Silver Solder > Why Braze? > Strength of Silver Solder Joints

Strength of Silver Solder Joints

Common questions asked relate to the strength of brazed joints made with silver solder. The answer is normally ''stronger than the parent materials''. The overall joint thickness is greater than either of the parent materials and hence is subject to less stress. Failure normally occurs outside the joint. This presupposes that the joint is sound ie no voids, has been been designed, assembled, fluxed and heated to achieve the fundamental principle of brazing - capillary flow of the silver solder.

All silver solder used in the manufacture of copper boilers, be they from us or another supplier, will withstand 35.000 psi 290x boiler pressure.

The European Pressure Equipment Directive 97/23/EU demands certain features of brazed joints regarding joint integrity and strength. Our 842 alloy, used in conjunction with copper grade C103 and C106, and tested independently to destruction by a DTI approved body, produced joints that fully met their requirements.

Before attempting to evaluate joint strength, consideration must be given to seven factors. The slightest variation in them can produce great differences in joint strengths being achieved in assemblies which, to the casual observer, are identical.

There are seven factors that can contribute to producing joints of lower strength.

1. Composition and strength of parent materials
Most silver solder is manufactured to the international standard BS EN 1044. Because of the demands in meeting the specification regarding cadmium free alloys most manufacturers will only use virgin metal to make the silver solder. However the user must ensure that there are no elements within the parent materials that can be alloyed with a constituent in the silver solder that will form a brittle inter-metallic eg alumimium, titanium. Such compounds can cause catastophic failure of the joint.

There are grades of copper, that can be embrittled by the effects of a gas torch during brazing. Unfortunately this occurs in one of the most readily available forms of copper. These grades of copper are often referred to as ''tough pitch'' or ''electrolytic copper'' and ''high conductivity copper'' and is designated C101 in British Standards. Most suppliers of silver solder, copper and independent authorities eg Copper Development Association all warn of this danger. The problem is caused by the presence of oxygen in the structure in the form of cuprous oxide. During heating, the flame can reduce the oxide to produce a porous metallic structure and steam to induce cracking. This phenomenon is referred to as ''hydrogen embrittlement''. Catastrophic failure can occur without significant deformation or obvious deterioration of the component

Care should be exercised that brazed assemblies use only phosphorus de-oxidized copper eg C106.
References;
Copper Development Association Technical Note TN4
Outokumpu Data Sheet OF-OK

2. Shape of the parts to be brazed.
Sudden changes in thickness of the parent materials should be avoided. If two components are of widely different thickness, wherever practicable there should be a gradual change in the thickness of the thicker component.

3. Geometry and surface condition of the joint
The rate of capillary flow into a joint is proportional to the joint gap. If the joint gap is not consistent, the molten alloy will first fill that part of the joint with the largest gap. It is possible that what appears visually appears to be a sound joint is not. The result can be a flux filled void within the joint. Care should be exercised to ensure that the joint gap suitable for the alloy/material combination is adhered to.

Joint geometry is an important consideration of joint design.
The edges of joints should not be subjected to combined tensile and bending stresses.
The assembly should be shaped and designed such that in service, or during cooling, the silver solder should be placed under compressive stress or pure shear. Joints placed under pure tension eg butt joints, should be avoided.
Avoid joint designs that could result in high stress concentration at joint edges.

4. Joint soundness
The absence of voids and inclusions are of paramount importance when assessing joint strength. A completely sound joint is one in which 100% of the joint area has been wetted and completely filled by the brazing alloy. A brazed joint can be considered as a small casting. Only a small amount of material, in the form of a fillet, is available to feed the casting. Narrow melting range alloys are not renowned for their fillet forming characteristics. However, the fillet often freezes first and shrinkage cavities can form within the joint. The larger the volume of alloy in the joint the greater the risks. The risks are reduced by using small joint gaps.

A more prevalent reason for joint unsoundness is flux entrapment. If there is insufficent flux in the gap, the flux can become saturated with oxides, becomes more viscous and cannot be displaced by the brazing alloy. Avoid this problem by cleaning the work initially, removing all scale and heavily oxidized material. Keep the heating cycle to a minimum and using sufficient flux suitable for the task.

