Laser brazing of reaction-bonded silicon carbide with a silicon-aluminum-titanium filler alloy

Abstract

This work focuses on the feasibility of joining reaction-bonded silicon carbide (RB-SiC) with a laser heat source. A ternary Si-Al-Ti alloy was used as filler metal. Research consisted of four phases: laser metrology, damage free preheating, wettability testing, and lap joint testing. Results indicate joining is feasible but can be further optimized. Laser metrology consisted of measuring beam diameter by ablating holes in a polyimide film. Knowing exact beam parameters was required to determine the threshold for heating the substrate without damaging it. Cracking is not the only damage mechanism; surface porosity is created due to thermal exposure. Rastering the beam allows for better control of temperatures and mitigates surface damage. Thus, most testing in this work was done with a rastered beam. Heating for the sessile drop wettability testing involved a preheat raster of the RB-SiC coupon, pause time for a controlled and uniform preheat temperature in the RB-SiC coupon, and a localized secondary raster around the filler metal location. The atmosphere was controlled to a maximum of 15 ppm oxygen, balanced argon. All contact angles were greater than or equal to ninety degrees. Oxidation of the liquid filler metal surface preventing spreading is the cause of high contact angles. Cross-sectioning the brazed joints revealed good wetting between the filler metal and substrate. Significant diffusion of alloying elements also occurred during testing. Lap joint brazing followed the same preheat, pause, localized heating pattern as in wettability testing with a limit on oxygen concentration. Joint strength was calculated by shearing the joints to failure and dividing the load at failure by the nominal joint area. Lap joints failed either inside the joint, through the joint and substrate, or only within the substrate. Strength was greatest in substrate failures and least for in joint failures, but never exceeded 20 MPa. Low strength is attributed to incomplete gap filling and surface damage. Since the filler metal was delivered as a foil preform covering the entire joint area, incomplete gap filling was a result of the loss of material. Material loss from the joint occurred through a combination of elemental diffusion and overflowing the gap and solidifying on the outside surfaces. Oxidation of the filler metal prevented overflowing most of the time, but the oxide layer also locally ruptured. Joint strength was mostly affected by factors influencing how much filler metal leaves the gap.