TASK 70-41-03-310-004
1 . General.
A.Brazing is the term for a special group of welding procedures in which base metals are bonded by heating them to sufficient temperatures above 800°F (426.7°C) and using a nonferrous filler metal with a melting point below that of the base metals. The filler metal is pulled between the closely fitted surfaces of the joint by capillary attraction. High temperature furnace brazing is usually done at 1750°F (954.4°C) or more in a vacuum.
B.Because of the unique design and fabricating advantages of brazing procedures, they are used frequently to manufacture aircraft components. But the details of the procedure must be accurately controlled to make sure of consistently high joint strength. Requirements for specified conditions such as part cleanliness, joint clearance, and application of the brazing materials must be closely followed.
C.Of the many fabrication repairs that are done with brazing processes, the replacement of honeycomb is most frequent.
D.This Standard Practice does not give sufficient detail for a specific part application. When brazing is required and authorized, refer to the Engine/Shop Manual repair for specific details.
2 . Equipment.
Subtask 70-41-03-310-041
A.Thermal Requirements.
Furnaces used for brazing should meet with the minimum requirements for accuracy of temperature control, temperature uniformity within the hot zone, and temperature heat-up and cool-down rates when they are loaded with the maximum total mass of fixturing and parts for which they are qualified.
Make sure that the furnaces agree with the requirements for temperatures through the maximum temperature at which they will be used and/or the temperatures that will be set as requirements for the specific repairs to be made. These minimum requirements are as follows:
(1)Temperature Capability: Braze temperature + 100°F (56°C).
(2)Maximum Thermal Nonuniformity: Nominal ±25°F (±14°C) in all areas of the furnace hot zone, as set on controller.
(3)Minimum Heat-up Rate: From 1000°F (538°C) to braze temperature, 25°F (14°C) per minute.
(4)Minimum Cool-Down Rate: From braze temperature to 1000°F (538°C), 10°F (6°C) per minute.
B.Vacuum Furnace Requirements.
Furnaces which are qualified for vacuum brazing must agree with minimum vacuum performance requirements as follows:
(1)The furnace shall be capable of maintaining a maximum pressure, with the vacuum pumps in operation, of less than 10-3 torr (0.13 Pa) throughout its qualified temperature range.
(2)The ambient temperature leak up rate of the furnace shall be determined with the furnace empty, at a pressure less than 5 x 10-4 Torr (0.5 micron of mercury) (6.5 Pa) and isolated from the effects of the pumping system. The leak up rate shall not be greater than 30 x 10-3 Torr (30 microns of mercury) (4.00 Pa) per hour. Bakeout before the leak up rate is permitted. After the initial qualification, the leak up rate shall be performed at least every seven days. These requirements shall not be required for oil quenching vacuum furnaces used for the heat treatment of steels.
(a)Alternative Leak Up Rate. The following alternate leak up rate may be performed: the leak up rate may be taken with a loaded furnace after the load has been baked out to remove the effects of moisture in the load. The leak up rate may be taken at any temperature from room temperature to 350°F (177°C).
(3)Vacuum furnace brazing may require a pre-braze bake-out run before brazing starts to remove absorbed gas from porous materials. This removal prevents out-gassing from causing contamination of the parts before brazing.
(4)Inert gas may be put into the furnace for cooling if it agrees with the dew point requirements of paragraph C. The inert gas may be argon (99.995 percent purity) or nitrogen (99.98 percent purity), but nitrogen may not be put into the furnace above 2000°F (1093°C).
C.Atmosphere Furnace Requirements.
(1)Special brazing atmospheres are designed to minimize the formation of chromium and other stable oxides on the component surfaces during the brazing operation, and to remove superficial oxides which may form below brazing temperature. High purity and sufficient dryness (dew point) must be controlled. In some cases, vacuum brazing is used to better control the presence of surface oxide during the brazing operation.
(2)Equipment qualified for hydrogen or inert gas atmosphere brazing shall have a dew point of -60°F (-51°C) or colder measured at an accessible point where gas enters the single furnace farthest from the source of the gas. This measurement shall be made at room temperature, at 1200°F (649°C), and at maximum brazing temperature. These measurements shall be made after sufficient gas flow time at the test temperature has permitted the dew point to become stable. (This is to eliminate the effects of out-gassing and/or oxide reduction of the furnace load.)
D.Instrument Requirements.
(1)Temperature measuring equipment shall have an accuracy of ±0.5 percent of their span. Thermocouples measuring the part temperature are usually shielded from direct furnace radiation and are put as near to the braze joint as safely possible.
(2)Pressure measuring equipment shall be accurate to ±0.45x10-3 torr (0.06 Pa) at a pressure of 1x10-3 torr (0.13 Pa) or less. This will require an ion gage type of instrument.
(3)Dew point measuring equipment shall be accurate to ±10°F (±6°C).
(4)Instruments that agree with these requirements should be in position during the equipment qualification testing and during brazing operations.
3 . Procedure.
Subtask 70-41-03-310-042
WARNING:
DO NOT BRAZE IF YOU ARE NOT APPROVED (TRAINED AND QUALIFIED). USE PERSONAL PROTECTION EQUIPMENT AND APPROVED SAFETY PROCEDURES.
