AgW70 and AgW50 Silver Tungsten Strip for Arc-Resistant Contacts

What Silver Tungsten Strip Is and Where It Fits

Silver tungsten strip is a two-phase composite in which a sintered tungsten skeleton is infiltrated with silver and then cold rolled to a controlled thickness. Tungsten and silver are mutually insoluble in the solid state, so the finished product is not an alloy in the metallurgical sense: it is a tungsten skeleton whose interconnected porosity has been filled with a continuous silver network. That architecture explains the commercial value of the material. The silver phase carries current at low resistance, while the tungsten phase absorbs the thermal and mechanical punishment delivered at the arc root. Strip is the delivery form used when contacts are blanked, coined, formed or brazed, and it is the reason a fabricator can hold a thin section with flat, parallel faces instead of machining every part out of solid bar.

In switching hardware the strip usually becomes one half of a contact pair. A single pair performs two jobs at once: it must carry rated current without breaching the temperature-rise limit of the switchgear, and it must interrupt fault current without excessive arc erosion or contact welding. Silver tungsten is selected mainly for the second job and tolerated for the first. Where the design calls for a current-carrying joint that must stay cool, a silver-rich grade such as AgW50 is the sensible choice. Where the arc root is severe and the contact must survive repeated interruptions, a tungsten-rich grade such as AgW70 or AgW75 is the sensible choice. The strip grades covered here, AgW70 and AgW50, sit on opposite sides of that trade-off and are frequently specified as a matched pair inside the same breaker.

Reading the Grade Designation Correctly

The designation system used for these materials is simple but often misread. In AgW70 the number is the nominal tungsten content by weight, not the silver content. AgW70 therefore contains about 70% tungsten and 30% silver, while AgW50 contains about 50% tungsten and 50% silver. Confusing the two numbers is one of the most common specification errors in contact procurement, and it is expensive: a buyer who assumes AgW70 is a high-silver grade will receive a harder, less conductive material than the design intended, and the contact temperature rise of the finished device will be higher than calculated. Silver-tungsten electrical contact materials of this family are specified under ASTM B631, which tabulates compositions and typical property ranges for the AgW50 to AgW80 series. When a drawing cites ASTM B631 together with a grade, the grade governs composition and the standard governs test method and tolerances.

Because the tungsten skeleton is produced by powder metallurgy, composition is controlled by weighing tungsten powder and silver rather than by melt analysis. Residual porosity, silver distribution and the continuity of the silver network are the variables that separate a good strip from a mediocre one at identical chemistry. Two suppliers can both report 70% tungsten and still deliver measurably different resistivity and arc behaviour, which is why the acceptance criteria in a purchase specification should include density, hardness and electrical resistivity, not chemistry alone.

Composition and Property Comparison of the AgW Series

The table below gives typical values for press-sinter-infiltrate silver tungsten materials in the AgW50 to AgW80 range, consistent with the property ranges published for ASTM B631 contact materials. Values are typical for sound, low-porosity product and are intended for selection and comparison, not as guaranteed minimums for a specific lot.

Grade	Silver wt%	Tungsten wt%	Density g/cm3	Resistivity micro-ohm-cm at 20 C	Conductivity % IACS	Hardness HBW	Bending strength MPa
AgW50	50	50	13.15	3.0	57	105	not typically specified
AgW55	45	55	13.55	3.2	54	115	not typically specified
AgW60	40	60	14.00	3.4	51	125	not typically specified
AgW65	35	65	14.50	3.6	48	135	not typically specified
AgW70	30	70	14.90	3.8	45	150	about 650
AgW75	25	75	15.40	4.2	41	165	about 690
AgW80	20	80	16.10	4.6	37	180	about 730

Two numbers in that table deserve attention from a design engineer. First, resistivity rises almost linearly with tungsten content, from roughly 3.0 to 4.6 micro-ohm-centimetre across the series; that is a 50% penalty in conductivity between AgW50 and AgW80. Second, hardness rises in the opposite direction, from about 105 to 180 HBW. The practical consequence is that a single breaker cannot be optimised for both low temperature rise and long arc life with one grade, and the usual engineering answer is to split the duty: a silver-rich contact on the current-carrying side and a tungsten-rich contact on the arcing side, or a bimetal arrangement in which each half does the job it is good at.

Why Strip Form Changes the Design Rules

Rolling does more than reduce thickness. Cold rolling closes residual porosity, orients the tungsten skeleton in the rolling plane and work-hardens the silver network. A well-rolled strip typically reaches 97% of theoretical density or better, and the improvement in density translates directly into lower resistivity and less silver spitting under arcing. The same rolling operation, however, introduces anisotropy: transverse bend ductility is lower than longitudinal bend ductility, and heavy reductions without intermediate annealing can open edge laminations. A purchase specification for strip should therefore state the rolling direction on the drawing, define which direction the bend test applies to, and require edge inspection for delamination.

