Reactive Fusion Welding for Ultra-High Temperature Ceramic Composite Joining.

摘要

Plasma and pulsed plasma arc welding (PAW and PPAW) processes were used to fusion weld ZrB2 containing 20 vol% ZrC. Varying welding parameters resulted in changes in weld pool shape and size, and the size of ZrB2 grains within the fusion zone (FZ). For PAW processes that resulted in a keyhole fusion zone (full penetration), the arc to workpiece power transfer efficiency was estimated to be <24%. Power transfer efficiency was estimated to drop to <18% for non-keyhole welds. The power transfer efficiency was not estimated for PPAW processes, but keyhole formation was observed for PPAW FZs. ZrB2 grain lengths were observed to decrease between binary PAW FZs (~1 mm in length), and binary PPAW FZs (~0.8 mm in length), and an increasing aspect ratio for ZrB2 grains in PPAW welds (up to 40:1) compared to PAW ZrB2 grain aspect ratios (up to 25:1) revealed that growth of ZrB2 was hindered in PPAW FZs. Grain growth was also observed to decrease with decreasing arc power. A high arc power resulted in high growth rates of ZrB2 and a textured FZ, whereas lower arc power FZs did not exhibit texture. A high current plasma arc (222 A) was used to increase the temperature of the weld pool, such that ZrB2 growth would follow the arc thermal gradient. ZrB2 growth occurred in the basal plane, resulting in grains with a plate-like morphology, where grain thickness increased by ledge growth. ZrB2 grain sizes within the FZ were observed to affect the strength of weldments. PM flexure strengths were measured to be ~660 MPa and strengths were observed to drop to ~140 MPa for PAW weldments and ~170 MPa for PPAW weldments. Diffusion of C into the melt pool was observed to hinder ZrB2 grain growth significantly (~150 m maximum ZrB2 grain size). The decreased maximum grain size resulted in average flexure strength of ~250 MPa for ZrB2-ZrC-C weldments.