Brazing, soldering and adhesive bonding

Wetting and capillary flow, brazing and soldering fillers, fluxes and clearances, adhesive types and limits, and lap-joint design for equal strength.

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Why it matters

Car radiators, refrigeration and AC copper lines, carbide-tipped tools, printed circuit boards and aircraft panels are joined without melting the parts. Brazing, soldering and adhesive bonding join thin, dissimilar or heat-sensitive parts with little distortion. Their strength comes from joint design — gap, lap length and loading direction — which is where most failures start.

Key ideas

Common principle. A filler — molten metal in brazing and soldering, a liquid polymer in adhesive bonding — must wet clean surfaces and be drawn into the joint gap by capillary action, then solidify or cure. The base metal is never melted. Wetting is measured by the contact angle θ: θ < 90° means the liquid spreads and is pulled into the gap; good brazing needs θ well below that.

Brazing. Filler melts above 450 °C but below the solidus of the base metals.

  • Fillers: copper and brass (often called spelter), silver alloys (Ag–Cu–Zn, flow at about 600–800 °C), aluminium–silicon (for Al), nickel-based (for high-temperature parts). Take liquidus values from your data book.
  • Joint clearance is small, typically about 0.025–0.125 mm; too small and the filler cannot flow in, too large and capillary force cannot hold it, leaving voids and a weak, cast-like filler layer.
  • Heating: torch, furnace (often in controlled atmosphere or vacuum, no flux), induction, resistance, dip. Flux (borax, fluorides) dissolves oxides and stops re-oxidation.
  • Joint strength can exceed the filler's own bulk strength because the thin layer is constrained by the stronger base metal, and lap joints load the filler in shear over a large area.
  • Braze welding: a groove is filled with brazing filler without relying on capillary action (repair of cast iron); it is not true brazing.

Soldering. Filler melts below 450 °C.

  • Solders: Sn–Pb (63Sn–37Pb is the eutectic, melting sharply at 183 °C), lead-free Sn–Ag–Cu (about 217–220 °C) now required for most electronics, Sn–Sb, Sn–Zn.
  • Fluxes: rosin (electronics, mild), acid/zinc chloride (sheet metal and plumbing, must be washed off — corrosive).
  • Methods: iron, torch, wave soldering and reflow soldering (solder paste + oven) for circuit boards.
  • Soldered joints are weak and creep at room temperature, so they are for electrical continuity and sealing, not structural loads. Joint strength relies on mechanical interlock (crimping, folding, through-holes) plus the solder.

Adhesive bonding. A polymer adhesive bonds by surface adhesion and its own cohesion.

  • Types: epoxies (structural, two-part or heat-cured), cyanoacrylates (fast), anaerobics (thread locking), polyurethanes, acrylics, hot-melts, pressure-sensitive tapes.
  • Surface preparation (degreasing, abrading, etching, priming) decides success.
  • Advantages: joins any material pair (metal–composite–glass), spreads stress over the whole area (good fatigue), seals, insulates, damps vibration, no heat damage, smooth appearance, light weight.
  • Limits: low service temperature (often below 150–200 °C), cure time, creep, sensitivity to moisture and solvents, hard to inspect, and very low strength in peel and cleavage. Design joints so the adhesive is loaded in shear or compression.

Joint design. Lap and scarf joints are preferred for brazing, soldering and adhesives because they put the filler in shear over a large area; butt joints rely on a tiny area in tension. Lap length is chosen so the filler's shear capacity matches or exceeds the base member's tensile capacity — commonly about three times the thinner member's thickness for brazed sheet, but always check by calculation.

Formulas

τ · L · w = σ · t · w → L = σ · t / τ Flat lap joint of equal strength: τ = shear strength of filler or adhesive (Pa), L = overlap length (m), w = joint width (m), σ = tensile strength of the base strip (Pa), t = thinner strip thickness (m).

τ · π · D · L = σ · (π/4) · (D² − d²) Tube inserted in a socket: D = tube outer diameter, d = tube inner diameter, L = insertion depth.

