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Vacuum Brazing

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Vacuum Brazing ISSUES IN VACUUM BRAZING Successful vacuum acuum brazing is usually a high Cooling motor temperature (typically 1700 - Double wall brazing relies on using V 2250°F, or 930 - 1230°C) fluxless vessel the proper techniques, process using nickel-base, pure copper, and, less frequently, pre- correct materials, and cious-metal composition BFM. furnace capabilities to Why braze under vacuum con- ditions? The purity level of the continuously control the atmosphere (vacuum) can be precisely controlled; at- Hot brazing cycle. More than zone mospheres of much higher Vacuum pumping 90% of all brazing purity can be achieved than can be problems reportedly are a obtained in regular atmosphere fur- Schematic of vacuum brazing furnace as- nace, in effect; there is less residual sembly. result of not paying close oxygen to contaminate the work ment, compatibility with base mate- attention to basic brazing piece. Oxide layers on the part sur- rial and braze filler metals, tempera- face are decomposed in a vacuum at ture requirements, and cooling fundamentals. high temperature, which improves speed. The most popular insulation base metal wetting resulting in better and heating element materials are Janusz Kowalewski** joint properties (e.g., increased graphite and a refractory metal such Seco/Warwick strength, minimum porosity, etc.). as molybdenum. Graphite is a more Meadville, Pa. Part distortion is minimized due to economical material with lower and heating and cooling at precisely con- maintenance requirements. Graphite Janusz Szczurek trolled heating/cooling rates. In ad- purity has improved over the past dition, the repeatability and relia- several years, and can now be con- Dallas Airmotive Inc. bility of brazing in modern vacuum sidered for many vacuum brazing Dallas, Tex. furnaces makes it suitable for a applications. A furnace using lean/agile manufacturing system. graphite requires a larger pumping system to operate at the same Vacuum Brazing Equipment vacuum levels as the furnace with a Two types of vacuum furnaces are metal hot zone because graphite is available based hot-zone (heating el- hygroscopic. Generally, graphite fur- ements and insulation) construction naces heat more slowly compared **Member ASM International and member, material, the choice of which de- with one having an all-metal hot ASM Heat Treating Society pends on the vacuum level require- zone. Molybdenum does not absorb Representative vacuum brazing furnace system with front end parts loader (left) and view of an all metal hot zone (right). HEAT TREATING PROGRESS • MAY/JUNE 2006 41 moisture and heats faster, and it is • Vacuum cleaning (removes ox- increases processing cost and can add still recommended in the aerospace ides from stainless steel and some Ni- to distortion difficulties. Tack industry to process titanium, alu- base alloys). welding, poke welding, gravity lo- minum, and materials having high • Hydrogen partial pressure cating, swaging, and stacking can be Ti and Al content. cleaning (HPPC) (removes oxides used when self-jigging is not pos- from stainless steels, Co-base super- sible. Maintaining a proper gap clear- Cleaning before Vacuum Brazing alloys, and some Ni-base alloys, but ance is one of the most important fac- Clean, oxide free surfaces are im- not Ni-base superalloys). tors in the assembly of parts for perative to ensure sound brazed • Fluoride ion cleaning, or FIC (the brazing. joints of uniform quality. Uniform only way to remove Ti and Al oxides Fixtures must be capable of main- capillary action is possible only when from gamma prime precipitated taining proper braze clearance, have all grease, oil, dirt, and oxides have nickel base superalloys, such as In- dimensional stability, and have com- been removed from both the filler conel, Rene, Nimonic, etc.). patible coefficients of thermal expan- metal and the base metal before • Flash nickel plating (can be used sion with parts being brazed. In de- brazing. The choice of cleaning in lieu of FIC; it covers, and thus pre- signing fixtures, use thin sections process depends on the nature of con- vents oxidation of high Al and Ni with the required rigidity and dura- tamination, specific base metal to be content base metals; also improves bility. The total weight of all fixtures cleaned, degree of cleanliness required surface wetability). should not exceed 50% of the total for brazing, part configuration, and weight of the assemblies being the need to remove or provide a bar- BFM Requirements and Assembly brazed per furnace run. Fixtures rier for coating for undesirable ele- & Fixturing should be made of materials that are ments, such as Al, Ti, N. A suitable brazing filler metal not reactive with the material of the Chemical cleaning methods in- (BFM) must be able to wet the base assembly being brazed. Inconel 600, clude emulsion cleaning in insoluble metal, must have melting and flow Alloy 230, MA956, molybdenum and hydrocarbons and water (good for properties to permit distribution by graphite are recommended, as these removal of oils and cutting fluids), capillary