Experience in HIP Diffusion Welding of Dissimilar Metals and Alloys

HIP solid-state diffusion welding is a controlled production operation at all the processing stages. Unlike other known solid-state welding techniques the HIP allows to provide strong and dense bonding with stability properties irrespective of the sizes and a configuration of the contact surfaces of materials welded. Here we present some special pilot examples of HIP diffusion welding of dissimilar metals and alloys: steel XM19-to-steel 316L, bronze Cu-Cr-Zr– to-steel 316L, copper M1–to-steel Fe-18Cr-10Ni-Ti-C, titanium alloy Ti-6Al-4V–to-steel Fe18Cr-10Ni-Ti-C, single-crystal molybdenum-to-polycrystal molybdenum and titanium alloy-toaluminum alloy. Introduction Solid-state diffusion welding (DW) is a main way to make a bimetallic structural material for space and nuclear application where a strong and dense bonding of materials with different chemical compositions is needed. This technology produces a monolithic joint resulting from a maximum closing of the contact surfaces due to their local plastic deformation at the increased temperature as well as the formation of metallic bond at the atomic level followed by a mutual diffusion of the components through the surface layers of the materials bonded [1]. Solid-state diffusion welding includes the following obligatory stages: the oxide film removal from contact surfaces, the actual contact formation, the surfaces activation, the chemical bond formation and diffusion. This sequence is true for all known methods of solid-state welding: cold bonding, explosion welding, percussion vacuum welding, friction welding, vacuum roll welding, induction and ultrasonic welding, etc. However, only the diffusion welding is the most universal and reliable method that allows controlling all four key technological parameters of process: temperature, pressure, dwell time and diffusion medium. The method of diffusion welding (DW) with use of hot isostatic pressing (HIP) can be considered as a kind of classical DW wherein technological parameters can be controlled within a wider range. Below we examined the influence of the HIP DW technological parameters on a welded joint quality. Influence of HIP parameters Temperature and pressure Temperature and pressure are mutually dependent parameters in HIP technology. Specified pressure values in a HIP installation chamber are achieved by thermal expansion of working gas as the temperature increases. Thus, with computation of the necessary amount of gas at the room temperature performed, it is possible to reach the HIP operation conditions both in the temperature of 200 °C to 1200 °C and pressure of 20 MPa to 200 MPa ranges under any parameter combination. As the pressure is created by gas, the pressure value will be the same in Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 65-72 doi: http://dx.doi.org/10.21741/9781644900031-9 66 any point of the HIP product contact surfaces despite the sizes and configurations of the parts bonded. As it is well known [2], if all-round compression pressure is applied to a crystal the concentration of vacancies in this case will then be equal to Ср =Со ехр (-РΩ/kТ), (1) Where, Со is the equilibrium component concentration at P=0; Ω is the atomic volume; Р is the all-round compression pressure; k is the Boltzmann's constant; T is the temperature. In this case the "minus" symbol denotes compression. That is, the amount of vacancies decreases with increase in pressure, such that the diffusive flow of atoms decreases too. In 1954, S. Storchheim et al. [3] established that the phase Ni3Al2 was not formed even at pressure of 170-300 MPa, only the phase Ni3Al was formed at pressure higher than 300 MPa, and intermetallic phases were not observed at a pressure about 500 MPa. Thus it is possible to increase or reduce diffusion rate with pressure increasing or decreasing. In so doing it is possible to reach such process conditions wherein the nucleation and growth of undesirable phases can be depressed at the contact zone. Dwell time Theoretically the duration of a HIP DW technological parameters can be unlimited and depends only on the end result required. HIP DW excludes the void volume along a boundary of the dissimilar metal diffusion bonding that caused by distinction in partial component diffusion coefficients, for example, nickel and copper (Kirkendall’s effect [3]), as owing to constantly applied pressure the formed vacancies are replaced with metal atoms having the largest diffusion velocity, here copper (Fig.1). Therefore, it is possible to create quite a wide transitional area in a contact zone of dissimilar metals (up to several hundred microns) by operating of HIP DW duration. Increasing the transitional area width will give the positive effect, for example, as damping layers between metals of greatly different coefficients of thermal expansion. а b Fig.1 – Voids in copper of Ni-Cu diffusion bonding according to Le Claire A.D. and Barnes R.S. [3], A is the initial line of contact (a), absence of voids after HIP DW [4] (b) Environment Under the fine vacuum and at the high temperatures the dissolution of oxides promotes the formation of juvenile contact surfaces of the joints welded. Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 65-72 doi: http://dx.doi.org/10.21741/9781644900031-9 67 Experimental procedure The following materials are used in this study: steel ХМ19 (chemical composition in wt%: 22 Cr, 12 Ni, 5 Mn, 2.5 Mo, 2.5 Nb, 0.2 V and the reminder Fe) in forging, steel 316L (in wt%: 17 Cr, 12 Ni, 2.5 Mo and the reminder Fe) in forging, stainless steel in wt%: 18 Cr, 11 Ni, 0.5 Ti and the reminder Fe in bar and sheet, bronze in wt%: 0.9 Cr, 0.1 Zr and the reminder Cu in sheet, titanium alloys in wt%: 6 Al, 4 V and the reminder Ti in bar; 4.5 Al, 5 V, 2 Mo, 1.2 Cr, 0.6 Fe and the reminder Ti in sheet, copper alloy M1 in bar, aluminum alloy in wt%: 6 Mg, 0.7 Mn and the reminder Al in sheet, single-crystal and especially pure polycrystal molybdenum in bars. To manufacturing of samples for test of mechanical properties and research of structure used one HIP diffusion bonding from party, and in a design of structural assembly of the diverter and mirrors were provided with special places for cutting of samples witnesses. Mechanical tensile strength testing was carried out according to requirements of the ISO 6892:1984, ISO 783:1989, ISO 783-89 standards. Microstructure was observed by of an optical microscope Zeiss Axio Observer with ImageExpert system and a raster electronic microscope JSM-6610LV equipped with Advanced AZtec EDS Detector. Metallographic samples were made with use of a combination of the machines which includes the Delta AbrasiMet Abrasive Cutter, SimpliMet 3000 Mounting Press and EcoMet 250 Grinder-Роlisher. Results Steel XM19-to-steel 316L HIP Diffusion Bonding Within an International Thermonuclear Experimental Reactor (ITER) program the diffusion welding has been performed of large-size parts of corrosion-resistant stainless steel AISI 316L and high-strength steel XM19 intended for pre-fabrication of the diverter attachment fitting (Fig. 2a). The structural assembly mass is equal to 760 kg and the summary diffusion bonded surface area is nearly 770 cm (Fig. 2b) and 1260 cm (Fig. 2c). Failure of the bimetallic tension specimens takes place on the base metal of steel 316L outside the diffusion bonding zone (Fig. 3a) as tensile strength of the HIP diffusion bonding zone is higher than tensile strength of steel 316L. In microstructure of the diffusion bonding zone steel XM19-to-steel 316L (Fig.3b) the presence of oxide phases is not detected. Besides, the common grains were observed in a contact area.