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Home , Brazing, Heat treating

ISSUES IN 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 -base, pure , and, less frequently, pre- correct materials, and cious- 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. layers on the part sur- rial and braze filler , 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 and a metal such Seco/Warwick strength, minimum porosity, etc.). as . 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 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 and some Ni- to distortion difficulties. Tack industry to process , alu- base alloys). , poke welding, gravity lo- minum, and materials having high • partial pressure cating, , and stacking can be Ti and Al content. cleaning (HPPC) (removes used when self-jigging is not pos- from stainless , 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 (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 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 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 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 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 . lowing chemical cleaning) include grit blasting using chilled cast , or grits or powders and or grinding provided that joint clearances are not disturbed. Grit blasting using nonmetallic ma- terials ( oxide, 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). thin materials. For extremely thin For critical materials (e.g., Ni-base metal, fillet type joint (e.g., honey- superalloys), particularly with comb seals), a brazing temperature heavily applicated parts (e.g., surface equal to or slightly below the liq- braze build-up), a cement burn-off uidus temperature is used to avoid soak is strongly recommended to excessive flow and erosion. avoid too high a pressure rise. In general, time at brazing temper- The soak temperature should be ature should be long enough to en- about 50ºF (28ºC) below the braze sure that all sections of a work piece slurry solidus temperature for 15 to and all parts within the load reach 30 minutes, or until the pressure the desired brazing temperature. drops below the desired level, Brazing temperature and time are whichever is longer. The soak allows considered critical events. It is the temperature throughout the load strongly recommended that guaran- to equalize so all parts in the load will teed soak point be used in the pro- reach brazing temperature at approx- gram. imately the same time during the For materials that do not require next heating cycle and it ensures that solution heat treatment or , HEAT TREATING PROGRESS • MAY/JUNE 2006 43 it is recommended that the load be workload thermocouples. should be run at a temperature 100°F vacuum cooled from brazing temper- A furnace instrument accuracy higher than the highest temperatures ature to a temperature at least 50ºF check consists of a visual comparison used during the previous week prior below the solidus temperature of the between the temperature indicator/ to conducting the leak rate check. brazing slurry before initiating the controller and the furnace chart Measure the leak rate after a vacuum gas system. If the part re- recorder readouts. Both must be of 5 × 10-4 torr is achieved. A more ac- quires heat treatment from brazing within specified furnace system tem- curate leak check that takes into con- temperature, then the gas quench perature accuracy (usually ±5°F). sideration the vacuum chamber size must be initiated at the end of the A furnace system temperature ac- using the expression brazing soak period. curacy test is conducted by inserting In theory, parts can be unloaded the temperature-sensing element Q = (P2-P1) × V/t when they are below 400ºF (205ºC) within a maximum of three inches without discoloration. However, to (or less) of the furnace control ther- where Q is the leak rate, P2 is the ensure that there is no possibility of mocouple. Temperature accuracy vacuum after the survey end, P1 is discoloration on critical, heavier should be measured every 30 days the vacuum at the start of the survey, parts, it is recommended that all load by the installation of a probe thermo- V is the furnace volume (liters), and thermocouples be well below 200ºF couple in the hot zone within 3 in. (76 t is the survey time (no less that 3,600 (~95ºC) before opening the furnace. mm) from the furnace control ther- seconds). Today, vacuum furnaces mocouple. The