Laser surface treatment not only for automobiles
Czech job shop wants to bring laser hardening to more industries
Czech job shop wants to bring laser hardening to more industries
The town of Plzen (Pilsen) in the Czech Republic is well known for its excellent beer, Pilsner Urquell. However, there is also a group of heavy industry factories; the Skoda concern, large machine tools, steam turbines, electric locomotives and even ship parts are produced here.
|FIGURE 1. Segments for cutting tool with hardened blades.|
The Czech Republic (and Czechoslovakia before) has always been a country of heavy industry with its production range similar to Germany, Sweden or Finland. After 1989, massive growth in the automotive industry came. Car producers Skoda, TPCA, Hyundai, and Tatra are located in the country, with Kia and VW in Slovakia. Toolshops and other subcontractors are located in every town and even in most villages.
|FIGURE 2. One bigger cutting/forming tool.|
In 2005 Tomáš Mužík and Stanislav Nemecek left a university research center and founded their own company, MATEX PM, offering laboratory analyses and expert work for industrial partners. Late in 2008 the company invested in laser technology and transformed into a laser job shop. Because laser metal sheet cutting and car body welding were already widely available in the region, the company focused on laser surface hardening and laser welding.
The company’s key equipment is a 3.6 kW diode laser, fiber coupled with optics mounted on an industrial robot. Application development and process optimization are done in-house because the company has operated well-equipped laboratories for many years.
Laser surface hardening is an old technology, first developed in the 1970s, and there have been a few excellent applications, primarily in the automotive industry, but broad commercial success, until recently, has been more difficult. Because the laser hardening technology seems like a new process to many, MATEX’s main challenge has been to show the economical advantages of the process. It is frustrating because hardening is usually the last production operation, many times on very expensive tools. In today’s tight economic times, it is somewhat easier to bring something new into large factories; but even then, nobody wants to be the first customer. And any ideas about support or joint R&D projects are not subjects for discussion.
Induction vs. laser hardening
Most often MATEX is asked to replace induction or flame hardening, so let’s take a closer look into the technological differences.
Generally, to transform hardened steel or cast iron, the material has to meet three conditions: 1) reach austenitizing temperature above Ac3; 2) achieve rapid cooling down from it to obtain martensitic transformation; and 3) have certain amounts of dissolved carbon in the material (as the main parameter influencing final hardness).
Both conventional technologies bring heat to the surface and let it penetrate into the material in the first step. In the second step, rapid cooling from the surface requires assistance, usually by water-based liquid.
A laser beam produces the surface heating also, but much faster, so that it doesn’t penetrate so deeply. A steeper temperature gradient heats the surrounding material less, resulting in lower distortion with a smaller heat affected zone. Then, in the second step, cooling is produced by conduction into the underlying cold material by a self quenching process.
The total amount of energy applied is lower in the laser hardening process, which offers valuable benefits such as better surface quality and lower part distortion. However, the main technological superiority is in cooling from below, which means the cooling rate is almost constant over the hardened depth, avoiding the occurrence of surface cracks. Additionally, the transformation to a martensitic structure starts from the base material, so the crystals grow epitaxially.
Induction or flame hardening is hard to control, especially on complex shapes and the process is still based on experience and subjective adjustment. Laser hardening has to solve a similar problem: temperature-based control. In real world applications, power-based control cannot be used because of differences in reflectivity, angle of surface irradiation, differences in the base material temperature, and its thickness. Power control is accomplished by an on-line measuring pyrometer and suitable computer software. The best choice is a “two color” pyrometer which is less dependent on surface emissivity. Pyrometer optics have to be built into the laser optics “on-axis” to insure that the incoming IR signal comes from the whole laser irradiated area. This allows the temperature to be held within a few degrees during the hardening process on commonly machined surfaces.
The process is not only controlled by set temperature, but also by the travel speed of the laser spot. For laser hardening, a rectangular laser spot with homogeneous power density is mostly used to prevent surface melting, even when sharply contoured edges are hardened.
Tools for the automotive industry
Many of MATEX’s customers are tool producers for the automotive industry that make tools used for cutting or pressing of plastics and metal parts in mass production. There are many such tool shops in the area, mostly subsidiaries of global companies.