F_max = τ · A_lap Lap-shear failure load of an adhesive joint: A_lap = overlap length × width (m²).

h = 2 · γ · cos θ / (ρ · g · c) Capillary rise between parallel plates: γ = surface tension of the liquid filler (N/m), θ = contact angle, ρ = liquid density (kg/m³), g = 9.81 m/s², c = gap (m). Shows why small gaps and good wetting pull filler into joints.

Q = m · c_p · ΔT Sensible heat to bring a part up to brazing or soldering temperature (J), with m in kg, c_p in J/(kg·K) and ΔT in K.

Worked examples

Example 1 — brazed lap length (standard). Two steel strips, 2 mm thick, tensile strength 400 MPa, are lap brazed with a filler whose shear strength in the joint is 150 MPa. Find the overlap for the joint to be as strong as the strip.

  1. L = σ · t / τ (width cancels).
  2. L = 400 × 2 / 150 = 5.33 mm. Use about 6 mm — close to the three-times-thickness rule.

Example 2 — soldered copper tube (GATE level). A copper tube of OD 20 mm and wall 1 mm (tensile strength 220 MPa) is soldered into a fitting. The solder's shear strength is 30 MPa. Find the minimum insertion depth so that the tube fails before the joint.

  1. Tube tensile capacity F = σ · (π/4)(D² − d²) = 220 × (π/4)(20² − 18²) = 220 × (π/4)(76) = 220 × 59.69 = 13 132 N.
  2. Shear area of joint = π · D · L = π × 20 × L mm².
  3. L = F / (τ · π · D) = 13 132 / (30 × π × 20) = 13 132 / 1885 = 6.97 mm; use about 7–8 mm.

Example 3 — capillary rise. Molten silver-braze filler: γ = 1.0 N/m, θ = 20°, ρ = 10 000 kg/m³, gap 0.05 mm.

  1. h = 2 · γ · cos θ / (ρ · g · c) = 2 × 1.0 × 0.9397 / (10 000 × 9.81 × 0.05 × 10⁻³).
  2. h = 1.879 / 4.905 = 0.38 m — capillarity easily fills a 0.05 mm gap; at 0.5 mm the rise falls ten-fold.

Example 4 — adhesive lap shear. Aluminium strips 25 mm wide overlap 12.5 mm and are bonded with an epoxy of lap-shear strength 20 MPa: F_max = τ · A_lap = 20 × 25 × 12.5 = 6250 N (6.25 kN). Real joints fail lower because shear stress peaks at the overlap ends.

Common mistakes

  • Saying brazing melts the base metal. Only the filler melts; otherwise it is welding.
  • Mixing the 450 °C boundary: above is brazing, below is soldering.
  • Using the strip width instead of the tube circumference, or the bore instead of the outer diameter, for a socket joint's shear area.
  • Assuming a wider gap gives a stronger brazed joint; strength drops above the optimum clearance.
  • Loading adhesive joints in peel or cleavage.
  • Forgetting flux or surface cleaning; oxides stop wetting completely.

For GATE PI

Expect MCQs on temperature ranges (450 °C boundary), flux functions, eutectic solder, choosing brazing, soldering or bonding for a given job, joint types and why lap joints are used. Numericals cover lap length or insertion depth for equal strength, adhesive failure load, capillary rise and heat needed to reach brazing temperature.

Quick check

  1. What separates brazing from soldering?
  2. Why is 63Sn–37Pb solder popular?
  3. A 1.5 mm strip (σ = 300 MPa) is lap joined with filler τ = 100 MPa. Minimum overlap?
  4. Why are adhesive joints designed to avoid peel?
  5. What is braze welding?

Answers: 1. Filler liquidus above 450 °C is brazing; below is soldering. 2. It is eutectic, so it melts and freezes at one temperature (183 °C) with no pasty range. 3. 300 × 1.5/100 = 4.5 mm. 4. Peel concentrates load on a thin line at the joint edge, where adhesives are weakest. 5. Filling a groove joint with brazing filler without capillary flow, typically for repair of cast iron.

Interview questions

All Casting, Forming and Joining interview questions

Try answering each one aloud before you open it.

