Magnesium Brazing

Magnesium is considered to be the ninth most abundant element in the universe by mass, constituting about 2% of the Earth's crust . It is also the third most abundant element dissolved in seawater. Magnesium and its alloys are used in variety of processes and industrial applications. One most common use of magnesium and its alloys is in brazing. Magnesium too makes use of similar brazing techniques applied for aluminum. It employs furnace, torch & dip brazing procedures for its proper functioning.

Properties
Magnesium & its alloys are widely used as they are light in weight in case of structural metals apart from possessing fine properties which are :

• High specific strength
• Low density
• Economical processability with cast technology
• High recycling potential

Materials
Magnesium alloys demand certain materials which can facilitate the brazing procedure. These are :

• Base Metal
  The nominal composition & physical properties of base metal in case of brazing magnesium alloys are namely :
  • Aluminum
  • Zinc
  • Manganese
  • Zirconium
  
  
• Filler Metal
  The filler metal in this case of magnesium brazing is a magnesium base alloy. Torch, dip & furnace brazing processes are perfect for brazing filler metal BMg-1.

Joint Design
The joints are designed in a way which can take full advantage of the capillary action. The flux is displaced by the brazing filler metal as it continues to flow into the joint. Lot of care is taken to lessen the flux entrapment. Lap & butt joints are both suitable for magnesium assemblies. About 0.004 to 0.010 in. of joint clearances are satisfactory at the brazing temperature.

Brazing Processes
Brazing processes meant for application in magnesium alloys are defined in detail which we are going to look at apart from highlighting their processes.

• Furnace Brazing
  Application of electric or gas heating equipment is made when it is furnace brazing AZ31B alloy. This equipment has automatic controls which are required for minimizing incipient melting of the baser metal apart from aiding in decreasing a potential magnesium fire. Filler metal flow are reduced & avoided with the help of atmosphere produced by the products of combustion in gas-fired furnaces & sulfur dioxide.
  
  
• Torch Brazing
  Neutral oxyfuel or air-fuel gas flame is required for carrying out torch brazing. A lot of care & attention is needed as it is difficult to use Bmg1 filler metal for any manual torch brazing. Since, the solidus temperature of the base metal & the brazing filler metal's flow point are quiet in close proximity, extreme skill is necessary for the filler metal's face feeding. Prior heating, the filler metal is placed on the joint & is fluxed. Heat application continues till the flux melts & the brazing filler metal melts & wets the surface thereby flowing into the joint by means of capillary action.
  
  
• Dip Brazing
  This particular brazing process involves the immersing of the assemblies in a molten brazing flux which is fixed at a suitable brazing temperature. The dual function of heating & fluxing are performed by the flux. A minimum of 0.004 to 0.010 in. joint clearance should be there in this brazing. Dip brazing yields greater results because of large volume of flux & uniform heating.

Flux
In case of magnesium alloy brazing, AWS brazing flux is used where due to the corrosive characteristic of the flux, complete removal is vital which aids in maintaining the good corrosion resistance in the brazed joints.The galvanic corrosion of brazed joints is minimized as magnesium base alloy is the brazing filler metal in this case.

The corrosion resistance, however, of brazed joints primarily depends on two vital points which are :

• Thoroughness of flux removal
• Prevention of flux entrapment by the adequate joint design

Brazing Temperature Preferences
Lot of considerations are kept in mind when deciding upon the brazing temperature in case of magnesium alloys. The filler metal's melting temperature should be lower than the base metal's.

In this case, the lowest permissible temperature for brazing is required necessary for points like :

• Heat effect is minimized on base metal
• The base metal-filler metal interaction is decreased
• The life of fixtures, jigs or other tools are enhanced
• Requirement of economic heat energy

Higher brazing temperature is suitable in case of :

• Brazing is combined with annealing, stress relief or the base metals' heat treatment
• For elevated temperatures, subsequent processing is a must
• Increases the remelt temperature of the joint
• Removes surface contaminants & oxides with vacuum or atmosphere brazing
• Stress cracking is avoided

Applications
Magnesium alloy has immense applications in the field of science & technology. Some of the usages are :

• Torch brazing magnesium hydraulic life float
• Magnesium battery container
• Magnesium microwave antenna