060070103-Rifat Yılmaz-9th Week Definitions

Laser Cladding(Previous) GROUP: Surface Modification Technique

Laser Cladding involves bonding of an overlay material to the surface to be hardened. The overlay material may be pre-positioned o it can be continously fed. The laser beam melts a thin sirface layer, similarly to the case of laser glazing. The overlay material is then metallurgically bonded to the surface. In contrast to glazing, the overlay material does not intermix with the surface. Only enough of the surface melts to form the bond with the overlay material. The process is also called hardfacing. Again, the laser would probably be a multikilowatt CO₂ laser.

(Industrial applications of lasers, John F. Ready, p. 381)

Laser Cladding(New)(Better)

Laser cladding is a melting process in which the laser beam is used to fuse an alloy addition onto a substrate. The alloy may be introduced into the beam-material interaction zone in various ways, either during or prior to processing. Figure 12.1 shows one technique, in which alloy powder is blown into the laser beam and is deposited as a molten coating on the rim of a valve. Very little of the substrate is melted and so a clad with the nominal alloy composition is created. Surface properties can then be tailored to a given application by selecting an alloy with good wear, erosion, oxidation or corrosion properties. The molten clad solidifies rapidly, forming a strong metallurgical bond with the substrate. Most substrates that tolerate laser melting are generally suitable for cladding: carbon-manganese and stainless steels, and alloys based on aluminum, titanium, magnesium, nickel and copper. Popular cladding alloys are based on cobalt, iron and nickel. They may also contain carbides of tungsten, titanium and silicon, or ceramics such as zirconia, which form a particle-reinforced metal matrix composite surface on solidification, endowing additional wear resistance. The type of laser used depends on the surface area to be covered, the thickness of clad required, and the complexity of the component. CO₂ lasers are ideal for large areas requiring dads several millimeters in thickness over regions with a regular geometry. A robot-mounted diode laser beam, or Nd:YAG laser light delivered via a fiber optic cable, is more suitable for precision treatment of complex three-dimensional components requiring a coating less than one millimeter in thickness. The overall aim is to produce a clad with appropriate service properties, a strong bond to the substrate, with the maximum coverage rate, the minimum use of alloy addition, and minimal distortion.

(John C. Ion, Laser Processing Of Engineering Materials: Principles, Procedure And ... Industrial Application, pages 296,297)

Laser Alloying(Previous) GROUP: Surface Modification Technique

Surface alloying involves spreading of a powder containing alloying elements over the surface to be hardened. Then, the laser beam taverses the surface. The powder and a thin surface layer melt and intermix. After passage of the beam, the surface resolidifies rapidly. A thin layer containing the alloying elements remains, with hardness greater than that of the original material.

(Ready, J., Industrial Applications of Lasers, p.380)

Laser Alloying(New)(Better)

Laser alloying is a fusion process in which a pulsed or a CW laser melts the alloy material applied to the surface. A convenient method is spraying powder over the surface and simultaneously melting it by a CW laser beam. The structure of the laser alloyed layer contains super saturated solid solutions and sometimes intermetallic compounds.[1]

Laser alloying of the material surface layer with selective elements, allows for modification of the properties of inexpensive products, instead of using very expensive highly alloyed materials. The potential of surface alloying to reduce consumption of expensive alloying elements is both strategically and commercially significant. The principal aim of the application of laser alloying technique in materials surface processing is to improve their properties due to formation of hard, homogenous and ultrafine structure of the surface layer by changing its chemical composition. Metallurgical changes that occur in the laser-modified layer are in the form of grain refinement, supersaturated solid solutions, and fine dispersions of panicles. These can contribute to the hardening and strengthening of the surface layer. This technique is more and more popular, as there are several different ways to introduce alloying elements into the thin surface layer melted by the laser beam: direct injection into the melt pool at time of the treatment (powder, wire, gas); pre-placed adherent coatings deposited prior to laser treatment (electroplating, diffusion coating, thermal spraying, sputtering, etc.); pre-placed non-adherent coatings (foils, pastes, powder slurries). Recent results showed that laser surface alloying of carbon inexpensive steels and cast iron with Cr, Si or C can convert microstructure and properties of the surface layer into ones characteristic of highly alloyed steels. Also, laser alloying of metallic substrates with alloying compounds, such as: W-Co-Cr-V, Ni-Cr-H-Si or Ni-Cr-AI-Fe, has been demonstrated as an effective method that modifies their surface layer properties significantly. To date, a variety of surface coatings have been easily achieved by using this non-contact and clean method. One of the main interests in the surface alloying of Fe-base substrates is the production of a stainless steel surface equivalent.[2]

([1] R. Kesavamoorthy,   Akhilesh K. Arorac,C.  Babu Rao,   P. Kalyanasundaram, Laser Applications in Material Science and Industry, page 176 [2] Marc A. Meyers,Gareth Thomas,Robert O. Ritchie,Mehmet Sarikaya, Nano and Microstructural Design of Advanced Materials, page 36)