CNC processing of tungsten copper alloy

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  Why tungsten copper alloy does not machine like either metal
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  Which tungsten copper grades machine well and which do not
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  Tooling and cutting parameters that hold up
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  How tungsten copper parts actually fail during machining
• 5
  Design limits and when another process wins
• 6
  Tungsten copper grade selection by part and process

Tungsten copper alloy is a pseudo-alloy, not a true alloy. Tungsten and copper do not dissolve into each other in solid form. Powder metallurgy presses tungsten particles into a skeleton, then infiltrates or melts copper into the remaining pore space. The result is a two-phase composite: hard tungsten grains around 3,400 MPa in hardness terms, held in a copper matrix that is soft, ductile, and thermally conductive.

Every machining problem on this material comes from that split. The cutting edge meets alternating hard and soft phases thousands of times per second. In the hard phase the edge is abraded; a few micrometres later it is pulled into soft copper that wants to smear and stick. On a softer grade such as WCu 70/30, the copper matrix dominates the response and chips are continuous and gummy. On WCu 85/15 or 90/10, the tungsten network is continuous and the material behaves closer to a sintered carbide.

Copper also conducts heat far better than steel. Depending on the grade, thermal conductivity sits in the range of 180 to 230 W/m·K. Heat generated at the shear zone leaves through the chip and the workpiece, not through the tool. The workpiece stays relatively cool while the edge tip still reaches high temperature, so the failure mode moves from crater wear to adhesive wear and micro-chipping.

That combination is why the same insert that runs a copper contact all day may fail on a tungsten copper electrode within a few minutes. The grade, not the geometry, sets the tool life, so the first job on any new part is to confirm the actual WCu ratio.

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  Hard phase abrades the edgeTungsten grains act like a fine grinding medium at the cutting interface.
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  Soft phase grabs the edgeCopper smears, welds to the rake face, and tears off as built-up edge.
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  Heat exits through the partHigh conductivity protects the tool tip less than it does on steel.
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  Grade sets the behaviorWCu 70/30 and WCu 90/10 need different parameters and different tooling.

Grade is written as tungsten content over copper content by weight. WCu 70/30 means 70 percent tungsten, 30 percent copper. As tungsten rises, density, hardness, and wear resistance rise while thermal conductivity and ductility fall. Machinability follows the copper. That single relationship explains most of what you see on the shop floor.

WCu 70/30 and WCu 75/25 machine the most predictably. Chips form continuously, surface finish is easy to hold, and thin walls are less likely to crack. WCu 80/20 is the common middle ground for resistance welding electrodes and EDM electrode stock; it machines well but demands sharper edges and light radial engagement. WCu 85/15 and 90/10 are where the trouble starts. The tungsten skeleton becomes continuous, the material turns brittle, and any tensile stress at the tool exit can open a crack.

Sintering route matters too. Infiltrated material, where copper is melted into a pre-sintered tungsten skeleton, usually has fewer internal pores than a liquid-phase sintered grade pressed from mixed powders. Pores act as stress risers. A part cut from infiltrated stock will survive a sharper edge and a lighter finishing pass that would edge-crack a porous grade.

Ask for the density and the copper content in writing before quoting. If a drawing only says tungsten copper, the shop is guessing. A grade change from 80/20 to 90/10 can move tool life by a factor of two or three, and it changes whether a 0.5 mm wall will survive the last pass.

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  WCu 70/30, 75/25Most forgiving. Continuous chips, easier finish, best for thin features.
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  WCu 80/20Workhorse grade. Sharp edges and light radial cuts hold tolerance.
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  WCu 85/15, 90/10Brittle and abrasive. Expect edge chipping and crack risk at exits.
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  Infiltrated over sinteredFewer pores, so less cracking and more consistent surface finish.

Carbide is the starting point, but not any carbide. Use a sub-micron or ultra-fine grain grade with a cobalt content around 8 to 10 percent, uncoated or with a thin PVD coating. Thick CVD coatings spall quickly against the abrasive tungsten phase. Diamond-coated or PCD tooling is worth the cost on long runs and on grades above WCu 80/20, especially for finishing where dimensional stability matters.

Geometry matters more than grade. A sharp, positive rake angle around 8 to 12 degrees reduces cutting forces and limits the built-up edge that plagues copper. Keep the edge honed but not dulled; a heavy hone simply rubs the tungsten grains instead of shearing them. Two-flute and three-flute tools clear chips better in deep pockets than high-flute-count tools, which pack copper into the flutes.