Strip also determines how the contact is attached. Thin strip in the 0.5 to 2.0 mm range is normally blanked or fine-blanked into discs and rectangles, then brazed or resistance-welded to a copper or brass carrier. Medium strip in the 2.0 to 5.0 mm range is used for coined profiles, formed arc runners and heavier current-carrying pads. Coil form matters for high-volume stamping because progression dies need consistent camber and thickness across the full coil length; a coil with a thickness step will produce intermittent welding defects that are difficult to trace back to the raw material.

Manufacturing Route: From Powder to Finished Coil

The standard route for AgW50 and AgW70 strip is press, pre-sinter, infiltrate, roll. Tungsten powder of controlled particle size and oxygen content is blended, pressed into a skeleton of the required green strength, and pre-sintered in a hydrogen atmosphere at a temperature high enough to develop necks between tungsten particles without closing all porosity. The porous skeleton is then contacted with silver and heated above the melting point of silver so that capillary action draws the liquid metal into the pore network. After infiltration the billet is cooled, cleaned and rolled in several passes with intermediate anneals, then slit to width and degreased.

Three process variables dominate quality. Sintering atmosphere quality, usually expressed as dew point, controls residual oxygen in the skeleton; oxygen left behind reacts during infiltration and produces gas porosity that shows up later as blisters during brazing. Infiltration completeness determines whether isolated pores remain; incomplete infiltration produces localised high-resistivity regions that overheat in service. Rolling schedule determines density and internal stress; excessive reduction per pass without annealing produces edge cracking and, in severe cases, silver exudation at the surface. A supplier that can document dew point control, density measurement per lot and a rolling schedule with defined anneals is a supplier whose strip will behave predictably in a stamping die.

Specifying Strip Dimensions and Tolerances

Silver tungsten strip is a powder-metallurgy product, so dimensional tolerances are wider than those of wrought copper alloys of the same thickness. Commercial practice is to hold thickness to plus or minus 0.02 mm on thin strip and plus or minus 0.05 mm on heavy strip, with tighter bands available by arrangement where a coining operation depends on exact volume constancy. Width is normally held to plus or minus 0.2 mm on slit coil, camber to a few millimetres over a two-metre length, and burr height to a fraction of the thickness on the sheared edge. Because contact resistance depends on the true contact area, thickness variation along a coil is more damaging than a small uniform offset, and the specification should therefore limit thickness spread within a coil rather than only the nominal tolerance.

Characteristic	Typical commercial control	Practical acceptance method
Thickness	plus or minus 0.02 mm thin strip, plus or minus 0.05 mm heavy strip	Micrometer at defined intervals along the coil, minimum five positions per coil
Width and camber	plus or minus 0.2 mm width, low camber on slit coil	Tape measure and straight-edge check on a flat plate
Density	97% of theoretical or better for sound strip	Archimedes method per ISO 3369 on coupons from each lot
Hardness	Within the range for the ordered grade	Brinell hardness per ISO 6506-1, tungsten carbide ball recommended
Electrical resistivity	Consistent with the grade and density	Four-terminal measurement on a straight coupon at 20 C
Surface and edges	Free of laps, blisters, silver exudation and edge laminations	Visual inspection at magnification plus bend test

Joining: Brazing, Welding and Bimetal Assembly

Silver tungsten strip is almost never used as a free-standing part; it is joined to a conductive carrier. Furnace or induction brazing with a silver-copper eutectic filler of the BAg-8 type, melting near 780 C, is the conventional route for breaker contacts because the brazing temperature stays well below the point at which the silver network begins to coarsen severely. Joint design matters more than filler choice: a thin, uniform braze line of 0.05 to 0.15 mm gives lower thermal resistance than a thick, voided one, and voids above roughly 10% of the joint area are usually grounds for rejection because they create hot spots that accelerate contact erosion.

Resistance welding and projection welding are used for high-volume production of small contacts, particularly where the carrier is a copper strip and the assembly is later stamped. Here the risk is different: excessive electrode force and current can squeeze molten silver out of the composite, leaving a tungsten-rich, brittle zone at the interface. A weld schedule should be qualified by shear test and by sectioning the first parts of each production run. Where the duty includes very high currents, a three-layer trimetal of silver tungsten facing, copper core and silver tungsten or silver backing is used; the copper core carries current and the two composite faces handle make-and-break duty on both sides.

Soldering is generally avoided on arcing faces because soft solder creeps under thermal cycling and because flux residues contaminate the contact surface; where soldering is unavoidable for a current-carrying joint, a high-lead or silver-bearing solder with a compatible flux system and a defined cleaning step is used. In all joining processes the silver tungsten part should be degreased and, where the process allows, given a light reducing treatment immediately before heating, since silver tarnish and handling oils both degrade braze wetting.

Application Map for AgW50 and AgW70 Strip

AgW50 strip, with the highest silver content of the press-sinter-infiltrate family, has the lowest contact resistance and the highest conductivity. It resists contact welding well and is used primarily as a current-carrying contact in power circuit breakers that must pass high transient currents, and as the non-arcing half of a contact pair. In oil circuit breakers and in air-break disconnectors, the silver-rich grade keeps the continuous current path cool while the tungsten-rich grade on the opposite side takes the arc.

