Formation of TiC by the application of Ti6Al4V machining chips as flux compounds of tubular wires

Titanium carbonates (TiC) have high hardness and wear resistance, being thus widely present as a phase present in various mechanical components subject to wear mechanisms. Its application on a large industrial scale becomes relatively low, due to the high cost of titanium commercial alloys. On the other hand, large amounts of chips are generated in the manufacture of prostheses and orthodontic implants, where titanium alloys (ASTM F67 and ASTM F136) are widely applied. The attractiveness of these residues lies in the fact that titanium is the major element present in alloys (at least 90% by weight) and are discarded at low cost when compared to commercial alloys. In order to re insert these residues in the production chain, ASTM F136 (Ti6Al4V) alloy chips were subjected to grinding processes to obtain powders with a grain size of less than 40 mesh and used as flux components in tubular type wires MCAW for manufacturing consumables, promoting the formation of TiC in the welding metal. The deposited cords presented low weld discontinuity index with a uniform distribution of TiC particles along the microstructure, resulting in considerable fractions of carbonate areas in the welds and presenting a considerable increase in micro hardness.


Introduction
The deposition of hard coatings composed of metallic carbides has been presented with an efficient alternative in resistance to the mechanisms of wear [1,2]. The carbides have a high melting point, are chemically stable and have a high hardness. In welding, chromium has been one of the main elements present in addition metals with the purpose of reacting in the form of carbides (Cr7C3), establishing high hardness and resistance to abrasive wear [3][4][5][6]. On the other hand, studies show that titanium carbide (TiC) have better mechanical properties and resistance to wear, however, they have a higher commercial value when compared to chromium [3,7].
Due to the biomechanical properties of titanium [8], its alloys have been widely used in the manufacture of orthodontic implants and surgical prostheses which are generally obtained by machining processes, thereby generating large quantities of titanium alloy chips, which in turn are discarded as scraps and low economic value.
ASTM F136-Grade IV (Ti6Al4V) alloy chips were converted into powders by grinding methods and used as components of the metal fluxes used in the formulation of MCAW type tubular wires to form a microstructure composed of titanium carbides with applications aimed at resistance to abrasive wear.

Methodology
ASTM F136 -Grade IV (Ti6Al4V) titanium chips were subjected to cleaning processes to remove the machining fluids and subsequently left in the oven for drying. The grinding was carried out in a grinder and classified in sieves with granulometry in the bands of -400+250μm, -250+150μm, -150+50μm and -50μm (Figure 1 and 2). In the present work the fines in the -400+250μm range were used. The fines were subjected to X-ray diffraction tests to identify the present phases.
Chemical composition of the metal flux was determined from the stoichiometric calculations for the formation of TiC, where the mass ratio of chips per mass of carbon (graphite powder) of 10:3 was determined. Tubular wires were fabricated in a forming and drawing machine by forming a SAE 1008 thin strip into a U-shape, filling it with the flux and continuing to roll it into a tubular wire and drawn through dies until the diameter of 2.2mm was reached ( Figure 3). Nominal chemical composition of thin strip, fine chips after milling and tubular wire formed are shown in Table 1.
Welding was performed in an Aristo U82 (ESAB) welding machine with the torch coupled in a speed controlled portable car, Figure 4. The welding parameters are presented in Table 2. As base metal a flat bar was used with section of 10x50mm in SAE 1020 low carbon steel.      Samples were taken from the weld beads formed as shown in Figure 5. After cutting, the ends were discarded (A 'and B'). Samples with the sections indicated by A and B were embedded in Bakelite and sent to macrographic analyzes (cord geometry), microhardness tests and microstructural characterization. The central samples, C, were prepared in the longitudinal direction of the bead and sent to X-ray diffraction (XRD) analyzes.
Metallographic preparation was made from conventional sanding and polishing techniques. The attack was performed in 2% NITAL solution.
Microstructural characterization was performed in an optical microscope where the generated images were processed in the ImageJ to study the particle size and fraction in the area of the formed carbides. To measure the microhardness, a Shimadzu micro durometer with indentation in the Vickers HV0.3 scale will be used. The determination of the microhardness was made from a line orientation with three indentation rows, starting from the top of the strand until it reaches the base metal and beyond the ZTA, spaced 0.3mm between each measurement, Figure 6.

Results and discussion
The grinding made from the use of a milling apparatus allowed the formation of fines with a grain size less than 50μm. The XRD analyzes performed on samples of the fines did not identify the formation of titanium oxides, Figure 7. Comparisons by XRD analyzes made on a titanium bar and on the generated fines indicated that the phases present in the fines are identical to those present in the titanium bar.
Wires presented satisfactory aspects as to the form and the efficiency of filling of the flow, [9]. The wire presented an external diameter of 2.252±0.004mm and a thickness of 0.473±0.005mm, resulting in an internal diameter of approximately 1.26mm, Figure 8. The wire filling efficiency presented an average of 10%, Figure 9. Although it is reported in the literature that tubular wires facing the hard coating application show efficiency in the range of 30-50% [10], the addition metals used in the present work have in the flux components with low density and considering the internal space of the wire formed, the maximum possible efficiency will be close to 18%.   The deposited cords presented a good finish to the presence of spatter indicating that the transfer method may have been short-circuited, Figure 10. In the section samples it is noted that the cords were absent from discontinuities such as pores and cracks, Figure 11 and 12, The mean dilution rate of 50.5±12.29% was higher than those recommended in the literature, where studies indicate that for the application of hard coatings the dilution should be between 10% and 20% [11,12].
In addition to the high dilution rate, the flux share in the weld is highly influenced by the thickness of the metal strip (responsible for the conformation of the wire) that affects the amount of flux deposited per unit length. TiC are characterized by a microstructure consisting of dendritic branches with a rounded or starred morphology [13,14]. However, for welds formed with one pass, it can be seen from the micrographs, Figure 13, that the carbides have a prismatic shape and can thus present a high concentration of stresses at the interface with the matrix, reducing the properties of the wear resistance microstructure [15].     The formation of titanium carbides was well distributed throughout the ferritic matrix, although it presented higher proportions of fine particles (0.75μm to 3.2μm), promoted a volumetric fraction with variations between 11% a 13%, Figure 13. The formation of the TiC may also be evidenced by the