During the production of spray formed billets of CuCrZr, large hot cracks can form in the vicinity of the billet head. From the crack positions, it is concluded that crack initiation is the result of the attendant cooling conditions. At the completion of the spraying process the cooling conditions change rapidly because the molten metal and gas flow are terminated abruptly. Observation of the hot cracks confirms that residual alloy melt is available at the time of crack initiation. Numerical calculations of the temperatures developed inside the billet confirm the possibility of the formation of a "hotspot" after the completion of spraying. These "hot spots" are regions inside the billet of residual alloy melt surrounded by solidified material. From the theoretical results and experimental observations, the following conclusions are drawn with respect to hot cracking: Hot cracks do not appear during spraying but after completion of spraying. The surface of the billet cools and solidifies finally at the top of the billet, when spraying is complete. Inside the billet residual alloy melt is captured and with further solification, shrinkage of this region is suppressed if the surrounding solid alloy is strong enough. This leads to an increase in residual stress in the semi-solid material to a level sufficient to initiate hot cracks. The temperature difference between the core and the surface of the billet can be used as an indicator of the probability of hot cracking. A lower specific enthalpy of the impacting particles and a reduction in the molten metal flow result in a lower temperature difference and in the probability of hot cracking. This correlation was confirmed experimentally.
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