Braze Life after Cadmium

Braze Life After Cadmium

Good braze properties are attainable from a variety of non-cadmium brazing alloys

Since December 14, 1992, when a revision to OSHA Standard 29 CFR Parts 1910, 1915, and 1926 went into effect establishing new cadmium standards, many users of cadmium containing brazing alloys have switched to alternate alloys without cadmium. The change in the law basically reduced the permissible exposure limit (PEL) in a total weighted average (TWA) of 8 h from 100 µg/m3 (fume) to 5 µg/m3 (fume and dust). The action level was also reduced to 2.5 µg/m3 (fume and dust). The action level is the exposure limit which, when reached triggers specific medical and work surveillance procedures. For further explanation and details of the regulation, reference should be made to OSHA Standard 29, which has been listed in the Federal Register.

Cadmium Alloys of Years Past

It has been well documented over the years that the addition of cadmium to the silver-copper-zinc (Ag-Cu-Zn) ternary system acts as a temperature depressant, which reduces the solidus-liquidus melt range, and improves wetting (primarily on iron-based alloys). Each of these properties is a factor to consider when brazing, but possession of all three is seldom an absolute necessity in the majority of brazed assemblies.

The toxicity of cadmium was established many years ago and the trend since has been to switch to a non-cadmium containing alloy. The first cadmium containing alloy was patented in the early 1930's and is known today as BAg-1a of the AWS A5.8 specification. In the mid 1930's Engelhard Corporation recognized the need for an alternative alloy, and after extensive research, developed and patented a tin-containing silver brazing alloy known today as BAg-7. Since the original patent, a whole series of alloys containing tin have been developed. These, and other non-cadmium alloys, will be compared to the cadmium alloys to assist in the selection of an alternative alloy.

The first step in selecting a non-cadmium containing alloy is to identify the characteristics or limitations of your assembly and process. For example, if the braze is restricted to an extended heating cycle because of the large mass of components, then the preferred replacement selection will be one with a lower melting temperature. If the fitup or capillary spacing of your design requires an alloy capable of filling a large joint clearance, then the replacement alloy chosen should be of similar viscosity. If you are joining iron-based materials or carbides with a nickel-containing brazing alloy, the replacement alloy should also contain nickel. Each assembly and process should be critiqued individually to establish the specific characteristics of the brazement before considering non-cadmium containing alloys.

Non-Cadmium Containing Alloys

(Silver-Copper-Zinc Family) There are a considerable number of silver-copper-zinc (Ag-Cu-Zn) ternary alloys from which to choose a potential alternative. A suggested starting point in the selection process would be with an alloy of similar silver content. (specifications can be found here on our High Silver Alloys section) Depending upon the cadmium alloy now being used, a non-cadmium alloy can be selected with parameters best approaching that of the cadmium alloy. The selection process will probably be influenced by the melt temperature and/or melt range and how it relates to present practice. Today, with technical improvements in heat sources, reaching brazing temperature is not a significant problem whether it be torch, furnace, induction or any other methods of heating.

(Silver-Copper-Zinc-Tin Family) As previously indicated, an extensive research effort was made in the development of the Ag-Cu-Zn-Sn quaternary system. Through this effort, it was discovered that tin performed like cadmium as a temperature depressant, contributed to a reduction in the solidus-liquidus range, and improved wetting (primarily with the iron-based alloys). Although tin acts like cadmium, it is not a direct replacement. The degree of effectiveness is slightly less, but in conjunction with a marginal increase in silver content, properties approaching those of the Ag-Cu-Zn-Cd alloys can be achieved.

AWS A5.8 BAg-7 has become the first choice of many when switching to a non-cadmium alloy. This alloy has a solidus temperature approaching those of the cadmium family and liquidus temperature lower than most of the more common cadmium containing alloys. A review of the table below of our cadmium free alloys shows that as the silver content is lowered and the tin content altered, the melt temperature and range increase slightly.

SILVALOY®	Ag%	Cu%	Zn%	Sn%	Ni%	Mn%	Solidus	Liquidus	AWS A5.8
SILVALOY A56T	56	22	17	5	---	---	1145º F	1205º F	BAg 7
SILVALOY A49NM	49	16	23	---	4.5	7.5	1260º F	1290º F	BAg 22
SILVALOY A45	45	30	25	---	---	---	1225º F	1370º F	BAg 5
SILVALOY A40T	40	30	28	2	---	---	1200º F	1310º F	BAg 28
SILVALOY A40L	40	30.5	29.5	---	---	---	1245º F	1340º F	---
SILVALOY A38T	38	32	28	2	---	---	1200º F	1330º F	BAg 34
SILVALOY A30	30	38	32	---	---	---	1250º F	1410º F	BAg 20

Although tin provides properties similar to cadmium, it should be noted tin also has the effect of a slight reduction in ductility. The lowering in ductility is more apparent at elevated temperature. This is most noticeable when joint design places the brazing alloy under severe tension upon cooling. Experience and tests show within the Ag-Cu-Zn-Sn family the ductility decreases with a decrease in silver content. If the joint design cannot be modified to place the brazing alloy under compression, and ductility is a factor, then an alloy from the Ag-Cu-Zn family may be the proper choice. Before the selection of a replacement alloy is made, each individual brazement should be analyzed and rated relative to the importance of each of the braze properties. The apparent economical selection will normally be the brazing alloy with the lowest silver content which still allows for cost-effective fabrication and yields a quality brazed joint.

(Silver-Copper-Zinc-Nickel Family) For a specialized application, where the brazing of tungsten-carbide tips to steel shanks for arduous service is undertaken, there are alternate non-cadmium, nickel containing alloys available. AWS A5.8 BAg-24 with a low liquidus temperature is the preferred choice. When improved strength is desired at elevated temperature applications, BAg-4 should be considered. Present practice or procedures require very little change, if any, when selecting one of these alternate alloys.

(Silver-Copper-Phosphorous Family) A few isolated applications remain in which cadmium containing alloys are used in joining copper and copper-based alloys. Within the Ag-Cu-P family, there are approximately six readily available alloys that can be considered in the selection of an alternate material. This family of brazing alloys is self-fluxing on copper to copper material, and can be used on iron and nickel-based materials, or when the nickel content exceeds 10% in a copper alloy.

Summary

The selection process of an alternate non-cadmium alloy should be similar to one of the principles of troubleshooting. That is, go back to the basics. The basic factors, other than filler metal, affecting joint integrity are as follows: Design, Cleaning & Fluxing, Assembling & Fixturing, Heating, Cooling and Flux removal. Evaluation or rating of each individual brazement should be made relative to these basic factors. If the present brazed assembly quality is marginal because or not following each basic principle, then the number of potential replacement alloys will be limited. When all factors approach optimum, the choice will be from several brazing alloys.