Gold Brazing Alloys

Brazing is a very efficient metal bonding process. It can join similar as well as dissimilar metals with equal efficiency. Brazing is used in multitude of industries to braze a number of products. A number of brazing alloys are used to perform the brazing through a number of brazing processes. Gold brazing alloys are an important category of brazing alloys. It is mainly used in the electronics and jewelery industry.

Commercially Available Gold Brazing Alloys

Composition wt. percent					Melting Range, °C
Au		Cu	Ni	Ag	Solidus		Liquidus
100		--	--	--	1063		1063
94		6	--	--	965		970
81.5		15.5	3	--			-910
80		20	--	--	910		910
80		19	--	--	905		910
75		25	--	--	910		914

Gold-Copper Alloys
Gold brazing alloys are constituted mainly of gold with a sound percentage of copper in some of them. They are called gold-copper alloys and are used in a number of industries. Electronic industry is the largest consumer of gold-copper brazing alloys. Here they are largely used in the fabrication of electron tubes, valves, wave guides, radar equipment, vacuum devices, etc. Apart from that it is widely used in the jewelery industry.

Properties
Some useful properties of gold-copper brazing alloys include the following:

• The gold-copper brazing alloys have the ability to wet nickel, copper, iron, cobalt, molybdenum, tantalum, niobium, tungsten, and their alloys.
  
  
• They can produce ductile joints without excessive inter-alloying. The latter factor is important in cases when excessive erosion of the work piece by molten brazing alloy could affect the dimensional accuracy of brazed parts or unduly weaken thin-walled structures.
  
  
• These alloys remain free from volatiles and constituents that form refractory oxides. Freedom from volatile constituents come handy when brazing is used to fabricate parts that operate in vacuum at higher temperatures. Freedom from constituents that form refractory oxides, facilitate easy brazing without a flux in reducing and neutral atmospheres or in vacuum.
  
  
• Gold-copper brazing alloys show exceptional resistance to corrosion, particularly in the case of gold-rich alloys.
  
  
• They have the ability to be produced in wrought forms, like strip, foil, and wire. It is a very important consideration in case of furnace brazing applications where the brazing alloy must be placed in or near the joint area before brazing.
  
  

Mechanical and Physical Properties of Gold-Copper Brazing Alloys

Property					Alloy Composition
					80% Au - 20% Cu		62.5% Au - 37.5% Cu
Melting Range, °C					910		930-940
U.T.S., tons/sq. inch					30		27
Elongation/centimeter					17		38
Hardness(D.P.N.)					250		112
Electrical resistivity, µO-cm					14.2		13.9
Density					17.25		15.2

Gold Brazing Alloys in Jewelery Industry
Gold brazing alloys are used in the production of a full range of jewelery products. The gold brazing alloys ensure permanent bonding of two metallic parts, by heating at a temperature higher than the liquidus temperature of the brazing alloys, but lower than the Solidus temperature of the parent materials to be joined. High liquidus temperature of the brazing material create problems because high temperature treatment deteriorate the mechanical properties of the parent alloy by annealing and can fade the surface of the jewelery.

Historically jewelery brazing alloys have been made by combining gold-silver-copper alloys with metals such as zinc (Zn) and cadmium (Cd). Cadmium, a low-melting metal, is added to lower the melting range and to add desirable wetting and flowing properties. Cadmium is toxic to human beings.

Hazards of Cadmium in Brazing Alloys
During the melting process, cadmium combines with the atmospheric oxygen and forms dangerous cadmium oxide (CdO). Studies have shown that exposure to CdO fumes is very irritant to the respiratory system and it must be controlled. International Agency for Research on Cancer has classified Cd as a carcinogen. The temperatures attained in brazing exceed the Cd boiling point (797°C), and under these conditions the vapor pressure of Cd is very high. There is a need by the industry for making cadmium-free brazing alloys. Cadmium can be substituted by other low melting point metals, such as tin(Sn), gallium(Ga), indium(In),zinc(Zn). Gallium and Indium have very low melting points (156°C and 30°C respectively) and high boiling points (2080°C and 2403°C).

Gold-Nickel Alloys
Gold-Nickel alloys are another important group of gold brazing alloys. It is metallurgically analogous to the gold-copper system. The number of gold-nickel alloys that are really useful as brazing media is relatively small compared to the gold-copper alloys. Gold-nickel alloys having more than 35 per cent nickel aren't used at present . They have comparatively deficient joint filling properties because of their high viscosity and wide melting ranges. These alloys have all the useful properties of gold-copper alloys.

They heavily rely on nickel for their high-temperature strength and their resistance to oxidation at higher temperatures. Gold-nickel alloys can withstand high-temperature. This property can be modified by the addition of alloying elements. For example, chromium can be added to these alloys to increase their oxidation resistance but at the expense of their free- flowing characteristics. Alloying can also be used to decrease the noble metal content and so the intrinsic cost, without losing any of their useful properties.