Heat treating and cladding are used to improve the surface properties of metal parts in a wide range of industries. Over the past few years, laser based methods have often proven to deliver better results and reduced costs compared to traditional techniques. This article provides a brief overview of the benefits of high power, diode laser-based heat treating and cladding.
by Frank Gaebler, and Heiko Riedelsberger
Long used in data storage and telecommunications, diode lasers are semiconductor devices that directly convert electricity into laser light. A typical, individual diode laser emitter might produce at most a few watts of output power in the near infrared. However, numerous emitters can be fabricated on a single monolithic semiconductor substrate or “bar” with a total output up to 100 W. These bars can then be combined in horizontal and vertical stacks to produce high power direct diode laser systems up to 10 kW. The small size of high power diode lasers makes them easier to integrate into workstations than other laser types. Moreover, their waste heat is produced in a small physical area, enabling cooling with a small volume of circulating water and a chiller.
In laser heat treating, a spatially well-defined beam of intense laser light is absorbed near the work surface, causing rapid heating that is highly localized, and which does not penetrate very deep into the bulk material. The bulk heat capacity of the material typically acts as a heat sink for the extraction of heat from the surface, therefore enabling self-quenching. The ability to precisely control the spatial extent and short timing of the energy transfer into the surface give rise to the main benefits of laser surface modification. Specifically, these are rapid processing, precise control over case depth and minimal part distortion. In fact, distortion is so minimal that subsequent processing to restore dimensional accuracy, such as grinding or machining, is typically unnecessary. These characteristics of laser hardening contrast significantly and favorably with flame hardening and induction hardening techniques. For example, flame hardening is limited by poor reproducibility, poor quench characteristics and environmental issues. Induction hardening typically produces deeper thermal penetration thus requiring an active quench, leading to undesirable and uncontrollable distortion. The high power diode laser is an ideal source for heat treating for several reasons. First, the near infrared output wavelength of diode lasers is well absorbed by most metals, avoiding the need to “paint” parts with an absorptive coating before processing. The beam shape of a high power diode laser is a line or rectangle which is well matched in size to many hardening tasks, and can be readily sized to match the dimensional requirements of a specific application. Diode lasers also offer attractive cost characteristics; these lasers have very high electrical efficiencies, so much of the input electrical energy is converted to useful light output. And because they have instant “on” capability, there is no standby power consumption. Plus, high power diode lasers provide solid state reliability and low maintenance costs.
High power diode laser systems offer advantages for cladding applications over both types of conventional cladding technology. Compared to arc welding techniques, diode laser systems offer lower heat distortion, reduced dilution of the clad material into the substrate metal (~ 4%), lower porosity (< 1%) and better surface uniformity. These advantages largely eliminate the need for post-processing. Plus, the high quench rate with laser cladding produces a finer grain structure in the clad leading to better corrosion resistance. Furthermore, these benefits generally apply at any power level and hence, deposition rate. In contrast, with arc welding the clad quality suffers with increasing power and deposition rate. Finally, the laser line beam shape can process large areas rapidly with a high degree of control over clad width and thickness. In particular, it facilitates the production of wide, flat clads having low dilution. Compared to traditional thermal spraying techniques, the key advantage of laser cladding is the formation of a true metallurgical bond with the base material. This results in better adhesion and wear resistance. Furthermore, metallurgically bonded clads produced with the diode laser limit the cracking and de-lamination often associated with mechanical coatings. As with heat treating, the near infrared wavelength output of the diode laser is better absorbed by the underlying base material and clad alloy than most other lasers, including fiber lasers, Nd:YAG lasers, and especially, CO2 lasers. The high electrical efficiency, combined with this absorption rate, translates into lower operating costs, a smaller carbon footprint and increased deposition efficiency. Plus, the very highest output power diode laser systems, such as the 10 kW Coherent HighLight™ 10,000D enable high deposition rate cladding (>9 kg/h). Because this is an emerging technology, applications support is often critical in developing new heat treating and cladding processes. For this reason, some high power diode laser manufacturers provide extensive applications development support. For example, in Summer 2014, Coherent will open a new applications center near Vienna, Austria, which will be equipped with a laser to perform testing and demonstrations, and which will have rail facilities to handle very large parts, such as are typical in the mining, and oil and gas industries.
By Frank Gaebler, Director of Marketing, Coherent Inc. and Heiko Riedelsberger, Market Development Manager Europe - High Power Lasers