG lasers, with both lamp and diode-pumping, provided an alternative source. One of the primary advantages of the Nd:YAG laser source is that the wavelength of the laser light (1.06µm) allows the beam to be delivered via an optical fiber with relatively small energy losses. This allows flexible delivery of the beam at the processing head. Consequently, Nd:YAG lasers, providing high levels of laser power, can be manipulated using a robot, thus making them ideal for three-dimensional processing. In recent years, GSI Lumonics has undertaken a number of customerdriven projects to better understand laser heat treatment with Nd:YAG lasers. Work has centered on high average power (500-3,000W) continuous wave (CW) lasers. Results from these projects are incorporated into the following findings. As the beam wavelength decreases for example from 10.6µm for a CO 2 laser to 1.06µm for Nd:YAG the absorptivity of metal surfaces increases, eliminating the need for an absorptive coating. This simplifies the operation considerably. More recently, multi-kilowatt diode lasers have been developed with wavelengths of 0.8µm. These are compact and able to be mounted directly on a robot for hardening of components with exceptionally complex geometries. Process Parameters A round beam is often used for hardening with both CO 2 and Nd:YAG laser beams. This shape, a satisfactory solution for many applications, is created by de-focusing the beam. The depth profile of the hardened region can be approximated as the mirror image of the beam intensity distribution, with reduced amplitude and some rounding of the edges resulting from lateral heat flow. Fig. 3 shows the beam profile of a fiber-delivered CW, Nd:YAG laser both at the end of the fiber and at focus. By using beam-shaping optics, the HEAT TREATING PROGRESS MAY/JUNE

4 Table 1 Transformation hardening results (low carbon steel). Hardened area Power Spot size Speed Width Depth Hardness Surface kw mm m/min (mm) (mm) HV 0.2 conditions ±30 Uncoated ±31 Uncoated ±20 Graphite coated ±43 Uncoated ±10 Uncoated ±57 Graphite coated ±35 Uncoated ±92 Graphite coated ±22 Uncoated Table 2: Transformation hardening results (low carbon steel) Hardened area Power Spot size Speed Width Depth Hardness Surface kw mm m/min (mm) (mm) HV 0.2 conditions ±20 Uncoated ±9 Uncoated ±31 Uncoated ±40 Uncoated ±35 Uncoated Table 3: Typical industrial application of laser transformation hardening Industry sector Component Material Automotive Axel bearing seat AISI 1035 steel Automotive Blanking die Tool steel Automotive Engine valve Alloy steel Automotive Gear teeth Steel Automotive Shaft Steel Automotive Piston ring Steel Automotive Steering gear housing Malleable cast iron Domestic good Typewriter interposer AISI 1035 steel Machine tools Cutting edge Steel Machinery Gear teeth AISI 1060/ low alloy steel Machinery Mandrel Martensitic stainless steel Machinery Press brake tools Steel Machinery Tool bed Cast iron Power generation Turbine blade edge Grey cast iron Railway Diesel engine cylinder Cast iron Table 4: Summary of laser surfacing techniques & uses Process Laser type Process description Transformation hardening CO 2 and Nd: YAG Local hardening, with case depth up to 2mm Cladding CO 2 and Nd: YAG Deposition of a second material onto surface Surface alloying CO 2 and Nd: YAG Local alloying to change the surface properties Shock hardening CO 2 and Nd: YAG Produce hardened surface layer by using pulses of laser energy Surface homogenization CO 2 and Nd: YAG Alter the microstructure by remelting shape of the hardened sections can be varied and it may be possible to harden with higher coverage rates. If a uniform depth profile of constant width is needed, a kaleidoscope is usually the least expensive solution. Shield gas serves two functions in laser heat treating. It shields the heated/melt zone from oxidation, while protecting the optics from fumes. Argon and nitrogen shield gases are normally used, with typical flow rates of about 20 l./min. The flow rate will depend on the method of shielding as well as the diameter of the nozzle used to deliver the gas. The length of the beam in the travel direction is fixed by the power density and track width requirements. A power level in the 1-4kW range is normally used. A high power enables high feed rate to be used, with correspondingly high coverage rates. However, the practical range that can be used varies considerably as risk of both overheating, which can lead to surface melting, or an insufficient peak temperature, which results in insufficient hardening, can occur. Feed rate is the variable that is normally changed when fine-tuning the process in order to achieve the required hardened depth and degree of homogenization. The range of alloys that can be transformation hardened by laser techniques includes all those that can be conventionally hardened. The response of steel to hardening increases with elevated carbon content, with hardness values exceeding 700HV for steels containing 0.75% carbon. In addition, because of the high cooling rates, plain carbon steel containing 0.2% carbon will harden. The hardenability of cast irons is controlled by the amount of pearlite present while only martensitic stainless steels will respond to heat-treating. Trial Results A summary of the surface treatment conditions used and results achieved on the transformation hardening of low carbon steel at different power levels and processing speeds is presented in Table 1. For low carbon steel, using a 5mm spot diameter, the minimum speed at which transformation hardening without melting 50 HEAT TREATING PROGRESS MAY/JUNE 2005

5 could be achieved was 0.5 m/min. Transformation hardened sections displayed a primarily martensitic structure with some bainite and had a typical micro hardness of about 740 HV 0.2. The initial trials on transformation hardening using a 4kW CW laser extended the processing capabilities as seen in Table 2 in terms of transverse speed when compared to a 2kW CW laser. The minimum speed at which transformation hardening could be achieved was 3.5 m/min. A micrograph of a sample produced at this speed is shown in Fig. 4, with a maximum treated depth of 0.7 mm. The maximum hardness in the laser treated zone was measured at 755 HV 0.3, with 6 mm spot diameter at 4.5 m/min compared to a parent material value of 300 HV 0.3. Surface condition affected the size of the hardened areas of the samples. At the same processing parameters, a graphite-coated sample tended to show more surface melting than uncoated samples. This is to be expected due to the higher absorptivity of the coated surface and the effect of carbon on the melting point of steel. At the same processing speed, the hardened zones were generally wider but not much deeper in the coated samples. It should be noted that the samples were coated to assess the effect of surface conditions rather than the possibility of alloying the steel with carbon. In general, with CO 2 samples are always coated to improve their absorptivity. Applications The automotive and machine tool industries have been responsible for much of the laser heat treatment process development that has taken place within industrial markets. Some of the more popular applications are listed in Table 3. The availability of new laser sources, CAD software improvements, process control equipment advances, and improved understanding of the laser heat treatment process have all resulted in the growth of a Fig. 4 Transverse section of laser hardened track in low carbon steel, 3.5 m./min., 6mm spot size. number of industrial applications. The principle laser surface engineering applications are summarized in Table 4. Author Mohammed Naeem, material processing manager of GSI Lumonics Rugby, England facility, is an active researcher who has published and spoken widely on laser material processing, an area in which he enjoys an international reputation. He holds his M.Tech. degree in metallurgical quality control from Brunel University and his Ph.D. in glass fiber composites from Loughborough University of Technology. For additional information contact GSI Lumonics, Northville, MI Tel: (248) ; Fax: (248) ; or visit Circle 17 or visit 51