In-situ synthesis of titanium carbides in iron alloys using plasma transferred arc welding
Introduction
The in-situ synthesis of metal matrix composites (MMC) has attracted much attention in the last two decades [1]. The reported advantages over conventional MMCs are mainly related with a smaller size and better distribution of reinforcing particles, their thermal stability and their high compatibility with the metallic matrix. In addition, direct metal deposition offers economic benefits over bulk production techniques because the MMC is only applied on the surface of the component while the bulk can be made of a lower-cost material or a more convenient one from a manufacturing point of view. This approach involves the reaction of alloying elements during deposition to form a dispersion of hard phases, usually carbides, in a metallic matrix [2], [3].
Direct metal deposition techniques have been described elsewhere [4], [5] and, more recently, the in-situ synthesis of MMCs by laser cladding and gas tungsten arc welding or plasma transferred arc (PTA) has been reported [6], [7], [8]. The latter technique is interesting because it is low-cost, widely available and accepted in industry. However, the higher pressure and the more complex fluid dynamics associated with the electric arc make it difficult to inject low density powders, such as graphite. The small particle size required for rapid synthesis of carbides in-situ often has poor flowing characteristics [4] and low injection and sticking efficiencies. On the other hand, graphite particles of larger sizes exhibit better flowability but remain un-reacted or un-dissolved in the matrix, particularly at the interface with the substrate. Some authors have used pre-placed powders, which are fixed on the substrate with a binder and subsequently melted with the laser beam or the electric arc to form an overlay [7]. Even though this approach is effective, the thickness of the overlays deposited in this way is severely limited and thick overlays exhibit poor metallurgical bonding with the substrate.
In recent work, MMC overlays were produced by feeding a carbon-rich iron alloy and titanium powders into a PTA system [9]. Grey cast iron was used to provide the iron matrix and the high carbon content for precipitation of titanium carbides. The resulting microstructure consisted of a martensitic matrix reinforced with TiC precipitates exhibiting a predominantly faceted morphology with little evidence of dendritic precipitates. Interestingly, this microstructure was different from the one described by other authors who used pure iron, titanium and graphite powders [10], [11], or a blend of FeTi master alloy and C powders [6], [7], [8]. Therefore, it was speculated that the alloying elements present in grey cast iron, namely silicon and manganese, were responsible for the differences in the TiC morphology and size.
The size and distribution of reinforcing phases are of utmost importance to the wear resistance of MMCs. Recent studies on the abrasive wear resistance of in-situ synthesised Fe–TiC MMCs indicate that, for a given volume fraction of TiC, a microstructure consisting of small TiC precipitates evenly distributed in the matrix is preferable to a more widely spread distribution of larger dendritic carbides [12]. The effect of deposition parameters on the carbide morphology was extensively studied and very valuable conclusions were extracted relating carbide morphology to parameters such as heat input [10], [11]. However, the heat input and the thermal cycle are sometimes difficult to control in industrial practise and the alloy chemistry offers a better way to control the carbide size and morphology.
In view of the preceding observations, the present work focuses on the in-situ synthesis of Fe–TiC MMCs with particular regard to the effect of silicon and manganese on the size and morphology of TiC precipitates.
Section snippets
Materials and methods
Mild steel AISI 1020 was selected as a substrate and slabs 100 mm long, 32 mm wide and 5 mm thick were cut from a bar of rectangular section. The slabs were ground with #120 emery paper and degreased with acetone prior to the deposition process.
Three different powder mixtures were used for the deposition and their chemical composition is reported in Table 1. These compositions were obtained by blending powders of pure iron (dmean ~ 120 μm), carbon (dmean ~ 15 μm), titanium (dmean ~ 15 μm), an FeSi
Alloy chemistry
The conditions used for this part of the assessment produced very little dilution of the substrate, in the order of 5% or less. At the same time, all the overlays exhibited some degree of porosity, and the blends containing graphite powder, i.e. overlays with Fe and FeSi matrices, also showed considerable amounts of un-reacted graphite flakes at the interface between the deposit and the substrate. The latter circumstance resulted in poor bonding of these coatings.
The typical microstructure of
Conclusions
From the present study on the co-deposition of iron-based powders and titanium using plasma transferred arc welding, the following conclusions can be drawn:
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Fe–TiC composite overlays can be produced by in-situ synthesis during plasma transferred arc welding using Ti, C and iron alloys containing Si and Mn.
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The morphology of TiC precipitates is affected by the chemical composition of the iron matrix. Additions of silicon reduce the occurrence of script carbides and manganese hinders dendritic
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