variation should not can achieve a leak rate in the range Vacuum Equipment: Qualification, be greater than ±0.75% of probe tem- of 10-3 torr/s or better. Testing & Calibration perature. The probe temperature A vacuum furnace used for should be taken within 200ºF (95ºC) Workload Thermocouples brazing must comply with specific of the lowest and highest qualifica- Thermocouple types are selected industry standards for brazing tion temperature of the furnace. To to suit the process, especially temper- processes. Vacuum furnaces should avoid excessive heat losses (heat ature capability. Three thermocouple be checked before use for instrument ), it is recommended to insulate types commonly used by Seco/ and temperature accuracy, tempera- the controlling thermocouple be- Warwick are Type K (to 2100°F, or ture uniformity, and leak rate. Fur- tween the hot zone and vacuum 1150°C), Type S (to 2900°F, or 1600°C) naces for brazing below 2000°F vessel. and W3 (to 3600°F, or 2000°C). (1095°C) that are using workload Temperature uniformity should be Type K is inexpensive and widely thermocouples should be qualified performed without a load. A min- used, but is susceptible to drift at el- every six months, and every 90 days imum of nine thermocouples should evated temperatures; accuracy and for those not using workload ther- be used for furnaces with hot zones reliability become increasingly poor mocouples. Vacuum furnaces used more that 10 ft3 (0.3 m3), located sym- with sustained exposure to higher for brazing above 2000°F should be metrically within the hot zone. The temperatures, especially above qualified every three months when qualification should be performed at 2000°F (1150°C). They are also atmos- working thermocouples are used, the lowest and highest operating phere sensitive; beaded types should and every 30 days for those not using temperature of the vacuum furnace be used in oxidizing conditions. and at an intermediate temperature Mildly oxidizing or reducing atmos- such that the difference between pheres can to Cr loss and lower qualification temperatures is not readings of EMF and error. Reuse of greater than 600°F (315°C). For type K thermocouples should be lim- vacuum furnaces operating above a ited according to the expression required range of less than 200°F, only the lowest and the highest tem- U = A + 2B + 7C < 30 perature should be used. The tem- perature should be recorded at five where A = number of uses below minutes interval starting 100°F (38°C) 1200°F, B = number of uses below below the first set point and con- 2000°F, and C = number of uses over tinued recorded at least 30 minutes 2000°F. after the controlling thermocouple Type N thermocouple is an alter- indicates the hot zone has reached native to K that avoids these prob- thermal equilibrium. lems without the expense of Types R A leak rate check should be con- or S. Susceptibility to drift is much ducted weekly, measured in an lower, accuracy is better (±1.5°C up empty, clean, cold and out-gassed to 1250°C), and it has a continuous furnace, and should not exceed 10 temperature capability of 1250°C mm/hr for general applications, 5 (or (2200°F). Seco/Warwick experience Hot zone with frame to test temperature even less) mm/hr for critical appli- shows that Type N thermocouples uniformity. cations. A furnace burnout cycle last much longer without failure than 44 HEAT TREATING PROGRESS • MAY/JUNE 2006 Type K thermocouples. Yet, Type N should be avoided if harmful forma- 0.1 1 10 102 103 104 105 106 thermocouples require re-calibration tion of occurs). or replacement every three months. Parts per million -5 -4 -3 -2 Load thermocouples need to be Post Brazing Inspection 10 10 10 10 0.1 1 10 100 placed in such a way that they reflect Aquality control system should be Percent true load behavior. The hot junction adequate for both general and critical 10-7 10-6 10-5 10-4 10-3 10-2 0.1 1 should be placed in a hole in the com- applications. Knowledgeable inspec- Atmosphere ponent or fixture by preference, but tion of finished brazed assemblies in- -4 -3 -2 if not feasible, then in a block of ma- cludes adequate inspection tech- 10 10 10 0.1 1 10 100 750 terial placed in the load and typical niques as applicable to a particular Torr 76 x of the component section thickness. application including visual inspec- 0.1 1 10 102 103 104 105 106 In any case, the hot junction should tion, leak testing, and radiographic Microns not see direct radiation