The Engineering of tecknology of soldering parts from tantalum and steel 12Cr18Ni10Ti

The article presents the results of developing a soldering technology of tantalum and stainless steel parts using VPR4 solder. The soldered assembly deign is a telescopic joint where a stainless steel ring encloses a tantalum ring with a diameter of 52 mm. Since tantalum forms intermetallic compounds with many metals, which lead to brittle fracture of soldered joints, it was necessary to determine the conditions that prevent the formation of continuous intermetallic layers in the solder joint. One of such conditions is an absence of contact between tantalum and the components of the solder and steel. As such a solution, a protective copper coating was selected. The coating was formed by surfacing followed by mechanical refinement. Experiments were carried out to determine the required thickness of copper coating when exposed to molted solder. Analysis was performed using electron microscopy on metallographic cross-sections of solder – copper samples and soldered joints to examine the interaction zone. Under the selected soldering parameters (holding at 1050 °C for 6 minutes in an argon atmosphere), the interaction between a cylindrical solder specimen (1 mm height, 5 mm diameter) and a 3 mm-thick copper plate resulted in a penetration depth of up to 0.6 mm. When the solder thickness was reduces to 0.3 mm, which corresponds to the soldering gap formed during heating the parts to the soldering temperature, no penetration through the 0.6 mm copper plate was observed. Based on experimental results, a minimum copper layer thickness of 0.6 mm was determined, which will exclude penetration to tantalum of elements forming intermetallic compounds during soldering of this assembly. Soldering of specimens for mechanical tests and required parts was performed. SEM and EDS of metallographic cross-sections confirmed the absence of intermetallic compounds in the tantalum – copper – solder – steel system after soldering under the selected parameters. The strength of copper-coated soldered specimens was almost twice as high (398 MPa) compared to uncoated specimens (212 MPa). For other constructions (dimensions, materials) and soldering temperature and time parameters it will be necessary to estimate the minimum thickness of protective coating, following the methodology applied in this study.