  1. 1.What is brazing and how does it differ from welding?Concept

    Brazing is a metal-joining process that involves melting and flowing a filler metal into the joint between two close-fitting parts without melting the base materials. Unlike welding, which melts the base materials, brazing uses a filler metal with a lower melting point than the workpieces. This allows for joining dissimilar metals and minimizes thermal distortion.

  2. 2.Explain the process of soldering and its typical applications.Concept

    Soldering is a process of joining two or more metal items by melting and flowing a filler metal (solder) into the joint. The filler metal has a lower melting point than the workpieces. Soldering is commonly used in electronics to connect components to circuit boards, as well as in plumbing to join pipes.

  3. 3.What are the key differences between brazing and soldering?Concept

    The main differences between brazing and soldering are the temperature and the strength of the joint. Brazing occurs at higher temperatures, typically above 450°C, and results in stronger joints compared to soldering, which occurs below 450°C. Brazing is suitable for structural applications, while soldering is often used for electrical connections.

  4. 4.Describe adhesive bonding and its advantages over mechanical fastening.Concept

    Adhesive bonding involves using a substance to hold two surfaces together. It offers several advantages over mechanical fastening, such as distributing stress over a larger area, reducing weight, and allowing for the joining of dissimilar materials. Adhesive bonding also provides a smooth surface finish and can seal joints against environmental factors.

  5. 5.Why is brazing preferred over welding for joining thin-walled components?Application

    Brazing is preferred for thin-walled components because it uses lower temperatures than welding, reducing the risk of thermal distortion and damage to the components. The process also allows for joining dissimilar metals and provides a clean, smooth joint without the need for extensive finishing.

  6. 6.What happens if the soldering temperature is too high during the process?Application

    If the soldering temperature is too high, it can damage the components being joined, especially in electronics where sensitive parts may be present. Excessive heat can also cause the solder to oxidize, leading to poor wetting and weak joints. Additionally, it may result in the burning of flux, which is essential for cleaning the surfaces.

  7. 7.In what situations would adhesive bonding be unsuitable?Application

    Adhesives are a poor choice where the joint will run hot (most lose strength above roughly 150–200 °C), where it will carry sustained load that causes creep, or where it will be loaded in peel or cleavage, because the stress then concentrates along a thin edge line. Long exposure to moisture or solvents can degrade the bond, surfaces that cannot be properly cleaned bond poorly, and the joint is hard to inspect non-destructively and cannot be taken apart. Structural epoxies are strong in shear, so the issue is the loading mode and environment, not strength as such.

  8. 8.Calculate the amount of heat required to raise the temperature of a 0.5 kg brazing filler metal from 25°C to its melting point of 600°C. Assume the specific heat capacity is 0.385 kJ/kg·°C.Numerical

    The amount of heat required (Q) can be calculated using the formula Q = m·c·ΔT, where m is the mass, c is the specific heat capacity, and ΔT is the change in temperature. Here, m = 0.5 kg, c = 0.385 kJ/kg·°C, and ΔT = 600°C - 25°C = 575°C. Therefore, Q = 0.5 kg × 0.385 kJ/kg·°C × 575°C = 110.6875 kJ.

  9. 9.A soldering process requires a joint to be heated to 350°C. If the initial temperature is 20°C and the specific heat capacity of the joint material is 0.9 kJ/kg·°C, how much energy is needed to heat a 0.2 kg joint?Numerical

    The energy required (Q) is calculated using Q = m·c·ΔT. Here, m = 0.2 kg, c = 0.9 kJ/kg·°C, and ΔT = 350°C - 20°C = 330°C. Therefore, Q = 0.2 kg × 0.9 kJ/kg·°C × 330°C = 59.4 kJ.

  10. 10.What are the potential environmental impacts of using adhesives in manufacturing?Application

    The use of adhesives in manufacturing can have several environmental impacts. Some adhesives release volatile organic compounds (VOCs) during application, contributing to air pollution and health hazards. Disposal of adhesive waste can also pose environmental challenges, as some adhesives are not biodegradable and can persist in the environment. Manufacturers are increasingly seeking eco-friendly adhesives to mitigate these impacts.

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