Too tight a joint fit, or the absence of a gap (which might occur when a tube does not align itself properly through a plate) will not have a consistent gap. There will be no flux, the silver solder will not penetrate. The result is, at worst, a dry joint - at best a weak joint that could fail due to thermal stresses if reheated. Assemblies with many joints can fail pressure testing many times as the weakest joint is repaired to expose another. If an assembly has to be clamped it should not be done so as to eliminate the joint gap. If there is not a gap, there can be no capillary flow and no joint. Modify the materials to ensure that a joint gap remains. Clamping components together can eliminate any gap. Centre punch plate or use a coarse file to maintain a joint gap.

Cavities or blow-holes can be created within the silver solder. Cavities can arise by the absorption of unburnt fuel gas from a torch as a result of using an unduly reducing flame. Blow holes can result from overheating an alloy particularly one containing cadmium. Cadmium boils at 767 deg C, temperatures readily achieved locally, particularly if an oxy-acetylene torch is used.

While it is very difficult to attain a 100% sound joint every time, it is not unreasonable to expect 90%. If this figure drops below 75%, something needs putting right.

5. Joint gap and joint area
Brazed joints are made by capillary flow of a filler metal. This implies flow between parallel surfaces ie a degree of overlap between component parts. Such joints then tend to operate with shear stresses.

It has been shown that the strength of a silver solder joint is affected by the thickness of the filler metal (joint gap). This is because a thin layer of filler metal which is bonded on both sides to a parent material deforms very differently to its normal state. It cannot deform in the normal ductile manner and does not fail until sufficient higher stress is applied to induce brittle fracture. Results have shown that the shear strength of a joint, made using a 40% silver quaternary brazing alloy, achieved maximum shear stress when the joint gap was 0.1mm. A variation in the joint gap produced lower figures. It is thought that the lower strength seen with smaller gaps is attributed to lower levels of joint soundness caused by flux deterioration, and or, poor metal flow.
The important figure is the gap at brazing temperature hence the need to heat all the joint. Consideration should be taken of the effects of thermal expansion and how it can effect the joint gap.

Another feature of joint strength in a brazed joint is the joint length or degree of overlap. Results show that increasing joint length does not produce greater joint strengths, more the opposite. The longer the joint gap increases the possibility of producing voids. Shear stresses (and hence strain) are highest at the edges of the joint. If the length of the joint increases beyond a figure dependent on material selection, there is filler metal within the joint that carries no stress. Its presence therefore serves no purpose and contributes nothing to joint strength.

Generally speaking the highest joint strength is attained with a joint length 3x the thickness of the thinnest component and the smallest gap possible commensurate with achieving good joint soundness.

6. Heating technique
Joint strength is dependent on attaining a high level of joint soundness. The manner in which a joint is heated can greatly influence the way a silver solder melts and flows. Capillary flow increases with increasing surface tension, decreasing viscosity and reduced contact angle of the molten filler metal with the parent material. Since all these features of a brazing alloy work beneficially with increased temperature, it is clear that the alloy will be encouraged to flow towards the hottest part of a joint. This creates a ''heat pattern''.

Heating should be carried out to ensure that all the joint is above the melting point of the silver solder. Failure to do this will result in a ''cold spot'' where the alloy freezes and stops entering the joint creating a void or does not penetrate the joint fully. In effect this reduces the joint length and hence strength.

Heating should be carried out at a rate quick enough to cause the alloy to melt without separation (liquation)

7. Alloy application
Alloy should be applied in a manner suitable to meet the requirements of the joint design. Best brazing practise does not involve the heating of the filler metal. The alloy is melted by absorbing heat from the workpiece. The dimensions of the filler material should reflect the heating capacity of the work.
It should be applied in such a manner that with the formation of the heat pattern, it flows towards the heat and through the joint.

Having heated an assembly to achieve a brazed joint care should be taken on cooling. A complex assembly, with varied weight distribution through it, should be allowed to cool naturally. Rapid cooling can induce distortion and cause materials to crack.

Failure to take account of these measures can only result in poor, weak joints.

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