Special Issue on Materials Joining: Best Paper Award

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. [20] 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. [21] reported the plasma arc welding of TiB₂ - 20 vol % TiC composites and ZrB₂-20 vol % ZrC composites [22] 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 1680^oC.

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. Click here for the full article related to this award.