Short CommunicationInfluence of overlapping tracks on microstructure evolution and corrosion behavior in laser-melt magnesium alloy
Introduction
Laser processing has recently generated intense research activities on improving surface properties of materials. Previous researchers have found that surface performance of metallic alloys, such as wear and corrosion resistance, is enhanced remarkably due to refined microstructure and enriched alloying elements following rapid solidification associated with the laser processing [1], [2], [3], [4], [5], [6], [7], [8], [9]. Mazumder and his co-workers [4], [5] have investigated effect of laser-cladding technique upon the microstructure and the corrosion resistance of treated materials, and found that corrosion properties of the laser claddings has been improved significantly compared with that of commercially used materials. Gray and Luan [6] have reviewed various techniques for protective coatings on magnesium alloys due to poor surface properties, and suggested laser processing is a very promising method for Mg alloys to extend their industrial applications. Audebert et al. [7] have examined the ability of production of glassy metallic layers on Zr- and Mg-based alloys by laser surface treatment. Man et al. [8] have suggested that laser modification technique employed in their study is capable of enhancing the feasibility and biocompatibility of NiTi samples used as orthopedic implants mainly due to improved wear resistance. Most recently, Sun et al. [9] have studied laser surface alloying to form wear resistant layers on steel rolls with powders, and concluded that the improvement in wear resistance is attributed to combined results of grain refining and solution strengthening effect.
In order to apply laser techniques for large surface components in real engineering applications, overlapping adjacent traces as a result of multiple passes using scanning laser beam is usually necessary for production of area coverage. It has long been realized that laser beam overlapping may play a significant role in influencing the final surface properties of laser-treated materials [1], [2], [3]. Lewis and Schlienger [10] have considered that overlapping plays an important role in determining quality control during laser assisted direct metal deposition. Particularly, overlapping is important in determining corrosion resistance due to microstructure in-homogeneities in the molten pool [11], [12], [13], [14], [15], [16]. Liu et al. [11] have demonstrated that overlapping results in microstructural non-uniformity within re-heated and re-melted area, which leads to preferred sites for corrosion development. Reitz and Rawers [12] have observed that accelerated corrosion occurs near laser beam overlap region, and iron element segregated near periphery of each molten pool is responsible for accelerated corrosion in zirconium alloys. Virtanen et al. [13] have showed that pits are initiated along the overlapped area of laser traces after surface melting of Al–7Si and Al–12Si cast alloys. Conde et al. [14] have reported that corrosion resistance of laser-melt steels depends critically on laser processing parameters, and care must be taken in the choice of parameters that leads to optimal properties in each material. In our previous work, coarse structure of laser-melt AZ91D Mg alloy is investigated in the overlapped area caused by scanning speed, and it provides preferential site for pitting corrosion in simulated body fluid [15], [16].
However, as of now, there has been no concentrated effort to study the effect of laser beam overlapping on kinetics of solidification microstructure evolution in the molten pool. The objective of this research is to study how overlapping tracks affect heat transfer and liquid flow, microstructure evolution as well as electrochemical behavior of laser melting AZ91D Mg alloy at optimized scanning speed. The heat transport mechanism in the molten pool was analyzed from a well-tested numerical heat transfer and fluid flow model by DebRoy and Yang [17], [18], [19].
Section snippets
Materials and methods
The material studied was an as-cast AZ91D Mg alloy with the following chemical composition (wt%): Al 8.97, Zn 0.78, Mn 0.31, Si 0.023, Cu 0.002, Ni 0.0005 and Mg balance. The specimens of 20 mm by 30 mm by 3 mm were extracted from the ingot, ground with progressively finer SiC paper (180, 400, 800, 1200, 2400 and 4000 grit), cleaned with alcohol, and then irradiated with Lumonics JK702 Nd:YAG laser system (with wavelength of 1064 nm) under high purity Ar gas protection. The fixed laser parameters
Results
Fig. 2, Fig. 3 indicate top view of microstructure evolution at AZ91D Mg alloy surface before and after laser melting by SEM and TEM, respectively. As shown in Fig. 2a, microstructure of as-received AZ91D Mg alloy contained bulk and lamellar β-Mg17Al12 phase distributed non-homogeneously in a matrix of α-Mg grains. After laser surface melting, solidification microstructure consisted of typical cellular/dendrite structure was observed in the molten pool, which was derived from refined α-Mg and
Discussions
Microstructure development in the molten pool is complicated due to physical processes that occur during interaction of the heat source with the metal, including re-melting, heat and fluid flow, vaporization, dissolution of gasses, solidification, subsequent solid-state transformation, stresses, and distortion [17], [18], [19], [23]. These processes and their interactions profoundly affect rapid solidification and microstructure evolution. In situations where laser processing on metals, atoms
Conclusions
Effect of overlapping tracks on solidification microstructure and electrochemical behavior of AZ91D Mg alloy was reported via irradiation by millisecond pulse Nd:YAG laser surface melting. The main conclusions are listed as following:
- (1)
With the increasing overlapping rates in laser-melt Mg alloy, morphology of solidification microstructure changed from cellular grains, to cellular–dendritic and equiaxed dendritic in the overlapped area of molten pool. Meanwhile, nano-sized β-Mg17Al12 precipitates
Acknowledgements
Support by Nanyang Technological University, Ph.D. scholarship and Singapore Institute of Manufacturing Technology, A*STAR (Agency for Science, Technology and Research, Singapore), Collaborative Research Project U09-M-006SU is gratefully acknowledged.
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