NOTE:
Authorized repairs that use furnace brazing are given in the Engine/Shop Manual. Always refer to the Engine/Shop Manual repair for specific information on specific parts. If the Engine/Shop Manual does not agree with this procedure, follow the Engine/Shop Manual.
A.Cleanliness and Surface Preparation.
(1)All part surfaces shall be free of oxides, oils, grease, dirt, paint and similar substances which may cause contamination of the furnace atmosphere and/or the part joint during the brazing operation. It is important that no contamination be introduced to the base metal surfaces to be brazed or into the fluxes or atmospheres which are designed to protect them from contamination. This does not apply to the use of binders or stop-off, when required, but stop-off must be used carefully to make sure that none accidentally goes on the surfaces to be brazed. It will prevent the flow of the filler material through the joint.
(2)The primary function of fluxes and protective, reducing furnace atmosphere is to keep the formation of oxides at a minimum during the brazing operation. Thus, parts must be correctly cleaned before they are brazed.
(3)The precleaning of parts before brazing is usually done by mechanical or chemical methods, since many surface oxide layers are highly resistant. After all chemical treatments, thorough rinsing is required to remove all remaining cleaning agents from the component surfaces. Cleaning agents that remain from mechanical cleaning shall also be removed. Refer to the Engine/Shop Manual repair for specific cleaning instructions. Wear clean white gloves when you touch parts after they are clean.
(4)The brazing operation should occur as soon as possible after cleaning, and parts should be stored in plastic bags during extended periods between cleaning and brazing. Base metal, atmospheric conditions, movement, storage, and other similar conditions have an effect on the time after which cleaning becomes necessary again.
(5)Plating of joint surfaces before brazing is controlled by the applicable Engine/Shop Manual procedure for the specific part being brazed.
B.Joint Clearance.
The gap (clear distance between the interfaces of the joint being brazed) is very important and must be controlled. This joint clearance has an important effect on the mechanical performance of the brazed joint. The clearance dimension is related to the capillary attraction that pulls the filler material into position after oxide films have been removed. Think of joint clearance in relation to conditions at a given moment, such as room temperature or brazing temperature, and the base metals being brazed. Refer to the applicable Engine/Shop Manual procedure for the gap required at the specific joint being brazed.
C.Application of Brazing Materials.
(1)The filler material must be prepared properly to make sure that it does not move before it melts. It must be put in a location from which it will flow through the joint area correctly. The location and quantity of the filler material is controlled to limit the size or location of the fillet. The quantity of filler material used is very important when such parts as honeycomb seals are brazed. The size of the fillet can have an effect on mechanical strength of the seal. It must be limited to prevent overfilling of the cells, which would have a detrimental effect on the function of the seal. If the filler material is put in the correct location, it will flow through and fill the joint, making fillets at all exposed joint edges. For honeycomb replacement, it is permitted to apply a filler material mixture of 85-90 percent of the specified braze alloy and 10-15 percent nickel powder for application along the outer row of cells. This provides many possible benefits. For example, to prevent pinholes, to slow flow-ability, to have better fill of larger joint gaps, and to eliminate reaction and poor bond on some braze alloys. Follow the requirements for filler quantity and location that are given in the applicable Engine/Shop Manual procedure for the specific part being brazed.
(2)Stop-off materials, to control filler materials outside the joint area, and binders, to keep powder-type filler materials in the correct location, may be used as specified in the Engine/Shop Manual repair.
(3)Use metallic shims, wire, or powders in non-honeycomb joints to improve joint fit, only if they are specified in the Engine/Shop Manual.
D.Braze Fixturing.
Brazing fixtures can have an effect on the part mechanically, and also on the brazing cycle. Refer to the applicable Engine/Shop Manual procedure for specific recommendations on specific parts.
E.Braze Run Procedure.
WARNING:
USE THERMAL GLOVES WHEN YOU MOVE OR TOUCH HOT OR COLD PARTS. HOT OR COLD PARTS CAN CAUSE INJURY.
CAUTION:
FAILURE TO FOLLOW THE SPECIFIC HEATING AND COOLING STEPS IN THE ENGINE/SHOP MANUAL COULD RESULT IN PART DISTORTION, STRAIN AGE CRACKING AND/OR MATERIAL PROPERTY CHANGES.
(1)Thermocouples shall be operational before the furnace is sealed.
(2)Set the overtemperature control at the brazing temperature +50°F (+28°C). Increase the temperature to the equilibration temperature as specified in the Engine/Shop Manual. Make sure that thermocouples equalize at +20°F (+11°C) in less than 10 minutes.
(3)Continue the heating cycle to the brazing temperature in steps and times as specified in the Engine/Shop Manual for the specific part being brazed.
(4)Cool to the furnace unload temperature, in the steps, at the rate, and in the atmosphere specified in the Engine/Shop Manual.
F.Rebraze Cycles.
Rebraze cycles shall be done only as specified in the applicable Engine/Shop Manual repair.
4 . Quality Assurance.
Subtask 70-41-03-310-043
A.Inspect the finished joint as follows:
(1)Visually inspect for continuity of braze alloy fillet around all exposed edges of the joint. Use 10-power magnification and sufficient lighting if necessary.
(2)Check the brazed joint for voids, porosity, discontinuities, erosion, and mechanical integrity. Use the amounts and methods that are specified in the applicable Engine/Shop Manual procedure.
(3)Make sure that all abrasive materials have been removed from the part.