Cutting speed falls as tungsten rises. On WCu 70/30, a coated carbide end mill can run around 120 to 180 m/min. On WCu 90/10, drop to roughly 50 to 90 m/min and expect the edge to chip if you push past it. Feed per tooth stays moderate, around 0.03 to 0.08 mm for a 6 to 10 mm cutter. Radial engagement of 5 to 8 percent of tool diameter with a full axial depth keeps heat low and forces the chip to carry it away.

Never run dry on this material. High-pressure coolant aimed at the cutting zone, or through-tool coolant, does two jobs: it cools the edge and it blasts the soft copper chip out of the flute before it welds back onto the workpiece. Trochoidal paths on pockets help as well, keeping the radial load constant and avoiding the sudden full-width engagements that crack brittle grades.

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  Sub-micron carbide, 8–10% CoUncoated or thin PVD. Avoid thick CVD coatings.
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  Positive rake 8–12°Lowers forces and reduces built-up edge on the copper phase.
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  5–8% radial engagementFull axial depth, constant load, heat leaves with the chip.
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  High-pressure coolantFlushes gummy chips and keeps the edge tip below failure temperature.

Edge chipping is the most common failure. It shows as a bright, irregular wear scar on the flank and a sudden jump in surface roughness. The cause is usually a grade mismatch: the tool was selected for copper, and the tungsten skeleton broke the edge. The fix is a tougher sub-micron grade, a lower speed, and a lighter radial cut.

Cracking at tool exit is second. On WCu 85/15 and above, the material has very little ductility, so the compressive stress ahead of the edge turns into tensile stress behind it. A corner breakout appears exactly where the cutter leaves the part. Reduce the feed at exit, use a chamfer or a radius rather than a sharp corner, and consider leaving 0.1 to 0.2 mm of stock for a light finishing pass.

Built-up edge is the third. It looks like a rough, smeared surface with a matte grey appearance instead of the expected semi-bright finish. Copper is welding to the rake face and then breaking off, taking workpiece material with it. Increase speed slightly, increase coolant pressure, and check that the tool is not rubbing rather than cutting.

Dimensional drift is the fourth and the quietest. Because the material conducts heat so well, the part stays cool and expands very little, so operators often skip the warm-up check. The real drift comes from tool wear on the abrasive tungsten phase. On a tight ±0.005 mm feature, measure the tool on a presetter every few parts and compensate, rather than trusting a nominal offset for a full shift.

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  Edge chippingTougher grade, lower speed, lighter radial engagement.
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  Corner breakoutChamfer or radius the exit, reduce feed as the tool leaves.
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  Built-up edgeMore coolant pressure, slight speed increase, sharper edge.
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  Tool wear driftMeasure the tool between parts and compensate on tight features.

Tungsten copper rewards simple, open geometry and punishes anything that traps stress. Keep wall thickness above roughly 0.8 mm on grades above WCu 80/20. Avoid sharp internal corners; a 0.5 mm corner radius or larger gives the cutter room to arc through instead of stopping and rubbing. Deep, narrow slots are slow because the tool has to be short and rigid, so a slot deeper than four times its width will add cost quickly.

Threads form well in the softer grades and poorly in the hard ones. On WCu 90/10, tapping tends to chip the crests. Use a forming tap where possible, cut threads with a thread mill rather than a tap for better chip control, and specify a coarser pitch. Very fine threads below M3 are a reliability problem on the high-tungsten grades.

There are cases where CNC is the wrong answer. A simple flat electrode blank with no tight features and no fine surface requirement is usually cheaper to buy as sintered stock and finish with a light grind. Large plates with only through-holes and a perimeter profile can be wire EDM cut to final size, avoiding the tool wear entirely. Complex three-dimensional shapes with thin ribs or deep pockets are where 5-axis milling is clearly the right process, because EDM cannot reach them efficiently.

If the part needs a mirror finish below Ra 0.4 μm, plan on a separate lapping or polishing step. Milling can reach Ra 0.2–0.8 μm on a stable setup, but holding it across a large face is a different problem. The high tungsten grades hold a polish well once the surface is clean; the risk is smearing copper across the surface during the last pass, which shows up later as an uneven etch or plating result.

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  Wall thickness ≥ 0.8 mmOn WCu 85/15 and above, thin walls crack at the last pass.
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  Corner radius ≥ 0.5 mmLets the cutter arc instead of stopping and rubbing.
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  Thread mill over tappingBetter chip control and less crest chipping on hard grades.
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  Wire EDM for simple platesNo tool wear and no cracking risk on flat through-feature parts.