FILLER METAL SELECTION
FILLER METAL SELECTION
The fundamental condition for achieving a correct brazed bond of two metals is the selection of a correct filler metal. The general requirements for filler metals may be summarised by the following points:
- the filler metal melting temperature should be below that of the brazed metals,
- the filler metal should spread easily across the surface of the brazed elements,
- the chemical relationship between the filler metal and the bonded metals should ensure the creation, at the boundaries, of solid solutions and intermetallic phases,
- the filler metal in its liquid state should pour easily,
- the scope of crystallization (that is the difference between the temperature at the start and end of coagulation) should not be too large,
- the filler metal should be sufficiently durable and plastic,
- thermal expansion factors of the filler metal and bonded metals should be similar,
- the filler metal and bonded metals should exhibit similar corrosion resistance properties,
- in a melted state, the filler metal should not oxidise (the occurrence of easily soluble oxides is acceptable),
- the filler metal should not contain expensive elements and those in short supply
- Depending on the purpose for the brazed elements, the filler metals should exhibit good electrical conductivity, good machinability, resistance to corrosive effects of various substances etc.
Filler metals with tin – sensitive to cooling, particularly for brazing elements with different thermal expansion coefficients,
Filler metals with silicon – silicon makes the bond more aesthetic. Not recommended for bonds which will be subject to impact, vibrations or increased pressures, as silicon may create brittle compounds.
Filler metals for salt rich environments – are such filler metals, which exhibit a high silver contents (> 40 %) as well as, but not always, some tin. With such filler metals, used for sea structures, tin losses in the galvanisation processes are reduced.
Multi-phase brazing – brazing alloys with narrow melting temperature ranges should be used. Filler metals of subsequently lower required brazing temperature are selected in order to avoid the phenomenon of re-melting of already completed bonds.
Brazing nickel and its alloys – nickel and its alloys are sensitive to cracking during brazing. A low tin content is recommended in such filler metals and an increased required brazing process temperature.
Filler metals with cadmium – Characterised by good liquidity and good mechanical properties and are more economical than those without cadmium. Used for bonding many materials – from copper to steel and even bonding acid-resistant steel and cemented carbides.
Filler metals for bonding carbides – The presence of nickel and manganese increases the wetability of bonded material. Filler metals with a lower melting temperature are recommended, particularly for smaller elements. Layered tape filler metals are highly recommended, with an interior copper layer able to absorb vibrations and impacts. Carbides are characteristic in that they have a lower thermal expansion coefficient than metal elements, thus uniform and slow heating up should be used in order to avoid cracks. Avoid rapid cooling after brazing.
Filler metals for stainless steels – in selecting the filler metal the conditions in which the bond will work have to be taken into account, with particular emphasis on humidity. Steels without nickel, low nickel content as well as series 400-410-420-430 steels are particularly sensitive as opposed to 300 series austenitic steel. Cadmium free filler metals with nickel yield better strength, particularly for brazing steel. On the other hand, filler metals with cadmium are often used in bonds exposed to surface and interior corrosion.