AgW70 strip is the workhorse arcing contact grade. The higher tungsten fraction raises the effective melting point of the surface and spreads the arc root over a larger area, which reduces depth of erosion per interruption and lowers the tendency of the contact faces to weld under short-circuit closing. Typical locations are arc-quenching contacts in large air and oil breakers, line disconnectors, reclosers, high-current switches and contactors, and short-circuit contacts in protective devices where resistance to welding is the design driver. Because AgW70 has roughly half the conductivity of AgW50, it should not be used as the sole current path in a device with a tight temperature-rise limit; the usual arrangement is AgW70 on the arcing face and a copper or AgW50 conductor behind it.

DC applications need a separate comment. Direct current does not have a natural current zero, so arc duration is longer and metal transfer is directional: material tends to move from anode to cathode. Silver tungsten resists this transfer better than fine silver, and matched pairs are preferred over dissimilar pairs in DC contactors to avoid asymmetric erosion that changes contact gap over life.

Incoming Inspection and Acceptance Testing

An incoming inspection plan for silver tungsten strip should be short, objective and reproducible. Verify identity first: check the grade marking against the certificate, and confirm that the reported silver content matches the designation, not its inverse. Measure density on a coupon by the Archimedes method; a density more than 2% below the grade typical indicates porosity that will show up as overheating or blistering. Measure hardness with a tungsten carbide ball, because a steel ball can deform under the load used on these hard composites and bias the reading. Check resistivity on a straight, deburred coupon with a four-terminal method at controlled temperature, since resistivity is the single most sensitive indicator of skeleton continuity and infiltration quality.

Then look at the material. Cross-section a sample, polish it and examine the silver network: a sound structure shows a continuous bright silver phase filling the spaces between tungsten grains, without clusters of isolated pores or streaks of silver-rich banding parallel to the rolling direction. Perform a 90-degree bend in both the longitudinal and transverse directions and inspect the outside of the bend for opening. Finally, run a braze trial on a few pieces using the production filler and cycle; blisters or silver beads appearing at the surface during the braze cycle are evidence of trapped gas and justify lot rejection. Recording these five results per lot builds the history needed to detect drift before it reaches the assembly line.

Common Procurement Mistakes

The most frequent mistake is buying on silver content alone. Silver is the visible cost driver, so a lower-silver grade looks like a saving, but the cost of a contact is dominated by the switchgear failure it prevents. A second common mistake is confusing silver tungsten with copper tungsten. Copper tungsten, with copper as the infiltrant, is used for resistance welding electrodes, electrical discharge machining electrodes and thermal management parts; it has high conductivity but poor arc erosion resistance at the arc root and is not interchangeable with AgW grades in a breaker. A third mistake is specifying strip where bar or a sintered near-net blank is technically and commercially better, which is common when a part has a thick, stepped profile that cannot be rolled from coil. A fourth is accepting a certificate of composition without dimensional and density data, which leaves the two variables most likely to cause production problems unverified.

Storage also deserves a line in the purchase specification. Silver tungsten strip tarnishes in sulphur-bearing atmospheres, and tarnish raises contact resistance and interferes with brazing. Coils should be kept in sealed packaging with a desiccant, handled with gloves, and consumed on a first-in-first-out basis. Strip that has been stored for a long period in a humid workshop should be re-inspected for surface condition before it enters a braze furnace, because a tarnish layer that is invisible on a machined edge can still spoil wetting across a wide contact face.

FAQ

Q: In AgW70, does the 70 refer to silver or tungsten content?
The number is the nominal tungsten content by weight, so AgW70 contains about 70% tungsten and 30% silver. AgW50 is the silver-rich end of the same series at roughly 50% silver.

Q: Why is silver tungsten used instead of fine silver for arcing contacts?
Fine silver has excellent conductivity but erodes quickly and welds easily under fault current. The tungsten skeleton raises the effective surface melting point, spreads the arc root, and suppresses contact welding, at the cost of higher resistivity and hardness.

Q: What density should AgW70 strip reach after rolling?
Sound rolled strip typically reaches about 97% of theoretical density or better. A density more than 2% below the typical value for the grade signals residual porosity that will raise resistivity and may cause blistering during brazing.

Q: Which brazing filler is normally used to attach silver tungsten contacts?
A silver-copper eutectic filler of the BAg-8 type, melting near 780 C, is conventional because it keeps the joining temperature well below the level at which the silver network coarsens severely.

Q: Can AgW50 strip be substituted for AgW70 in the same breaker?
Only after re-checking the arc erosion and welding duty. AgW50 has higher conductivity but lower hardness and less erosion resistance, so it suits the current-carrying side of a pair rather than a severe arcing face.

Q: How should silver tungsten strip be stored before assembly?
Keep it sealed with a desiccant, handle with gloves, and use first-in-first-out. Sulphur-bearing air tarnishes the silver network, which raises contact resistance and impairs braze wetting.