Induction brazing

An example 20.000 mm shaft in a 20.200 mm bore heats freely through 600 K, with shaft and sleeve expansion coefficients of 20 and 10 × 10⁻⁶/K. Diametral clearance falls from 0.2000 to 0.0812 mm, and radial clearance from 0.1000 to 0.0406 mm. Check actual temperature gradients, restraint, tolerances and filler flow.

Induction supplies heat. Molten brazing filler wets the solid base metals and flows through the joint by capillary action. Verify concealed fill independently of external filler. Lucas Milhaupt, brazing FAQ, undated

Coil, workpiece and power supply

Ambrell describes induction as non-contact electromagnetic heating of electrically conductive material. Coil coupling, workpiece material and mass, required temperature rise, heating time and losses all influence equipment selection. Magnetic materials can have an additional hysteretic contribution, while eddy-current heating remains fundamental. Do not interpret “induction” as requiring a ferromagnetic workpiece or assume that equal power settings heat unlike components equally. Ambrell, induction-heating essentials, undated

The coil's geometry and distance from the component are process variables. Define part location, coil access and removal, cooling provisions and neighboring conductive hardware during development. A coupling change caused by a fixture or replacement coil can alter heating even if the programmed time remains unchanged. Confirm the complete system and thermal outcome rather than qualifying a power-supply name alone. Ambrell, coupling and coil-design discussion

Joint design and hot clearance

Ambrell's Precision Induction Brazing brochure, Rev D, ©2022, identifies joint design, surface cleaning, fluxing, fixturing, heating and residue removal as linked steps. Its clearance guidance explicitly applies at the filler-flow temperature. Such supplier guidance is a starting point for the stated brazing system, not a universal gap for all fillers, alloys, atmospheres or joint geometries. Ambrell, printed p. 5 and back-cover revision

Lucas Milhaupt explains that unequal expansion changes clearance during heating. Evaluate whether the inner member closes the gap or the outer member opens it, using the actual geometry and thermal conditions. No separately verified overall issue date is assigned here to its guide. Lucas Milhaupt, fit guidance

Specify overlap, locating features and filler placement as well as nominal gap. Identify the surface pair that must bond and how the assembly is held during filler flow and cooling. The drawing should distinguish permitted external fillets from required internal coverage. Joint strength cannot be assigned from filler tensile strength alone; the Lucas FAQ identifies base metals, filler and clearance among the controlling variables. Lucas Milhaupt, joint-strength FAQ

Filler, flux and preparation

Select filler and oxidation control for both base metals and the service. Lucas Milhaupt describes self-fluxing phosphorus-bearing fillers for copper-to-copper joints but warns against copper-to-steel use because brittle phosphides can form. A familiar copper-brazing consumable is not a general-purpose substitute. Lucas Milhaupt, filler guidance

Cleaning is not replaced by flux: the guide explains that contamination prevents proper adhesion and capillary flow. Specify the qualified surface preparation and flux application. Treat omission of flux as a process change requiring review. Lucas Milhaupt, preparation guidance

Temperature evidence and a bounded application

Ambrell's application note dated February 2, 2016 describes a steel-and-carbide dental assembly, 0.032 in / 0.81 mm diameter, heated toward 1400°F / 760°C using an EASYHEAT 10 kW system and a specially designed channel coil. It reports temperature-indicating paint melting within the targeted seven seconds, with a braze shim and flux at the joint. Ambrell, dental-tool application note

This demonstrates an application-development result, not a seven-second cycle for other carbide joints or proof of a particular joint strength. The listed 10 kW is the system designation, not evidence that exactly 10 kW was absorbed by the part. Agree how joint temperature is measured and how the measurement relates to the required thermal cycle. Keep heating time separate from loading, cooling, cleaning and inspection time.

Original expansion example

Assume an invented concentric assembly at room temperature with a 20.000 mm shaft inside a 20.200 mm bore. Assume uniform 600 K temperature rise, constant average expansion coefficients of 20 × 10⁻⁶/K for the shaft and 10 × 10⁻⁶/K for the sleeve, and unconstrained expansion:

hot shaft diameter = 20.000 × (1 + 20 × 10⁻⁶ × 600) = 20.2400 mm

hot bore diameter = 20.200 × (1 + 10 × 10⁻⁶ × 600) = 20.3212 mm

The diametral gap decreases from 0.2000 mm to 0.0812 mm; the corresponding radial gap decreases from 0.1000 mm to 0.0406 mm.

Use temperature-dependent expansion data, measured thermal history, restraint and tolerances for the actual assembly. Verify filler flow separately.

Cleaning, qualification and acceptance

The Ambrell brochure includes removal of flux residue after brazing. Define the cleaning method for the actual flux, materials and assembly, including inaccessible internal surfaces. Do not transfer a generic quench or acid-cleaning instruction to a heat-sensitive, coated or crack-sensitive component without the corresponding process approval. Ambrell, post-braze step

ISO 18279:2023, edition 2, November 2023, classifies brazed-joint imperfections and provides quality-level guidance. Its scope explicitly leaves individual assessment to the particular joint requirements and notes that nondestructive testing alone may not detect every imperfection. It covers imperfections without additional service-load effects; the standard number is not a blanket service-performance rating. ISO, scope and edition

Provide the joint drawing, base-metal conditions, filler/flux specification, hot-clearance basis, fixture and coil arrangement, thermal measurement and cleaning route. Agree dimensional, coverage, leak/pressure or mechanical tests as applicable, their stage and acceptance criteria. An external fillet, temperature trace and leak test answer different questions; request evidence for the actual function rather than treating any one as complete proof of joint quality.