Brazing of aluminium

 

Brazing vs. soldering

Aluminum and aluminium alloys can be joined by brazing and soldering [1-4]:

• Brazing:
  Employs filler metal having a liquidus:
  – above 450 °С and
  – below solidus of the base aluminium alloy.
• Soldering:
  Solders melt below 450 ° C.

Brazing vs. welding

• Brazing differs from welding in that no much amount of the base metal is melted during brazing.
• The temperatures for brazing of aluminium are intermediate between those for welding and soldering.
• Brazed aluminium joints generally between welded and soldered joints in strength and resistance to corrosion.

Brazing basics

Fluxes and fillers

Brazing of aluminium alloys was made possible by development of:

• fluxes that disrupt the oxide on aluminium without harming the underlying metal and
• aluminium-base fillers that have suitable melting ranges.

Close temperature control

• The aluminium-base filler metals used for brazing aluminium have liquidus temperatures very close to the solidus temperature of the base metal.
• Thus, close temperture control is required in brazing aluminium.
• The brazing temperatueshould be approximately 40 °С below the solidus temperature of the base metal.
• Most brazing is done at temperature between 560 and 615 ° C.

Brazeability of aluminium aloys

Some common wrought and casting aluminium alloys are listed in Table 1 and Table 2. Note melting temperature ranges and brazeability ratings of various aluminium alloys.

Table 1 – Melting ranges and brazeability of common base metal aluminium alloys [1]

Table 2 – Brazeability of aluminium alloys [2]

Non-heat-treatable wrought alloys

The non-heat-treatable wrought aluminium alloys that have been brazed most good are [1-3]:

• 1xxx series alloys
• 3xxx series alloys
• low-magnesium alloys of the 5xxx series.

The alloys containing a higher magnesium cintent, such as:

• 5083, 5086, 5154 and 5456 are more difficult to braze.

Heat-treatable wrought alloys

• The heat-treatable wrought aluminium alloys of 6xxx series are commonly good brazed. Most frequently brazed alloys are 6061, 6063, 6101 and 6951.
• The 2xxx and 7xxx series of aluminium alloys have low melting temperature. Therefore, they are not normaly brazeable. The exception are 7072, 7004 and 7005 alloys.

Casting alloys

Brazeable aluminium casting alloys include:

• 356.0
• 357.0
• 359.0
• 443.0
• 710.0
• 711.0
• 712.0.

Alloys 710.0, 711.0 and 712.0 are the casting aluminium alloys most frequently brazed.

Brazing filler metals

Aluminium-base filler metals

• Common flller metals for brazing aluminium are aluminium-silicon alloys with 7 to 12% Si.
• Lower melting points are attained by adding copper and zinc. This results in some loss of corrosion resistance.

The composition and the solidus, liquidus and brazing temperatures of the most frequently used brazing filler metals for aluminium are given in Table 3.

Table 3 – Compositions and solidus, liquidus and brazing temperature ranges of brazing filler metals for use on aluminium alloys

Selection of a brazing filler metal [1]

• When torch or induction brazing, select a filler with a liquidus temperature as far below that of base metal ass possible.
• When it may be necessary to push the molten filler into a far corner, select a filler with a wide spread between its solidus and liquidus.
• When the importance of producing a high-integrity brazed joint, select a filler with a short melting range.
• When furnace and dip brazing, select a filler with short melting range.

Sources:

1. Aluminum and Aluminum Alloys, ASM International, 1996
2. Brazing of Aluminum Alloys / Y. Sugiyama // Encyclopedia of Aluminum and Its Alloys – Eds. G.E. Totten, M. Tiryakioğlu, and O. Kessler – 2019 – 171-182
3. Brazing/Joining / Aluminium Automotive Manual – European Aluminium Association – 2015
4. Introduction to Brazing of Aluminium Alloys / R. Mundt, Blast furnaces, Koblenz