Abstract
A high power direct diode laser, emitting in the range of near infrared radiation at wavelength 808–940 nm, was applied to produce a titanium matrix composite on a surface layer of titanium alloy Ti6Al4V by laser surface gas nitriding. The nitrided surface layers were produced as single stringer beads at different heat inputs, different scanning speeds, and different powers of laser beam. The influence of laser nitriding parameters on the quality, shape, and morphology of the surface layers was investigated. It was found that the nitrided surface layers consist of titanium nitride precipitations mainly in the form of dendrites embedded in the titanium alloy matrix. The titanium nitrides are produced as a result of the reaction between molten Ti and gaseous nitrogen. Solidification and subsequent growth of the TiN dendrites takes place to a large extent at the interface of the molten Ti and the nitrogen gas atmosphere. The direction of TiN dendrites growth is perpendicular to the surface of molten Ti. The roughness of the surface layers depends strongly on the heat input of laser nitriding and can be precisely controlled. In spite of high microhardness up to 2400 HV0.2, the surface layers are crack free.
Highlights
Titanium alloys are attractive structural materials, compared to other light alloys and modern high strength steels as investigated by A
The nitrided surface layers produced at the same heat input of 90 J/mm but at different laser power output, different scanning speed, different time of laser beam interaction, as well as different power density, have very different surface topography, roughness, and different morphology (Figure 6, Table 2)
The composite surface layers produced by HPDDL laser nitriding of Ti6Al4V titanium alloy specimens in a pure nitrogen atmosphere, in the investigated range of processing parameters, were of high quality and free of any surface and internal defects such as cracks or porosity even in the case of very thick layers with very high hardness, often reported by many researchers who have used other types of lasers
Summary
Titanium alloys are attractive structural materials, compared to other light alloys and modern high strength steels as investigated by A. One of the most promising and effective methods for improving the surface characteristics and properties of titanium alloys is laser surface treatment in a nitrogen atmosphere or rich in nitrogen-laser gas nitriding (LGN) [20,21] This is the result of the large chemical affinity of titanium for nitrogen at elevated temperatures, in a liquid state. Biswas et al [6] investigated the laser gas nitriding of Ti6Al4V alloy applying a high power diode laser They found that the residual stresses are mainly compressive in nature and strongly depend on process parameters; the lowest compressive residual stresses started from 50 MPa. To summarize, the advantages of the specific type of diode lasers beam for surface processing of metals are as follows:. Where: w, is laser beam width (or length, mm) in the case of a rectangular or square beam and a diameter in a case of a circular beam (just for calculation of the maximum interaction time τmax, as illustrated in Figure 1a), v, is scanning speed (mm/s)
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