Braze eutectic alloys

While the use of pure metals and eutectic alloys as brazes is generally good practice, exceptions do occur. Thus it may be necessary to use brazes that deviate from eutectic compositions to minimise the concentration of an embrittling temperature depressant, or to complicate the melting characteristics and accept degraded fluidity by introducing elements that enhance the corrosion resistance of the solidified braze. Thus recommended Al-Si brazes have hypo-eutectic compositions and Cr is introduced into Ni-P and Ni-Si-B braze alloys. 

The atomic radius of silver (144 pm) is within about 15% of many elements, permitting sofid solutions with Al, Au, Be, Bi, Cu, Cd, Ge, In, Mn, Pb, Pd, Pt, Sb, Sn, Th, and Zn. These metals form useful braziag, jewelry, and soldering alloys. Copper is the only metal with which silver forms a simple eutectic between two sofid solutions (Pig. 3). Silver has extremely limited solubiUtyia B, C, Co, Cr, Pe, Ge, Ir, Ni, Mg, Mo, Se, Si, Te, Ti, and W. Thus these metals may be brazed by silver alloys without serious erosion during welding (qv). 

Pure ruthenium powder or mixed ruthenium-molybdenum powders have been found able to effect good joints between molybdenum and tungsten. A eutectic melting above 1 900°C is formed, and joints produced in hydrogen atmospheres at 2 100°C operate satisfactorily at 1 500°C. A cobalt-palladium-gold alloy has also been reported to be useful in brazing molybdenum. 

The classical electronics solder alloy (Sn + 37 wt% Pb) contains the inhalation-toxic element lead and is being replaced in electronics for health reasons. A Pb-free solder is Sn-3.5wt% Ag indium and bismuth alloys are other eutectic-forming alloying additives to Sn. The classical brazing alloy is at the Cu-Ag eutectic (Ag-F 28.1 w% Cu) and is well suited for mechanically strong contacts with Cu and its alloys. 

In the Al-Si system no binary compounds of the general formula Al Si are known. The system has an eutectic at 12.6% Si and 577°C the maximum solid solubility of Si in Al is 1.65%. Commercial casting and brazing alloys vary from 5 to 20% Si in combination with other elements. They can be obtained by mixing the liquid elements. ... 

Silver alloys provide the ability to wet and join base metals at low temperatures while providing good strength. Because low temperatures can be used for silver soldering and brazing, the risk of surface oxidation and heat distortion is reduced. Silver alloys usually comprise eutectic mixtures of tin, zinc, or copper, with lead and cadmium being used in some applications. The Ag-Cu alloys have been used with ceramics to produce electrical parts, and they have been used to braze carbide inserts to drills. The carbide inserts can easily be adjusted or replaced by melting the alloy. 

Testing arid joining development should concentrate on BNi-5 and the eutectics Pd-Co and Pd-Nl. BNi-5 is promising because Haynes 230 already contains ail of these elements, whereas Pd-Co and Pd-NI are systems with complete solid solubility and should braze the materials well. Long-term testing should be conducted to identify any phase evolution between the braze materials and base superalloys. Pd does form an intermetallic phase with Mo, and at least 8 intermetalllc phases with Al, both of which are alloying elements of Haynes 230 (see Reference... 

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