Silicon bronze is copper with roughly 1–4% silicon, and it is the traditional bronze fastener material — the default for marine, architectural and boat-building fastening for a century. All three grades here are listed in ASTM F468 and F467 as Cu 651, Cu 655 and Cu 661; no other alloy family on this site can say that. What sets silicon bronze apart is that it declines most of the trades other copper alloys force on you: CDA rate it Excellent for cold working and Excellent for hot forming, and Excellent for gas shielded arc welding, spot welding, butt welding and brazing — there is no refractory oxide film to fight, unlike every aluminium bronze. It has no selective-leaching failure mode at all: no dezincification, no dealuminification. The price is conductivity, down to 7% IACS at C65500 — these are structural and fastening alloys, not electrical ones.
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All three grades on this hub are listed in ASTM F468 and F467, and no other alloy family on this site can say that. It is not an accident: silicon bronze is the traditional bronze fastener material. C65100 is Cu 651, the only alloy in the standard whose requirement splits by diameter — 483–690 MPa with 379 MPa minimum yield to 19 mm, falling to 379–621 MPa and 276 MPa from 22 mm, both at 8% minimum elongation. C65500 is Cu 655 at 345–552 MPa with 138 MPa minimum yield and 20% minimum elongation. C66100 is Cu 661 at 483–690 MPa with 241 MPa minimum yield and 15% minimum elongation. Read those against each other and the choice falls out: Cu 655 is the ductile option at two and a half times Cu 651's elongation, which is why it dominates marine and architectural bolting; Cu 651 and Cu 661 are the strong options at the same tensile range, with Cu 661 offering nearly twice Cu 651's ductility but carrying lead.
The fabrication ratings are the real story, and they are the best on this site. Most copper alloys force a trade: a leaded brass machines superbly and welds badly; an aluminium bronze resists seawater superbly but fights the alumina film at every joint; a phosphor bronze makes a fine spring but hot works Poorly. Silicon bronze declines most of them. CDA rate both documented grades Excellent for cold working and Excellent for hot forming — a combination almost nothing else here manages — plus Excellent for gas shielded arc welding, spot welding, butt welding and brazing, Good for oxyacetylene, and C65100 Excellent for soldering too. There is no refractory oxide film to remove before joining. CDA's listed fabrication processes for these alloys — blanking, drawing, forming and bending, heading and upsetting, hot forging, roll threading and knurling, squeezing and swaging — read as a description of fastener manufacture, which is exactly what they are used for.
Silicon strengthens by solid solution, and the price is conductivity. C65100 at 0.8–2.0% silicon carries 12% IACS; C65500 at 2.8–3.8% carries only 7% IACS and 36 W/m·K, among the lowest of any copper alloy documented on this site. Silicon in solution scatters conduction electrons hard. These are not electrical alloys — for conductivity with spring temper see C50500 at 48% IACS — they are structural and fastening alloys that happen to be copper-based. Machinability sits at 30 for both documented grades: modest against C36000's 100, but half again better than an unleaded phosphor bronze. Where machining dominates, C66100 carries a deliberate 0.20–0.8% lead addition for the purpose.
In service, silicon bronze has no selective-leaching failure mode at all, which is unusual. Zinc is incidental here and capped at 1.5%, so dezincification — the characteristic failure of the brasses — does not apply. There is no gamma-2 phase, so dealuminification — the corresponding weakness of the aluminium bronzes — does not apply either. The alloys resist seawater and brackish water, fresh water, neutral salt solutions, non-oxidising acids, alkalis, organic acids and marine atmospheres, and weather to a stable brown-bronze patina rather than green, which is why they are specified for exposed architectural metalwork. They are non-sparking and essentially non-magnetic. The limits are the normal copper-alloy ones: oxidising acids, notably nitric, attack them; ammonia and amines cause stress-corrosion cracking in material carrying residual tension, so stress relieve cold-headed and heavily formed parts; and they are cathodic to steel, aluminium and zinc in a galvanic couple.
