September_AMP_Digital

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 7 12 TEMPERATURE MEASUREMENT Temperature uniformity and consistent tempera- ture control are critical in all aerospace and medical heat treatment processes. Placing the thermocouple within the printed part is preferred to meet these requirements. Adding a printed hole 1.5 mm (0.0625 in.) in minimum di- ameter within the thickest cross section of the printed part provides the most accurate temperature profile during the heat-treating cycle (Fig. 1). If it is not possible to place a hole within the part, incorporate a dummy block (heat sink) into the build design that matches the material com- position and the maximum cross section of the part. This provides a more accurate temperature profile of processed parts than placing a standard dummy block on the build plate. Avoid using the build plate for thermocouple inser- tion due to a mismatch with the temperature profile of the parts. If building a heat sink is not possible, then use a separate heat sink matching the thickest part cross section and material composition (Fig. 2). UNVENTED CAVITIES Because AM produces parts with unique geometries and uses an argon or nitrogen atmosphere, designing parts with unvented blind holes, cavities, pockets, and sealed cooling channels should be avoided. Trapped gases cause differential pressure within the part and outside the part as the vacuum pressure in the hot zone decreases. This pres- sure differential becomes significant during the heating process, leading to cracks and eventually part destruction. It is imperative that all printed geometries include a means to evacuation of any gases used during the vacuum pump- down process. LOOSE POWDER Prior to heat treating, build plates should be invert- ed, and the entire build blown out using nitrogen until any free powder is fully removed from the external surfaces and, most importantly, from internal cavities such as blind holes and cooling passages. Loose powder remaining in the build is evacuated during vacuum pump-down and contaminates the furnace hot zone. Minor powder con- tamination might only result in the need to clean the fur- nace using a vacuum cleaner. However, in some high-tem- perature cycles, powder can cause severe reactions such as eutectic melting and diffusion bonding between the powder and furnace hot-zone components. Figure 3 shows titanium powder released from a build plate that resulted in severe diffusion bonding between the molybdenumhot- zone shields and heating elements due to incipient melt- ing at the sintering temperature for titanium. For safety reasons, adding a slow-pump valve to the roughing line helps to avoid such powder eruptions during the vacuum pump-down. ADVERSE REACTIONS OF METALS IN DIRECT CONTACT The maximum thickness of the printed part should closely match the thickness of the build plate, and the build-plate composition should closely match the build material composition. Matching build-plate composition and dimensionsminimizes cracking of the parts and avoids unwanted thermal reactions between the build plate and parts during post-build vacuum heat treatment. Surfaceoxides onmanymetals prevent incipientmelt- ing between the metal and the fixture by creating asperi- ties at the contact site. However, when heated in a vacuum, surface oxides diffuse into the bulk material or decompose at the processing temperatures, thereby eliminating such rough surfaces that limit direct surface-to-surface contact. With the two materials in direct contact, atoms can diffuse across the interface and react with similar or dissimilar at- oms of the opposing material to form a new lower melting phase of the two materials. Such interactions can occur at temperatures 50 to 90% below the absolute melting point of either metal. Therefore, sticking (diffusion bonding) and melting (eutectic and incipient melting) can occur at much lower temperatures than expected. Knowledge of the temperatures at which such reactions occur is essen- Fig. 3 — Damage to vacuum-furnace hot-zone components due to harmful reactions of loose powder hidden in blind holes of the build plate blown throughout the furnace during the vacuum evacuation. 9