Silicon aluminium bronze is aluminium bronze in which silicon, not iron or nickel, does the strengthening. A deliberate 0.7–3.2% silicon addition means less aluminium is needed for a given strength, and that single fact shapes the whole family. Every grade here sits below the 9.4% aluminium threshold at which the brittle, preferentially-corroding gamma-2 phase forms, so all of them are inherently dealuminification-resistant without needing nickel — and none is quench hardenable. The headline property is machinability: C64200 and C95600 rate 60 against 20–30 for almost every other aluminium bronze. C64200 is ASTM F468 Cu 642, a certified fastener alloy sitting deliberately between Cu 630 and Cu 614 on the strength-versus-ductility trade. C63600 goes the other way entirely — 64–67% elongation annealed, made for cold heading. The one caution is weldability, which is poorer than the nickel aluminium bronzes.
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Silicon does the job that iron does in a plain aluminium bronze and nickel does in a nickel aluminium bronze — but differently. In a plain aluminium bronze iron refines the grain and forms hard kappa particles; in a nickel aluminium bronze nickel additionally suppresses the gamma-2 eutectoid. Here a deliberate 0.7–3.2% silicon addition strengthens the alloy through solid solution and silicon-bearing phases, and the critical result is that less aluminium is needed for a given strength. These alloys reach 517–703 MPa at only 3–8% aluminium, where a plain aluminium bronze would need 10–11%.
That keeps the whole family out of the gamma-2 danger zone, and it simplifies specification considerably. The brittle, preferentially-corroding gamma-2 phase that causes dealuminification only appears above roughly 9.4% aluminium. Every grade here sits well below it — C63600 at 3.0–4.0%, C64200 and C64210 at 6.3–7.6%, C95600 at 6.0–8.0%. So there is no eutectoid to suppress, no quench-and-temper cycle to specify and certify, and no iron-to-nickel ratio to police, unlike the nickel aluminium bronzes. The corollary is that none of these alloys is hardenable by heat treatment: strength comes from composition and cold work alone, and the temper designation records it.
Machinability is the headline property and the main practical reason to choose this family. C64200 and the cast C95600 both rate 60 on the CDA scale, and C63600 rates 40. Set that against 20 for C95800, 30 for C63000 and 20 for C61400: two to three times more free-cutting at comparable corrosion performance, because the silicon-bearing phases break the chip where a conventional aluminium bronze smears. If a corrosion-resistant bronze part has to be machined in volume, look here first.
The cost is weldability, and it should influence grade choice. CDA rate C63600 and C64200 only Fair for gas shielded arc welding and for all three resistance processes, and Not Recommended for oxyacetylene welding; soldering is Not Recommended on both and brazing is Not Recommended on C63600. Compare C63200, which CDA rate Excellent for gas shielded arc welding. The cast C95600 is the exception and is rated Good for welding, soldering and brazing. The practical conclusion: specify silicon aluminium bronze for machined, forged, cold-headed and cast parts, and choose a nickel aluminium bronze or C61300 for a critical welded fabrication.
C64200 is ASTM F468 and F467 Cu 642, and it is positioned deliberately between the alternatives. Bolting is specified at 517–758 MPa with 241 MPa minimum yield at 75–95 HRB and 10% minimum elongation (the standard also sets a copper minimum of 88.65%). Read that against Cu 630, stronger at 690–896 MPa but allowing only 5% elongation, and Cu 614, more ductile at 30% but no stronger. Cu 642 gives the same tensile range as Cu 614 with twice the elongation of Cu 630, and the best machinability of any certified aluminium bronze fastener alloy. Where a bolted joint needs real strength but must still yield and redistribute load, it is frequently the right answer. C63600 is not an F468 grade, but CDA list cold headed nuts among its uses and rate it Excellent for cold working — it is the cold-heading alloy of this family.
This is the easy-machining branch of the aluminium bronzes, and it is the main practical reason to choose it. C64200 and the cast C95600 both rate 60 on the CDA scale and C63600 rates 40, against 20–30 across the plain and nickel aluminium bronzes — the silicon-bearing phases break the chip where a conventional aluminium bronze smears. It is still a bronze and not a leaded brass: C36000 is the 100 on that scale, and no leaded version exists because lead and aluminium together make the melt hot short. Keep the family practice — rigid setups, keen positive-rake carbide, a cut taken under the work-hardened layer rather than skimming it, moderate surface speed with generous feed, flood coolant throughout. On the forming side the family splits: C63600 is rated Excellent for cold working and cold heads readily at 64–67% elongation, while C64200 is Poor cold and Excellent hot at a forgeability rating of 80 — forge or extrude, then machine.
All composition, machinability and cold-working data from the Copper Development Association alloy records. Aluminium and silicon figures are read directly from the individual grade pages. Machinability is on the CDA scale where C36000 free-cutting brass = 100.
| Grade | Designation | Form | Aluminium % | Silicon % | Structure | Machinability | Cold Working | Chosen For |
|---|---|---|---|---|---|---|---|---|
| C63600 | Cold-Heading Silicon Al Bronze | Wrought | 3.0–4.0 | 0.7–1.3 | Single-phase alpha | 40 | Excellent | 64–67% elongation — cold headed nuts, connectors |
| C64200 | Silicon Aluminium Bronze | Wrought | 6.3–7.6 | 1.5–2.2 | Single-phase alpha with silicon-bearing phases | 60 | Poor | ASTM F468 Cu 642; forge then machine |
| C64210 | Silicon Al Bronze, Tightened Limits | Wrought | 6.3–7.0 | 1.5–2.0 | Single-phase alpha | — | — | C64200 with narrower Al and Si bands — density only |
| C95600 | Cast Silicon Aluminium Bronze | Cast | 6.0–8.0 | 1.8–3.2 | Cast alpha with silicon-bearing phases | 60 | — | CAST; best weldability here; machinability 60 |
Read the silicon column alongside the aluminium one — together they explain every row. Silicon does the strengthening, so the aluminium stays low, so there is no gamma-2, so no grade here is heat treatable and none needs a nickel addition to resist dealuminification. C63600 is the outlier at only 3.0–4.0% aluminium: Excellent cold working and 64–67% elongation, made for cold headed nuts and cable connectors, and the only grade here with the highest density and thermal conductivity because it is the least alloyed — though CDA list no product specification for it and publish no yield strength data at all, so that column is blank rather than filled with a borrowed figure. C64200 is the main engineering grade and the ASTM F468 Cu 642 fastener alloy. C64210 is the same alloy with aluminium capped at 7.0% instead of 7.6% and silicon at 2.0% instead of 2.2%, registered for consistency — CDA publish a density figure and nothing else, and we have deliberately not substituted C64200 data, because the tightened ranges are the whole reason the designation exists. C95600 is the cast member, sand castings and ingot only, with the highest silicon here, the best weldability on the hub, and a looser 99.0% copper-plus-named-elements minimum against 99.5% for the wrought grades.
We are committed to delivering zero-defect silicon aluminium bronze fasteners, components and castings. All products undergo rigorous testing including PMI (Positive Material Identification), chemical analysis with aluminium and silicon certified explicitly — together they set the strength, the machinability and the position relative to the gamma-2 threshold — along with the arsenic content, which carries a registered maximum on all three wrought grades; mechanical and hardness verification against the specified temper and section, stress-relief treatment on cold-headed and formed parts, dimensional inspection and full traceability with EN 10204 3.1 certification.
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