Machinability sits at 30 for both documented grades — modest against C36000's 100, but half again better than the unleaded phosphor bronzes at 20 and comparable with the wrought nickel aluminium bronzes. The copper-silicon matrix is tough and work hardens, so use sharp positive-rake tooling, generous chip breakers, firm feed and flood coolant, and never let the tool dwell. Where machining dominates, C66100 carries a deliberate 0.20–0.8% lead addition for exactly that purpose — though CDA register no machinability rating for it, so we quote none. Forming is where this family excels. CDA rate C65100 and C65500 Excellent for cold working and Excellent for hot forming, and list blanking, drawing, forming and bending, heading and upsetting, hot forging and pressing, roll threading and knurling, shearing, squeezing and swaging among their common fabrication processes. That list is essentially a description of fastener manufacture, which is why silicon bronze cold heads and roll threads so well.
Composition, conductivity, machinability and working data from the Copper Development Association alloy records; silicon and lead figures are read directly from the individual grade pages. The F468 column gives the ASTM F468 machined-specimen tensile range in MPa and minimum elongation. Machinability is on the CDA scale where C36000 free-cutting brass = 100.
| Grade | Designation | Silicon % | Lead % | F468 Alloy | F468 MPa / Elong. | Conductivity | Machinability | Cold / Hot Working | Chosen For |
|---|---|---|---|---|---|---|---|---|---|
| C65100 | Low-Silicon Bronze B | 0.8–2.0 | 0.05 max | Cu 651 | 483–690 / 8% | 12% IACS | 30 | Excellent / Excellent | Seven Excellent fabrication ratings; 12% IACS |
| C65500 | High-Silicon Bronze A | 2.8–3.8 | 0.05 max | Cu 655 | 345–552 / 20% | 7% IACS | 30 | Excellent / Excellent | The standard silicon bronze; most ductile bolting |
| C66100 | Leaded Silicon Bronze | 2.8–3.5 | 0.20–0.8 | Cu 661 | 483–690 / 15% | — | — | — | Free-machining; CDA data is composition only |
The F468 columns are the ones to read first, because all three grades are in the standard. C65500 — High-Silicon Bronze A, Cu 655 — is the one most people mean by “silicon bronze”: sixteen listed specifications, the broadest product range, 1000 MPa in spring wire, and a fastener requirement of 20% minimum elongation, two and a half times Cu 651's. That ductility is why it dominates marine and architectural bolting. C65100 trades it away: Cu 651 reaches 690 MPa in small diameters but at only 8% minimum elongation, and is the one alloy in F468 whose requirement changes with diameter. In exchange it carries nearly twice the conductivity and seven Excellent fabrication ratings — more than any other alloy documented on this site. C66100 is the free-machining grade, Cu 661, at Cu 651's strength with nearly twice its ductility — but CDA register composition, a density figure and the specification list for it and nothing else. Its mechanical table is taken from ASTM F468 rather than from typical-property data, no machinability rating is quoted because none is published, and we have not substituted C65500 figures despite the overlapping silicon range, because the lead changes the machining, welding and hot-working behaviour that is the whole reason the designation exists.
We are committed to delivering zero-defect silicon bronze fasteners, fabrications and machined components. All products undergo rigorous testing including PMI (Positive Material Identification), chemical analysis with silicon, manganese and lead certified explicitly — silicon sets the strength and conductivity, manganese is a deliberate addition in C65500, and lead distinguishes C66100 and governs acceptability under the Safe Drinking Water Act and RoHS. Where fasteners are supplied to ASTM F468 or F467 the copper minimum set by that standard is verified in addition to the CDA limits. Mechanical and hardness verification against the specified temper and section, grain-size verification on annealed tempers, stress-relief treatment on cold-headed parts, dimensional inspection and full traceability with EN 10204 3.1 certification.
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