BACKGROUND OF THE WORK
ZrB2-SiC based ceramics are attractive for aerospace applications, such as thermal protection systems, leading edges, trailing edges and propulsion components for hypersonic flight vehicles. Components of ZrB2-SiC composite are generally made by pressureless sintering (PS) or hot pressing (HP). However, their applications are limited by the challenges involved in fabricating large sizes or complex shapes.
Several studies have investigated the joining of (i) ZrB2 with different metals, and (ii) ZrB2-SiC, ZrB2-SiC9-SiC and ZrB2-SiC-C composites to themselves or to Ti / Nb and its alloys. These joining techniques involve the use of (i) glass powder (Ca-Al-Si-O, and Y-Al-Si-O), (ii) Ni metal powder, (iii) Ni foam, (iv) Ti inter layer for diffusion bonding and (v) Ni / Pd / Cu / Ag / Au base alloys for brazing. Due to the low melting points of these solders, the joints cannot be useful at temperatures above 1000 oC. Hairline cracks, substantial chemical interaction and interfacial cracking due to residual stresses have been observed. High temperatures, pressures and special fixtures and equipment are required for diffusion bonding.
Though arc welding of ZrB2 is possible due to its electrical conductivity, research on joining by gas tungsten arc welding (GTAW) or plasma arc welding is very limited. Brown et al.  reported fusion welding of ZrB2-20 vol% SiC and ZrB2-SiC-B4C composites to themselves. By pre-heating and controlled cooling under a protective atmosphere, the parts up to 3mm thick were joined by GTAW. The strength of the joints was ¼ of the strength of the parent material. Derek S King et al.  reported the plasma arc welding of TiB2 - 20 vol % TiC composites and ZrB2-20 vol % ZrC composites  by pre-heating of weld coupons. Even after pre-heating to a temperature of 1450 oC, and controlled cooling after welding, formation of porosity was observed at the weld interface.
NOVELTY OF THE PRESENT WORK
Molten liquid of ZrB2 composite will form upon striking an arc between a tungsten electrode and the solid surfaces of the composite. As the molten pool is cooled, the solid joint surfaces are bonded together. Due to shrinkage accompanied by solidification of melt pool formation of some porosity at solid-liquid boundary between parent material and fusion zone is possible. During welding, oxidation of the ZrB2-SiC parent material can also lead to escape of gaseous species like SiO, CO, and B2O3 and induce porosity in the fusion zone. To avoid cracks and pores during welding, a suitable filler material is required. Fillers form a liquid pool and fill the gap between the surfaces to be joined. It is similar to metal casting into a mold. By properly controlling the welding speed, the flow of the filler liquid into butt weld gap can be controlled to avoid cracks and pores that could form due to shrinkage during the solidification of molten filler.
There exists the necessity to develop a suitable filler composite material possessing oxidation resistance and thermal shock resistance to join ZrB2-SiC based composites. In the present work, a filler material of (ZrB2-SiC-B4C-YAG) composite possessing oxidation resistance and thermal shock resistance has been produced in the form of welding rods by pressureless sintering at relatively low temperatures between 1550 and 1680oC.
Using the filler, GTAW was performed to join hot pressed (ZrB2 - 20 vol.% SiC), and pressureless sintered (ZrB2 - 20 vol.% SiC - 8 vol.% B4C - 7 vol.% YAG) composites to themselves. Without any pre-heating, post-controlled cooling and extraneous protective gas shield GTA welding was performed manually. The weld interfaces for both the composites were very clean and coherent. The Vickers micro hardness across the weld interface was found to increase due to the increase in the volume % of both SiC and B4C in the filler material. The shear strength of the weld was about 50% of the flextural strength of the parent composite.
JUSTIFICATION FOR AWARD
Preparation of filler material for fusion welding of Ultra High Temperature Ceramic Composites. Demonstration of its oxidation and thermal shock resistance, mechanical behaviour and micro structural characterisation of welds is unique, novel and has not been reported elsewhere.
The award was selected by the editorial team, delegated by the chairman Dr. K. Prasad Rao and guest editor of the special issue, Dr. G. Magudeeswaran. Clickfor the full article related to this award.