action, must be able to make materials maintain high strength at water-base alkaline cleaners (good a strong, sound metallurgical bond, elevated temperatures and have for removal of oils and cutting fluids), must have chemical composition of good resistance to thermal shock. solvent cleaning in mineral spirits, sufficient homogeneity and stability Dissimilar metals should not be used alcohol, acetone, and chlorinated hy- to minimize separation by liquation where differences in thermal expan- drocarbons (good for removal of in brazing, should be compatible sion could affect assembly dimen- mineral oils and cutting fluids, but with the substrate, and must be able sions. Bolts and screws should not be poor for removal of water-soluble to produce a braze joint that will meet used as they may relax upon heating oils), and vapor degreasing in the specified service requirements or pressure weld in place. In addi- trichlorethylene, trichloroethane, (ex- and mechanical properties. tion, fixtures should be subjected to cellent for removal of mineral oils Brazements should be designed so the brazing environment prior to use, and cutting fluids, but poor for re- that the detail parts are self-fixturing to ensure stability and relieve moval of water-soluble oils). and self-aligning where possible. The stresses, and they must not interfere Mechanical cleaning methods (fol- use of fixtures in the vacuum furnace with the flow of cooling gas. lowing chemical cleaning) include grit blasting using chilled cast iron, or stainless steel grits or powders and machining or grinding provided that joint clearances are not disturbed. Grit blasting using nonmetallic ma- terials (aluminium oxide, silicon car- bide, etc.,) can ruin brazing! Surface Conditioning Before Brazing Some base metals and/or compo- nents require special methods of sur- face preparation prior to vacuum brazing despite being absolutely clean after cleaning using standard chemical and/or mechanical clean- ing techniques (e.g., most turbine en- gine hot section components that re- quire braze repair). Four well-known surface conditioning techniques are: Load of turbine vane parts for brazing. Photo courtesy of Dallas Airmotive Inc., Dallas, Tex. 42 HEAT TREATING PROGRESS • MAY/JUNE 2006 Vacuum safety interlock Vacuum safety interlock to ensure 8 x 10-4 torr to ensure 8 x 10-4 torr Braze soak Braze soak Brazing vacuum level Brazing vacuum level temp. temp. > Vacuum cool > Vacuum cool Brazing Brazing alloy alloy solidus Heating rate solidus Heating rate minus 35 - 75oF/min minus 35 - 75oF/min 50oF 50oF Preheat or Preheat or Temperature —— Temperature stabilizing —— Temperature Burn-off stabilizing Quench temp. Quench temp. soak Gas 500oF - soak quench o Heating rate 700 F Heating rate 20 - 30oF/min 20 - 30oF/min x 300oF x 300oF R.T. R.T. Vacuum pump down 5 x 10-4 torr Time ——> Time ——> Typical braze cycles; an intitial pumpdown to 8 × 10-4 torr is sufficient for relatively easy-to-braze materials (left); the furnace should be pumped down to below to 8 × 10-4 torr before commencing heating for difficult-to-braze materials, such as Ni-base superalloys containing ap- preciable amounts of Al and Ti, and a cement burn-off soak is strongly recommended to avoid too high a pressure rise (right). Vacuum Furnace Brazing Cycles vacuum pressure levels are low Braze-joint clearance has A properly designed vacuum enough before proceeding (ramping) brazing cycle is a critical step in the to brazing temperature. a significant effect on process. A brazing cycle consist of ini- The final heating rate to brazing mechanical properties of tial pumpdown, initial heating ramp, temperature is critical. It must be fast cement burn-off, stabilizing soak, enough to avoid excessive liquation the joint. Vacuum has an heating ramp to brazing tempera- of the brazing alloy and subsequent effect on the design of ture, brazing soak, and cool down. alloying with, and erosion of, the clearances for a specific For easy-to braze materials, evac- base metal. For thin materials (e.g., uating vacuum to 8 × 10-4 torr is suf- <0.010 in., or <0.25 mm thick), base and filler material. ficient, but the furnace should be heating rates of 50 to 75ºF/min (~28 Vacuum brazing requires pumped down to below 5 × 10-4 torr to 40ºC/min) are essential. Rates of (e.g., Ni-base superalloys containing 30 to 50ºF/minute are the most fre- lower clearances than appreciable amounts of Al and Ti) be- quently used in the industry. atmosphere brazing to fore commencing heating. It is desirable to use the lowest obtain optimum strength The initial heating ramp should be brazing temperature within the rec- 20 to 30ºF/min (~10 to 16ºC/min). ommended brazing range consistent in a joint. Faster rates are not recommended with producing a satisfactory joint. due to possible part distortion, Minimum brazing temperatures are spalling of the applied brazing slurry, essential in some applications such and the likely occurrence of excessive as when using pure copper brazing outgassing with large loads con- filler, filling large gaps with wide-gap taining an appreciable amount of nickel alloys, and when brazing very brazing slurry).
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