Laser processing of materials

Light amplification by stimulated emission of radiation (laser) is a coherent and monochromatic beam of electromagnetic radiation that can propagate in a straight line with negligible divergence and occur in a wide range of wave-length, energy/power and beam-modes/configurations. As a result, lasers find wide applications in the mundane to the most sophisticated devices, in commercial to purely scientific purposes, and in life-saving as well as life-threatening causes. In the present contribution, we provide an overview of the application of lasers for material processing. The processes covered are broadly divided into four major categories; namely, laser-assisted forming, joining, machining and surface engineering. Apart from briefly introducing the fundamentals of these operations, we present an updated review of the relevant literature to highlight the recent advances and open questions. We begin our discussion with the general applications of lasers, fundamentals of laser-matter interaction and classification of laser material processing. A major part of the discussion focuses on laser surface engineering that has attracted a good deal of attention from the scientific community for its technological significance and scientific challenges. In this regard, a special mention is made about laser surface vitrification or amorphization that remains a very attractive but unaccomplished proposition

Laser Surface Engineering of Metallic Components

The engineering solution to improve the surface dependent properties like wear, corrosion and oxidation resistance involves tailoring the surface composition and/or micro-structure of the near-surface region of a component without affecting the bulk. This may be achieved, using a high power laser beam as a source of heat, by surface hardening, melting, alloying and cladding. Fast heat-ing /cooling rate (104-1011 K/s), very high thermal grad-ient(106-108 K/m)and ultra-rapid resolidification veloc-ity (1-30 m/s) are the characteristics of this process which often develop exotic microstructures and composit-ions having large extension of solid solubility and meta-stable or even amorphous phases in the surface. This paper gives a brief review of the present status and future scope of laser assisted surface engineering with parti-cular reference to the authors' work

Studies on laser surface melting of Al-11% Si alloy

In the present investigation the effect of laser surface melting on wear and corrosion resistance of Al-11 wt.% Si alloy has been investigated. Laser surface melting has been carried out using a 2 kW continuous wave CO2 laser at an applied power of 2.3 kW and scan speed ranging from 6 to 12 mm/min. Following the laser surface melting, a detailed investigation of the melted zone in terms of microstructure, composition and phases were undertaken. Mechanical properties of the melted zone were evaluated so far as the microhardness and wear resistance were concerned. The corro-sion behaviour of the as-received and the laser surface melted surface was evaluated in 1(M) H2SO4, 1(M) HNO3 and 3.56 wt.% NaCl solutions. The microstructure of the melt zone consists of grain refinedAl andAl-Si eutec-tic colonies which results in an improved microhardness from 87 VHN as compared to 55 VHN of the as-received Al-Si alloy. The wear resistance of the melt surface was improved significantly as compared to the as-received Al-Si alloy. A detailed corrosion study in various environments showed that corrosion resistance was marginally less in the 3.56 wt.% NaCl and 1 M H2SO4 solutions, but was better in the 1 M HNO3 solution

Studies on residual stress developed in laser surface irradiated 0.6% carbon steel

Laser surface hardening is a process of microstructural modification of the near surface region of iron-based component by inducing martensitic transformation with a high power laser beam as a source of heat. The process is aimed at introducing a hard and wear-resistant layer on the surface, thereby increasing the service life of the component. Due to a rapid rate of heating and cooling and a large thermal gradient associated with the process, a measurable amount of residual stress is developed in the laser irradiated region. In the present investigation, an attempt has been made to surface harden medium carbon steel (0.6% Carbon) using 2.5 kW continuous wave CO2 laser as a source of heat using Ar as shrouding gas. The microstructure and phase analysis of the irradiated region have been carried out in details. Residual stress developed in the laser-irradiated region has been carefully measured. Effect of laser parameters on microhardness and wear resistance has been studied. Finally, the processing zone for the surface hardening has been derived following a detailed structure-property correlation

Laser composite surfacing of a magnesium alloy with chromium carbide

The present study concerns improving the wear resistance of a Mg alloy (MEZ) by melting the surface with a high power laser and simultaneously injecting hard particles, of Cr2C3 +(25mm -40mm) into the surface. The laser processing was carried out using a continuous wave CO2 laser, Model: Rofin Sinar, RS 10000, with a beam diameter of 4 mm and a focal point 30 mm above the surface. Following laser processing, a detailed investigation of the microstructures, compositions and phases were undertaken and mechanical (wear resistance) and electrochemical (pitting corrosion resistance) properties of the surface layer were evaluated in details. The microstructure of the surface layer consists of uniformly dispersed Cr2C3 precipitates in grain-refined matrix. The micro-hardness and wear resistance of the surface layer were significantly improved as compared to the base metal

