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Ceramics consisting of titanium diboride with titanium carbide additions were fusion welded to produce continuous joins. A welding current of 135 A, welding speed of 8 cm/min, and plasma flow rate of 0.75 L/min were combined with a preheat temperature of ~1450°C to fusion weld coupons of TiB2 containing 20 vol% TiC with a thickness of 4 mm. The resulting fusion zone (FZ) was 3.9 mm deep at the center of the joint and 10.4 mm wide. During cooling of the melt pool, four distinct regions of crystal growth and nucleation were observed due to thermal gradients. Regions at the top and bottom of the FZ exhibited smaller TiB2 crystals due to higher nucleation rates whereas regions in the middle of the FZ showed higher growth rates, with TiB2 crystals up to 1.2 mm in length. Thermal gradients also affected cooling of the eutectic phase, causing a cellular structure to appear in the cooled eutectic. Plasma arc welding was a viable method for joining diboride-based ceramics.