Aluminium, titanium or steel — choosing the right horn material

A horn is a tuned piece of metal, and the metal you tune matters. Get it wrong and you either pay too much or replace the horn every few months. This is one of the more common questions we get, usually from someone who has just worn out their first horn faster than they expected.

The three properties that decide it

Horn material selection comes down to a balance of three things:

• Acoustic loss. How much of the ultrasonic energy the metal turns into heat instead of passing along. Low loss is good — it means efficiency and a cool-running horn.
• Fatigue strength. The horn flexes 20,000 times a second. Over a shift that is 500 million cycles or so. Fatigue strength decides whether it survives.
• Surface hardness. How long the working face lasts against the plastic, especially filled plastic.

No metal is best at all three, which is why there are three answers.

Titanium alloy — the production standard

Ti-6Al-4V is what most serious production horns are made from, and for good reason. It has excellent fatigue strength, low acoustic loss, and enough surface hardness to survive normal use. It handles the highest amplitudes without complaint, which matters on semi-crystalline materials where you need every micron you can get.

Downsides: it is expensive, and it is genuinely difficult to machine. A complex titanium horn takes longer on the machine than the same shape in aluminium, and that shows in the price.

Use it when: the horn will run production shifts, the material is filled or semi-crystalline, the amplitude needs to be high, or the horn geometry has small features that would fatigue in a softer metal.

Aluminium — the practical workhorse

High-strength aluminium, typically 7075, is cheap, light and easy to cut into complicated shapes. Acoustic loss is low, which surprises people. What it lacks is surface hardness.

Aluminium comes into its own on large horns. A 15 kHz horn 170 mm across is a substantial lump of metal, and in titanium the cost can be hard to justify. In aluminium it is affordable, and because aluminium is light, the stack has less mass to drive.

It is also the obvious choice for prototypes and trials. When we are proving out a joint on a customer's sample part and we may need three shapes before the fourth one works, cutting those in aluminium is the sensible thing to do.

The wear problem has known fixes:

• Hard anodising — cheap, adds useful life on unfilled resins.
• Hard chrome plating — better, and re-platable.
• Carbide coating on the working face — the serious option for filled resins.
• A replaceable titanium or steel wear cap on the face — best of both, and you replace a small insert instead of a whole horn.

Use it when: the horn is large, the run is short, the material is unfilled, or you are still figuring out the geometry.

Hardened steel — narrow but real uses

Tool steels such as D2 and H13 are hard and wear beautifully. They also have high internal damping, which means a steel horn turns a meaningful chunk of your ultrasonic energy into heat. Run one hard and it gets hot.

So steel is not a general horn material. It is used where the working surface takes a beating and the acoustic path is short:

• Small staking and swaging tips.
• Anvils and nests, where the part sits.
• Some boosters, where wear at the mating faces matters.
• Knurled anvil rolls for non-woven fabric sealing, where the pattern has to hold its edge through millions of metres of fabric.

Use it when: wear resistance matters more than efficiency, and the part is small.

Side by side

How we usually specify it

For a customer running one part in volume on filled nylon, titanium, every time. The horn costs more once and lasts.

For a customer welding a 300 mm PP corrugated box on a 15 kHz machine, aluminium with a hard-coated face. Titanium at that size would nearly double the tooling bill for wear life they do not need on unfilled PP.

For a mask line running knurled anvils against non-woven fabric, hardened steel on the anvil and titanium on the horn.

For a customer who is not yet sure the joint design is final — aluminium, and we will cut it again when the design settles. That happens more often than anyone admits, and it is cheaper to plan for it.

A note on lead time and geometry

Whatever the material, the horn has to be tuned. It is cut to a calculated length, tested on a stack, and adjusted until it resonates properly at 15 or 20 kHz. A horn that is close but not right will run hot, sound wrong and fail early.

That is the part of horn making that takes experience rather than a machine, and it is why we cut horns in-house rather than ordering them in. See what we make in the horns and fixtures section, or send us the part and we will quote the horn with the machine as one number.

Questions we get asked