In-situ dispersion of titanium boride on copper by laser composite surfacing for improved wear resistance

The present study concerns the development of a hard in-situ titanium boride dispersed in a composite layer on a copper substrate with the objective of improving the wear resistance. Laser composite surfacing was carried out by melting the surface of a sand blasted commercially pure copper substrate using a continuous wave CO2 laser (with a beam diameter of 3.5 mm) and the simultaneous deposition of a mixture of K2TiF6 and KBF6 (in the weight ratio of 2:1) using an external feeder (at a feed rate of 4 g/min) and Ar as shroud. The process variables used in the present study were the laser power applied and the scan speed. Following the laser irradiation, a detailed characterisation of the composite layer was undertaken in terms of microstructure, composition and phases. Surface dependent mechanical properties such as micro-hardness and wear resistance were also evaluated in detail. Irradiation resulted in melting of the substrate, along with the delivered powder mixture, intermixing and rapid solidification. The microstructure of the composite layer consisted of uniformly dispersed titanium boride particles in a grain-refined copper matrix. The micro-hardness of the surface was improved threefold as compared to that of as-received copper substrate. There was a significant improvement in the wear resistance of the composite surfaced copper, as compared to that of the as-received copper. The mechanism of wear was investigated

Laser surface cladding of EN19 steel with stellite 6 for improved wear resistance

The present study concerns the generation of a wear resistant Stellite 6 CO2 laser clad layer on the surface of an EN19 steel substrate by means of laser surface cladding. Laser surface cladding was carried out by melting the Stellite powder (particle size 10 to 40 μm) supplied through a pneumatically driven powder delivery system (using a 4 MP powder unit) with a 9 kW continuous wave (CW) CO2 laser with the wavelength 10.6 µm. The microstructure of the clad layer was found to consist of three zones: a clad layer comprised of dendrites of Stellite 6; an alloyed zone comprised of a cellular microstructure, which was a mixture of Fe and Co; and the heat affected zone (HAZ), which was a mixture of pearlite and martensite. Compared to the EN19 steel substrate, the micro-hardness of the clad layer represented a significant improvement, increasing to 1200 VHN

Laser surface nitriding of Ti-6Al-4V for bio-implant application

The present study aims at enhancing the biocompatibility of Ti-6Al-4V by laser surface nitriding. Laser surface nitriding has been carried out by melting of sand blasted Ti-6Al-4V substrate using a high power continuous wave DIODE laser with nitrogen as shrouding environment (at a pressure of 5l/min). Following laser treatment, a detailed characterization of the surface has been conducted. Microhardness and biocompatibility have been evaluated. Laser surface nitriding led to formation of dendrites of TiN on the surface. The microhardness is improved to 900- 950 VHN (in laser surface nitriding) as compared to 260 VHN of as-received substrate. Biocompatibility behavior showed a better cell viability in laser surface nitrided Ti-6Al-4V sample as compared to as-received one

Laser surface treatment of Ti-6Al-4V for bio-implant application

The present study aims at enhancing the wear resistance of Ti-6Al-4V by laser surface melting and nitriding and subsequently, studying the influence of laser surface treatment on the corrosion resistance in a simulated body fluid and also the bio-compatibility. The laser surface treatment is carried out using a high power continuous wave diode laser with argon and nitrogen as shrouding gas. Laser surface melting leads to an increased volume fraction of acicular martensite and a decreased volume fraction of the β phase in the microstructure. Laser surface nitriding leads to the formation of titanium nitride dendrites. The micro-hardness could be improved up to a maximum of 450 Hv in laser surface melting and 900-950 Hv in the case of laser surface nitriding as compared to 260 Hv of the as-received substrate. Surface melting increases the corrosion potential (Ecorr) and primary potential for pit formation (Epp1) significantly as compared to the as-received Ti-6Al-4V. However, when processed under similar conditions, surface nitriding shifts Ecorr marginally in the more noble direction, and increased Epp1 as compared to Ti-6Al-4V. The biocompatibility behaviour shows a superior cell viability on surface nitriding and an inferior cell viability on surface melting as compared to the as-received Ti-6Al-4V

Exchange bias effect in alloys and compounds

The phenomenology of exchange bias effects observed in structurally single-phase alloys and compounds but composed of a variety of coexisting magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic, spin-glass, cluster-glass and disordered magnetic states are reviewed. The investigations on exchange bias effects are discussed in diverse types of alloys and compounds where qualitative and quantitative aspects of magnetism are focused based on macroscopic experimental tools such as magnetization and magnetoresistance measurements. Here, we focus on improvement of fundamental issues of the exchange bias effects rather than on their technological importance