Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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HTPRO talline and noncrystalline (or amorphous) ferrous and nonferrous materials for a wide variety of industrial applications (Fig. 4). The mechanisms primarily constrain dislocation motion that, in turn, enhances mechanical properties. An example is selective and total area hardening of carbon steel by inducing martensitic phase transformation that restricts dislo(a) 10 (c) (b) (d) Fig. 4 — Laser surface heat treating: (a) schematic of process, (b) crosssectional microstructure of laser processed AZ31B magnesium alloy, (c) as-received AZ31B with 50 Vickers hardness, and (d) refined grain structure in laser heat treated AZ31B with 150 Vickers hardness. (a) (b) Fig. 5 — Laser nitriding: (a) schematic of process, and (b) cross-sectional SEM back-scatter micrograph shows the distinct TiN layer along x–y cutting plane. (a) (b) Fig. 6 — Laser surface alloying: (a) schematic of process, and (b) crosssectional SEM back-scatter micrograph showing the TiC composite coating (brighter contrast) on aluminum (darker grey contrast). 46 ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 cation movement, further increasing mechanical properties. It is difficult to obtain a fine-grain microstructure using conventional methods due to low nucleation density associated with the processes. By comparison, LSM involves very high heating and cooling/quenching rates, which result in increased nucleation density, and fast quenching reduces the growth rate. Self-quenching is one of the unique features of LSM, in which the small localized surface experiences rapid melting and solidification, while the bulk material below the surface serves as a heat sink with no change in microstructure. Thus, unique characteristics of laser heat treatment generate a fine-grain microstructure and promote phase transformations that are not possible using conventional techniques. Altering surface chemistry This approach primarily processes the material surface under a reactive atmosphere (e.g., carbon, methane, or nitrogen) to promote carburization and nitrification. It is achieved by diffusing carbon/nitrogen interstitial atoms into the crystal lattice creating a barrier for dislocation/slip, thus increasing mechanical strength. Some drawbacks associated with conventional methods are longer processing times, toxic conditions, and large dimensional deviations. LSM overcomes these limitations and produces carburization and nitrification through complicated interactions between the laser beam, material surface, enclosure gas, and plasma/plume. LSM forms a more effective carbide/nitride layer on many commonly used materials (Fe, Ti, Al, Si) to enhance surface performance. For example, titanium nitride (TiN) is formed on titanium and Ti alloys to enhance tribological, corrosion resistant, and biological properties as shown in Fig. 5[8, 9]. Laser-nitrided Ti-6Al-4V has a lower corrosion rate than untreated samples (0.019 µm/y compared with 2.627 µm/y, respectively) under simulated body fluid solution (a mixture of H2O, NaCl, NaHCO3, KCl, K2HPO4·3H2O, MgCl2·6H2O, CaCl2, and Na2SO4)[9]. Adding a surface layer of strategic materials In this method, surface and subsurface properties are altered to meet requirements without sacrificing bulk material characteristics by adding just the right amount of surface layer of strategic materials (e.g., expensive, scarce metals and ceramics) on the bulk material. The bulk material is alloyed with additive materials (preplaced or powder feeder) by using the laser beam as a heat source. The alloyed region is confined to a shallow depth. Rapid solidification rates produce a sound, metallurgically bonded coating with a high concentration of key elements in the alloyed region along with favorable microstructures[6, 10]. Such microstructures with metastable phases or extended solid solubility characteristic of rapid solidification cannot be obtained using conventional methods. Also, due to localized heating, laser processing has advantages such as negligible distortion, low poros-

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