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FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 0 4 8 13 tial to avoid unexpected melting between parts, fixtures, and workbaskets. Eutectic melting and diffusion bonding are also time and force dependent. Although the temperature might be 110°C (200°F) below the eutectic temperature if the time at temperature is extremely long or the mass of the part largely exceeds the mass of the fixture, some diffusion bonding or incipient melting could still occur. The best way to prevent adverse reactions is to avoid heating at or near the eutectic melting point of the two materials. However, in instances where this is unavoidable, the use of a ceramic material between the part and the fix- ture is required. Ceramic separators can be in the form of sheet, blankets, plates, and paint. Some common stop-off paints consist of alumina, zirconia, yttria, and boron ni- tride. It is important to carefully select the ceramic separa- tor based on the part or fixture material, because not all ceramic-based separator materials are inert to all metals at high temperatures. Above a temperature of 760°C (1400°F), boron nitride reacts negatively with titanium and titanium alloys. Silicon oxide, used as an additive in some ceramic compositions, reacts with some metals such as titanium and even graphite in vacuum or under reducing conditions at high temperature. It is also important to ensure that the coating fully adheres to the graphite/metal fixture or that the sheet or fabric is intact. Over time, ceramic paints start to flake off due to continued heating and cooling. Loss of coating in- tegrity increases the risk of failure, and ultimately eutec- tic or incipient melting, or solid-state diffusion reactions (diffusion bonding) between materials. Exposed material could result in unexpected melting or atomic diffusion, which changes the chemical composition of the parts at that interface, thus changing the mechanical and physical properties of the part. Continuous successful reuse of the separator can lead to a false sense of security. For exam- ple, Fig. 4 shows how the weight of the load fractured the weaves of old or overused silica fabric, resulting in direct contact between the graphite rail and Inconel sheet and, ultimately, full eutectic melting as the process tempera- ture exceeded 1150°C (2100°F). The same precautions ap- ply during high-temperature bake-out cycles performed periodically for maintenance purposes. All iron-base alloy workbaskets, trays, and grids should be removed from the hot zone prior to heating; otherwise, these components and the furnace itself can suffer extensive damage due to eutectic melting (Fig. 5). ~HTPro Note: This article is an excerpt from “Vacuum Heat Treating Additively Manufactured Parts” in the recently published ASM Handbook, Volume 24: Additive Manufac- turing Processes, available in the ASM Digital Library at dl.asminternational.org . References 1. R. Hill, Vacuum Heat Treating of 3D-Printed Com- ponents, Industrial Heating, Sept. 12, 2018. 2. R. Fradette, V. Osterman, W. Jones, and J. Dossett, Vacuum Heat Treating Processes, Heat Treating Technol- ogies, Vol 4B, ASM Handbook, ASM International, 2014, p 182. For more information: Dr. Virginia Osterman, corporate chemist, Solar Atmospheres Inc., 1969 Clearview Rd, Soud- erton, PA 18964, 215.721.1502, ginny@solaratm.com . Fig. 4 — Damage to a graphite-metal fixture from eutectic melting enabled by deterioration of old or reused silica textiles used to separate the graphite and metal. Fig. 5 — Damage in the area of an Inconel alloy grid from eutectic melting due to exceedance of the eutectic temperature during furnace bake-out. 10