from heating examination. Visual inspection is the 10-5 10-4 10-3 10-2 0.1 1 1029.9 elements or hot zone insulation. most widely used nondestructive testing method. Fluorescent penetrant Inches mercury Vacuum Level during Brazing inspection (FPI) is used for machined -95 -58 -30 18 80 212 The ability to wet and flow freely surfaces only. Parts should be in- Dew point, oF over a surface is the most important spected for cracks or voids in brazed Conversion chart between vacuum level characteristic of any braze alloy. To joint or adjacent base material, surface and dew point used in brazing to compare secure good wetting, parts and porosity, completeness of brazed joint vacuum level and atmosphere quality. vacuum furnace cleanliness and band (excluding the fillet) -2 proper vacuum level are essential. around the circumference 10

Parts require protection when heated, or along braze application o Material line F which requires allowing adequate (visual only), and freedom -3 10 Mg Material points 5000

—sec •W 4500 time for thorough purging and evac- from excess braze filler 2 •Nb Li Ta• uation before heating starts. material on surface of as- •C 4000 •Mo 3500 To achieve the high vacuum nec- sembled parts (visual 10-4 Be •Rh essary and ensure it is maintained only). •Pt 3000 Center line index through the useful life, it is necessary Leak testing is most ad- Pb W II Al -5 Si Ca• 2500 to outgas/clean the vacuum hot zone vantageous where gas or 10 Sn Pt •Ni Fe B I Ti• •Fe Mo Ag Si••B periodically. Together with the neces- tightness of braze Cr Rh

Evaporation rate gm/cm Ni Cu C Ti Be• •Cr 2000 sity for low vacuum leak rate, joints is required. This Be Nb Cu• -6Ca •Sn 1900 vacuum tightness is very important technique is limited to 10 Ta •Al 1800 •Li to achieve quality brazing. low pressure applica- •Ag 1700 A vacuum level of 5 microns or less tions. To determine 1600 •Pb 10-7 Evaporation rate: 1500 should be properly controlled. A dew Radiography (or ultra- 1. Select the materials 1400 point of 67°F (-55°C) is required to sonic inspection) should from among the materials points, the temperature at •Mg braze stainless steel. Oxygen must be used for braze joints 1300 -8 right. Connect to intersect •Ba also be controlled to suit the process Class A to detect subsur- 10 an index point. 1200 2. Select the material on and materials processed. Vacuum face and/or internal de- the material line. Connect 1100 conditions in most cases reduce ox- fects. Special techniques with the index point to 10-9 intersect the evaporation. 1000 ides, so braze fillers can wet and flow are required to reliably in- over the surfaces of the cleaned spect joints of varying thickness. Chart showing metal evaporation under vacuum, which is critical to brazing quality. metal. Turbomolecular or cryopumps Multiple views and careful interpre- are used in special applications with tation is required. • Kay, D., Brazing Fundamentals - Dif- more demanding brazing specifica- Some NDT methods are used im- ferential Metal Expansion tions at a 10-7 torr vacuum level. properly. For example, FPI is limited • Brazing Handbook, Fourth Edition, American Welding Society, 1991 to inspect machined surfaces only, • Bottoms, S., Seco/Warwick Ltd., UK, Cooling but it is being used to detect defects 1998, private communication Cooling gases used to shorten the in braze joint fillets. Similarly, many • Introduction to Brazing, Sulzer brazing cycle and achieve proper braze joints should not be radi- Metco, 2004 metallurgical properties should be of ographically inspected because this For more information: high purity to prevent the formation technique requires sensitivities that Janusz Kowalewski is Vice President, of oxides. Inert cooling gases include are difficult to achieve (better Vacuum Systems, Seco/Warwick Corp., (99.995 percent purity with than 2%). PO Box 908, 180 Mercer St., Meadville, dew point lower than -60ºF, or -51ºC, PA16335; tel: 814-332-8491; fax: 814-724- inhibits evaporation and forms no Bibliography 1407; email: jkowalew@secowarwick. • Peaslee R.L., The Brazement – De- com; Internet: www.secowarwick.com. compound) and (99.998 per- sign and Application, ASM International Janusz Szczurek, Dallas Airmotive Inc., cent purity with dew point lower • Fundamentals of Brazing, Kay & As- Dallas, TX 75235; tel: 214-956-2946; e- than -60ºF should be acceptable, but sociates, May 2005 mail: [email protected]. HEAT TREATING PROGRESS • MAY/JUNE 2006 45