XRD and XPS studies on surface MMC layer of SiC reinforced Ti–6Al–4V alloy

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Abstract

Overlapping tracks were produced by laser processing using a powder SiC (∼6 μm) preplacement technique which has been developed to modify the surface structure of a Ti–6Al–4V alloy. A continuous-wave CO2 laser was used for the processing which produced six overlapping tracks covering 14 mm across the surface of a 10 mm thick plate. Under spinning beam conditions, a surface alloyed/metal matrix composite (MMC) layer over 300 μm in depth was produced on the alloy. The surface contained a complex microstructure, but with no cracks and only two pores at the melt/HAZ interface. Using XRD and XPS analysis, it was shown that the solidified melt consisted of α′-Ti, Ti0.55C0.45 and Ti5Si3 phases, which vary with melt depth and with the particular group of overlapping tracks examined. Therefore, no new phases to those previously identified in single track laser processing experiments were found in this work.

Introduction

One of the major obstacles to the widespread use of bulk titanium metal matrix composites (MMC) is the high cost. Major advances have been made in using creative processing techniques with alloys by introducing surface MMC layers. The use of SiC particles for producing bulk titanium and aluminium MMCs suggested that this approach could be applied for surface alloying by laser processing through the incorporation of the same or similar ceramic particles [1], [2], [3], [4]. To obtain a good dispersion of particles a smaller size of 3–7 μm SiC was added via a preplacement technique [3], [4] compared with 150 μm particles injected by Ayers [1] and Abboud and West [2]. In the former case the SiC dissolved in the Ti alloy and Ti5Si3 and TiC precipitated [5], [6], [7].

An increase in the potential life of a component may be associated with an improvement in the wear resistance [1], [4], [5]. This is normally dependent on an increase in both the hardness and the depth of the surface region. Laser processing can develop a deeper melt pool than other surface modification techniques. On solidification after laser processing a very different microstructure from the parent alloy may be produced through retaining SiC particles in the melt pool. Earlier work at Strathclyde University used large SiC particles, between 60 and 150 μm in size which were either preplaced or injected [3], [8]. The injected case led to rough surfaces with cavities and a poor distribution of SiCp which had undergone partial dissolution [8]. The preplacement technique resulted in a better particle distribution [3], [8]. Partially dissolved SiCp provides Si, which forms Ti5Si3, and C, which forms spherical TiC particles in the complex microstructure [5], [6], [7], [8]. Under some conditions, both Si and C can combine with Ti to form Ti3SiC2 during the laser processing [6], [7].

In the present work, a smaller particle size was employed with the aim of producing complete dissolution of the SiC particles and giving a high fraction of Si and C atoms available for new compound formation. A detailed microstructural characterisation of a Ti–6Al–4V alloy with SiC preplacement was undertaken. The main aims were to identify, after laser processing, the phases present as a function of the melt depth, due both to the production of overlapping tracks and in progressing from the first to the sixth track, in a sequence designed to cover an area of the specimen surface. This should develop a better understanding of the influence of the laser processing parameters on the microstructure and allow optimisation of the laser conditions for future industrial opportunities.

Section snippets

Specimen preparation

Commercial Ti–6Al–4V (IMI-318) gifted by Imperial Metals Industries, UK was used as the base alloy in the present study. The composition of the alloy was: <0.08 wt.% C, <0.25 wt.% Fe, <0.05 wt.% N, <0.2 wt.% O, <0.015 wt.% H, 5.5–6.75 wt.% Al, 3.5–4.5 wt.% V and Ti (balance). The base alloy specimens of size 100mm×100mm and 10 mm thickness were cut from the as-received material for laser processing. Laser surface alloying with 6 μm SiC powder preplaced on the alloy using 100% argon environment was

Surface condition

The external appearance of the laser alloyed Ti–6Al–4V specimen is shown in the macrograph of Fig. 1a. A shiny white colour is observed on the surface alloyed specimen. As seen in the macrograph, the morphology of the melt tracks can be characterised by a combination of slight surface tension lamellae and ripples in the radial directions of each track. From visual inspection, a small variation of surface roughness on the laser processed tracks is observed throughout the top surface compared

Discussion

The interactions in the Ti–SiC system have been studied extensively [21], [22], [28], [29], [30], [31], [32], [33] albeit in the solid state. It is generally accepted that in this state the interdiffusion of titanium, silicon and carbon results in the formation of Ti5Si3 and TiC [28], [29] in a fine layer between SiC and Ti. The formation of Ti3SiC2 was also reported by Martineau et al. [30] in a stoichiometric SiC filament/Ti matrix, by Morozumi et al. [31] in SiC–Ti–SiC diffusion couples and

XPS

The X-ray results discussed above have shown that in the top layer and at depths of 100 and 300 μm, the microstructure consisted of mainly α′-Ti with very little Ti0.55C0.45 and Ti5Si3. However, more Ti0.55C0.45 and Ti5Si3 was found to precipitate in the later melted tracks due to the effect of the preheat produced by the earlier melted tracks reducing the cooling rate. The present XPS results are in good agreement for the identification of TiC and α′-Ti. Due to lack of XPS data from the

Conclusions

From the results and discussion of an XRD and XPS study of the microstructure of Ti–6Al–4V laser surface alloyed with 6 μm SiC powder, to produce overlapping melt tracks the following conclusions were reached:

  • 1.

    A continuous CO2 laser used in the spinning beam mode, giving a 4 mm track width, to process a 10 mm thick plate of a Ti–6Al–4V alloy with a preplaced SiCp layer, produced a surface metal matrix composite to a depth of >300 μm. Six overlapping tracks with a 50% overlap, gave a total melted

Acknowledgements

MSS gratefully acknowledges SIRIM Berhad and the Malaysian Government for the financial support in this work. The contributions of Dr. C.F. Burdett for advice with the X-ray diffraction analysis and Dr. C. Hu for providing the laser alloyed specimen, are also acknowledged.

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