Swelling of copper powders during sintering of heat pipes in hydrogen-containing atmospheres

Abstract

Swelling of copper powders often occurs during sintering of heat pipes and causes problems in removing core rods from the fixture. To address this problem, this study examines the effects of sintering atmospheres (H2, N 2-10%H2, N2, and vacuum) and oxygen content in copper powders on the density and dimensional changes of loose-powder-sintered copper compacts. The results show that serious powder cracking occurs when high oxygen content is present in the as-received powder and when the sintering atmosphere contains hydrogen due to the hydrogen-oxygen reaction forming water vapors. The dilatometer curve indicates that such reaction starts between 673 and 723 K. Comparison of the pycnometer density and Archimedes' density of sintered compacts suggests that the swelling is mainly caused by cracking of the powder, and that the plastic deformation caused by the expansion of trapped gas in isolated pores plays a minor role. To prevent such cracking and swelling, copper powders with low oxygen content should be used, and the sintering should be carried out in an atmosphere with low hydrogen content or under vacuum or inert gas atmosphere. ©2010 The Japan Institute of Metals.