MATEX gets the tools just after their milling; after laser hardening, the tool goes back to the tool shop, where it is aligned, checked, and mounted to a frame. Or the tool may be finalized, tested for some time, then dismounted from the press, and sent to the company for laser hardening.
After customer acceptance, the tool is transported to a production site somewhere in Europe. All this takes only a few days and these tools may be used to form parts for Audi, BMW, VW, Opel, SEAT and other cars.
Basically, four types of tools are laser hardened for the automotive industry.
1) Cutting edges (pinch) for plastic materials. Such tools can consist of a lot of segments or one big piece that weighs tons. They are used for forming hot plastics and cutting them in one step by press. Plastics can contain reinforcing filler such as glass fibers, and cladding is applied to their surface. All this has to be cut perfectly, so sharp and hard blades are necessary (see FIGURES 1 and 2).
2) Cutting parts for metal sheets, mostly made from smaller segments that are not as sharp as for plastics. They are made from different materials that were traditionally hardened in a vacuum oven. Laser hardening of cutting edges is much faster and cheaper with lower distortion. Laser-hardened tools are proven to cut at least 2 mm thick metal sheets. Thus, the highest possible depth of hardening with low distortion is requested, if possible.
|FIGURE 3. Metal forming tool made of nodular cast iron with hardened edges.|
3) Round shapes for metal forming/drawing are mostly made from nodular cast iron. Much higher surface hardness is reached by means of laser hardening, assuring better wear resistance and longer lifetime of the tool. Good surface quality without cracking and stable results are achieved (FIGURE 3).
4) The moving parts of the tool, which slides side on side, are laser hardened instead of nitrided. Shallow hardening depth is needed, as perfect surface quality and no dimensional changes are expected. These parts are up to 200 kg in weight and very precisely machined from all sides of their complex 3D shape. Usually, they are equipped with cooling channels, ended by a brass cap or screw.
|FIGURE 4. Laserhardened tooth wheel segments.|
MATEX has developed a process to harden these tools with fitted caps. The induced heat is so small that they remain tight. This is a great advantage for the tool producers because no dismounting is needed.
Unfortunately, there is no mass production in tool hardening, where each tool is different. They have complex shapes, so programming of the robot’s path limits productivity. Off-line programming software based on 3D models is the solution.
Another challenge is the size and weight of the tools. There are two ways to compensate: either invest in powerful manipulation technology or harden larger parts at the client’s site. This is possible with new compact fiber guided lasers and smaller robots. Recently, MATEX took the second option and moved its equipment to a nearby forging plant, where two guidance rails of hammer heads were hardened. Each part weighed 40 tons and a total of more than 60,000 cm2 was processed flawlessly during a 72-hour period. It is possible to harden such hammer parts even when mounted without disassembling the hammer. This brings a vast saving of time and labor.
Other machinery parts
Many other applications for heavy industry clients are performed, for example, hardening of the tooth wheel, where the competition is induction hardening. If the batches are of smaller sizes, with high batch quantities, there is no chance for the laser, but the company finds success processing larger wheels where laser hardening is faster, cheaper and without risk of surface cracking. The largest wheel processed was about 8 m in diameter, delivered in eight segments. The wheel works in a huge coal harvesting machine (FIGURE 4).
Hardening of turbine blades for steam turbines is also a growing job. Laser hardening produces better results than other technologies; moreover, it is flexible, perfectly controlled, and reliable. Also very promising is hardening of straight cast iron parts such as lathe beds, which allow for simpler construction and increased lifetime of big machine tools.
The toolmaking industry for automotive applications drives the technology forwards as it always needs new solutions, shorter terms, lower prices, and better quality. Laser hardening already has a stable market position in this sector. However, this job is changing rapidly as new materials and new production processes come. An open mind and close cooperation between production engineers in industry and laser job shops are necessary to achieve new ideas and successful solutions.
Ing. Tomáš Mužík is director (CEO) and 50 percent owner of MATEX PM (www.matexpm.com/en/index.htm).