V. Muthupandi

Analysis and Characterization of the Role of Ni Interlayer in the Friction Welding of Titanium and 304 Austenitic Stainless Steel

Joining of commercially pure Ti to 304 stainless steel by fusion welding processes possesses problems due to the formation of brittle intermetallic compounds in the weld metal, which degrade the mechanical properties of the joints. Solid-state welding processes are contemplated to overcome these problems. However, intermetallic compounds are likely to form even in Ti-SS joints produced with solid-state welding processes such as friction welding process. Therefore, interlayers are employed to prevent the direct contact between two base metals and thereby mainly to suppress the formation of brittle Ti-Fe intermetallic compounds. In the present study, friction-welded joints between commercially pure titanium and 304 stainless steel were obtained using a thin nickel interlayer. Then, the joints were characterized by optical microscopy, scanning electron microscopy, energy dispersive spectrometry, and X-ray diffractometry. The mechanical properties of the joints were evaluated by microhardness survey and tensile tests. Although the results showed that the tensile strength of the joints is even lower than titanium base metal, it is higher than that of the joints which were produced without nickel interlayer. The highest hardness value was observed at the interface between titanium and nickel interlayers indicating the formation of Ni-Ti intermetallic compounds. Formation these compounds was validated by XRD patterns. Moreover, in tensile tests, fracture of the joints occurred along this interface which is related to its brittle nature.

The influence of aluminium intermediate layer in dissimilar friction welds

Abstract Friction welding of 304 austenitic stainless steel to commercially pure titanium was carried out using commercially pure aluminium as an interlayer of different thicknesses viz., 0.5, 1.1, 1.7 and 2.3 mm. The interlayer was aimed at preventing the occurrence of direct contact between the parent metals, which otherwise would lead to the formation of Ti-Fe intermetallic phases. These intermetallic phases can seriously affect the weld properties. The microstructure of the friction welded joint interfaces was characterized by means of optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction analysis. The maximum hardness at the Ti – Al interface was attributed to the presence of AlTi and Al3Ti intermetallics. Tensile tests were conducted to evaluate the strength of the joints and the results showed the maximum strength (272 ± 9 MPa) was achieved in the joint having 1.7 mm thick interlayer. The analysis of fractured surfaces revealed that the failure was confined to the aluminium layer at the Ti – Al interface and was transgranular with flat facets.

Tribological studies on laser surface melted Hastelloy C-276

Abstract Hastelloy C-276 was laser surface melted in a nitrogen environment in order to improve the wear resistance. The metallurgical characterisation was performed using optical microscopy, scanning electron microscopy with electron dispersive spectroscopy (EDS) and X-ray diffraction analysis. At a particular laser power and scan speed combination, significant grain refinement was achieved with finely dispersed cell-like structure which improved the hardness up to 390 HV0·3. The EDS point analysis indicated nitrogen diffusion at the cost of nickel depletion. The absence of Cr2N or CrN is a positive aspect in preserving the intrinsic corrosion characteristics of the alloy. The wear rates of the laser treated specimens were significantly lower by a factor of 2·5. Potentiodynamic polarisation corrosion test on the laser treated specimens also exhibited reduced corrosion rates compared to the base material.

Enhanced Relative Slip Distance in Gas-Tungsten-Arc-Welded Al0.5CoCrFeNi High-Entropy Alloy

Abstract Gas-tungsten-arc-welded (GTAW) Al0.5CoCrFeNi high-entropy alloy (HEA) was analyzed using scanning electron microscopy (SEM), microhardness, and tensile testing. The weld metal having refined equiaxed and elongated columnar dendritic microstructure experienced 6.38 pct reduction in strength and marginally reduced hardness compared to the base metal (BM). Lower work hardening with enhanced relative slip distance, which was observed through the Kocks–Mecking plot and slip distance–true strain plots, was attributed to the reduced bcc fraction in the weld.

Improving the corrosion properties of magnesium AZ31 alloy GTA weld metal using microarc oxidation process

In this work, the morphology, phase composition, and corrosion properties of microarc oxidized (MAO) gas tungsten arc (GTA) weldments of AZ31 alloy were investigated. Autogenous gas tungsten arc welds were made as full penetration bead-on-plate welding under the alternating-current mode. A uniform oxide layer was developed on the surface of the specimens with MAO treatment in silicate-based alkaline electrolytes for different oxidation times. The corrosion behavior of the samples was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy. The oxide film improved the corrosion resistance substantially compared to the uncoated specimens. The sample coated for 10 min exhibited better corrosion properties. The corrosion resistance of the coatings was concluded to strongly depend on the morphology, whereas the phase composition and thickness were concluded to only slightly affect the corrosion resistance.

Improving Corrosion Resistance of AA2014 Welds with Micro Arc Oxidation

Autogenous single pass full penetration welds of 2.8 mm thick AA 2014 plates were produced by un-pulsed and pulsed gas tungsten arc welding (GTAW) process. In pulsed current mode, two levels of pulsing viz., 3 and 6 Hz were employed. In order to improve the corrosion resistance of weld metals, micro arc oxidation (MAO) treatment was given in aqueous silicate solution containing either borax or tungstate. The MAO treatment was carried out for 10, 20 and 30 minutes. Irrespective of the electrolyte composition, surface roughness increased with increasing treatment duration. However, the effect was observed to be significant with borax addition. X-ray diffraction studies showed that γ-Al2O3 content in the coating continued to increase up to 20 minutes and for the coating produced with 30 minutes duration the α-Al2O3 content was found to be more than γ-Al2O3. The coating treated for 30 minutes exhibited relatively poorer corrosion resistance than the other coated specimens. The better corrosion resistance offered by the coating obtained after 20 minutes, compared with the other coatings, could be attributed to the nature of the oxide and its content. Even though tungstate addition improved hardness of the coating, borax addition was found to enhance the corrosion resistance.

Studies on Dissimilar Welding of AA5083 and AA6061 Alloys by Laser Beam Welding

Dissimilar welds were prepared between 4 mm thick AA5083 and AA6061 sheets using Nd-YAG laser welding with different laser power, beam spot sizes and welding speeds. The results show that the temperature gradient and cooling rates play a major role in resultant microstructures of the weldments. Room temperature hardness and tensile properties of the weldments in the as-welded condition were studied and correlated with the microstructure. The microhardness examination showed that there was significant variation in hardness values at AA6061 side when compared with AA5083 side due to steeper temperature gradient. It has been observed that welds prepared with 3.5kW laser power, 3.5 m/min welding speed and 180 µm beam spot size exhibited highest tensile strength of 241 MPa.