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Manufacturing Processes > Joining processes > Brazing

 

Brazing

 

Process description

Heat is applied to the parts to be joined which melts a manually fed or pre-placed filler braze metal (which has a melting temperature ≥450°C) into the joint by capillary action. A flux is usually applied to facilitate ‘wetting’ of the joint, prevent oxidation, remove oxides and reduce fuming.

Materials

Almost any metal and combination of metals can be brazed. Aluminum difficult due to oxide layer.

Branzing

Process variations

  • Gas brazing: neutral or carburizing oxy-fuel flame is used to heat the parts. Can be manual Torch Brazing (TB) for small production runs or automated with a fixed burner (ATB).
  • Induction Brazing (IB): components are placed in a magnetic field surrounding an inductor carrying a high-frequency current giving uniform heating.
  • Resistance Brazing (RB): high electric resistance at joint surfaces causes heating for brazing. Not recommended for brazing dissimilar metals.
  • Dip Brazing (DB): parts immersed to a certain depth in a bath of molten chemical or brazing alloy covered with molten flux. Commonly used for brazing aluminum.
  • Furnace Brazing (FB): heating takes place in carburizing/inert atmosphere or a vacuum. The filler metal is preplaced at the joint and no additional flux is needed. Large batches of parts of varying sizes and joint types can be brazed simultaneously. Good for parts that may distort using localized heating methods and dissimilar metals.
  • Infrared Brazing (IRB): uses quartz-iodine incandescent lamps as heat energy. For joining pipes typically.
  • Diffusion Brazing (DFB): braze filler actually diffuses into the base metal creating a new alloy at the joint interface. Gives a strong bond of equal strength to that of the base metal.
  • Braze welding: base metal is pre-heated with an oxyacetylene or oxypropane gas torch at the joint area. Brazing filler metal, usually supplied in rod form, and a flux is applied to joint area where the filler becomes molten and fills the joint gap through capillary action.
  • Filler metal can be in preforms, wire, foil, coatings, slugs and pastes in a variety of metal alloys, commonly the alloys are based on: copper, silver, nickel and aluminum.
  • Flux types: borax, borates, fluoroborates, alkali-fluorides and alkali-chlorides (for brazing aluminum and its alloys) in powder, pastes or liquid form.

Economic considerations

  • High production rates possible using FB and IB, but low with TB.
  • Cycle times vary. Long for FB and DFB, short for TB.
  • Very flexible process.
  • Large fabrications may be better suited to welding than brazing.
  • Economical for very low production volumes. Can be used for one-offs.
  • Tooling costs low. Little tooling required.
  • Equipment costs vary depending on process and degree of automation. Low for TB, high for FB.
  • Direct labor costs low to moderate. Cost of joint preparation can be high.
  • Finishing costs moderate. Cleaning of the parts to remove corrosive flux residues is critical.

Typical applications

  • Machine parts
  • Pipework
  • Bicycle frames
  • Repair work
  • Cutting tool inserts

Design aspects

  • All levels of complexity.
  • Joints should be designed to operate in shear or compression, not tension.
  • Typical joint designs using brazing: lap and scarf in thin joints with large contact areas or a combination of lap and fillet. Fillets can help to distribute stresses at the joint. Butt joints are possible but can cause stress concentrators in bending.
  • Lap joints should have a length to thickness ratio of between three and four times that of the thinnest part for optimum strength.
  • Joints should be designed to give a clearance between the mating parts of typically, 0.02–0.2mm depending on the process to be used and the material to be joined (can be zero for some process/ material combinations). The clearance directly affects joint strength. If the clearance is too great the joint will loose a considerable amount of strength.
  • Tolerances on mating parts should maintain the joint clearances recommended.
  • Parts in the assembly should be arranged to promote capillary action by gravity.
  • Machine marks should be in line with the flow of solder.
  • Joint strength between that of the base and filler metals in a well-designed joint.
  • Vertical brazing should integrate chamfers on parts to create reservoirs.
  • Jigs and fixtures should be used only on parts where self-locating mechanisms (staking, press fits, knurls and spot welds) not practical. If jigs and fixtures are used they should support the joint as far from the joint area as possible, have minimum contact and have low thermal mass.
  • Provision for the escape of gases and vapors in the joint design important.
  • Metals with a melting temperature less than 650°C cannot be brazed.
  • Minimum sheet thickness =0.1 mm.
  • Maximum thickness =50 mm.
  • Unequal thicknesses possible, but sudden changes in section can create stress concentrators.
  • Dissimilar metals can cause thermal stresses on cooling.

Quality issues

  • Good quality joints with very low distortion produced.
  • Virtually a stress free joint created with proper control of cooling.
  • Choice of filler metal important in order to avoid joint embrittlement. Possibility of galvanic corrosion.
  • A limited amount of inter-alloying takes place between the filler metal and the part metal, however, excessive alloying can reduce joint strength. Control of the time and temperature of the applied heat important with respect to this.
  • Subsequent heating of assembly after brazing could melt the filler metal again.
  • Filler metal selection based upon the metals to be brazed, process to be used and its economics, and the operating temperature of the finished assembly.
  • Surface preparation important to remove any contaminates from the joint area such as oxide layers, paint and thick films of grease and oil and promote wetting. Pickling and degreasing commonly performed before brazing of parts.
  • Smooth surfaces preferred to rough ones. Sand blasted surfaces not recommended as they tend to reduce joint strength. Abrading the joint area using emery cloth acceptable.
  • Correct clearance, temperature gradients and use of effective use of gravity promote flow of braze filler through capillary action.
  • Flux residues after the joint has been made must be removed to avoid corrosion.
  • Surface finish of brazed joints good.
  • Fabrication tolerances a function of the accuracy of the component parts and the assembly/jigging method.

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