Joining Processes

Question 1

Spot welding (RSW): Describe how spot welding works! Include at least two process parameters in your answer.

Answer 1

Principle:

Resistance Spot Welding (RSW) joins two (or more) metal sheets by pressing them together between two copper electrodes and passing a high electric current through the contact area.

The resistance at the interface generates heat → metal melts locally → forms a weld nugget upon solidification.

Main steps:
1. Sheets are clamped between electrodes (pressure applied).
2. A current pulse is passed for a short time.
3. The molten nugget forms between the sheets.
4. Current stops, and pressure is maintained to forge and solidify the joint.

Key process parameters:

Welding current (I): determines heat generation (∝ I²R).

Welding time (t): how long current is applied.

Electrode force (F): controls contact resistance and prevents expulsion.

Electrode tip size: affects current density and weld nugget size.

Question 2

In spot welding explain what determines the appropriate time for each of the following:
Tₐ – Approach time

Tₛ – Squeeze (welding) time

T_H + T_R – Hold and release time

Answer 2

Tₐ – Approach time:

Time for electrodes to move and apply contact pressure.

Must ensure full contact before current starts (to avoid arcing).

Tₛ – Squeeze (weld) time:

Duration of current flow through the joint.

Determines heat input and nugget size.

Too short → incomplete fusion; too long → expulsion or electrode wear.

T_H + T_R – Hold + Release time:

Time to keep electrodes pressed after current stops (T_H) + time to release pressure (T_R).

Needed for solidification of the weld nugget and to prevent cracks or voids.

If too short → weak or porous joint; if too long → slower cycle, lower productivity.

Question 3

Laser welding: Compare ordinary laser welding (continuous seam) with RSW. Shortly explain the pros and cons based on the functions or principles of each process.

Answer 3

Laser welding → high precision, speed, and automation, but expensive.

RSW → robust and economical for sheet joining, but limited flexibility and quality control.

Question 4

Clinching: Mention at least three advantages over spot welding, and three other general advantages.

Answer 4

Clinching vs. spot welding:

No heat / no melting: avoids thermal distortion or coating damage.

No electrical contact: works for coated or dissimilar materials.

Lower energy use: purely mechanical forming process.

Other advantages:
4. Environmentally friendly: no fumes, no sparks, no cooling water.
5. Low maintenance: no electrodes or consumables.
6. Simple automation: fast cycle time and easy integration.
7. Good repeatability: consistent joint quality.

Question 5

In soldering or brazing explain how the joining is made from the physical (metallurgical) point of view – how do the materials stick together and what mechanisms are involved?

Answer 5

Soldering and brazing mechanisms:

Both join metals using a filler material that melts and flows between the surfaces without melting the base materials.

Soldering: filler < 450°C (soft).

Brazing: filler > 450°C (hard).

Physical processes:

Wetting: molten filler spreads over base metal.

Capillary action: draws filler into the joint gap.

Diffusion and adhesion: atoms diffuse across the interface forming a metallic bond.

Solidification: upon cooling, the filler creates a strong metallurgical joint.

Difference:
Brazing generally gives stronger joints due to higher temperature and diffusion depth.

Question 6

What is wetting, and how can it be measured? Why is it important? How can you ensure that good wetting will take place?

Answer 6

Wetting:

The ability of a liquid (filler or adhesive) to spread over a solid surface.

Measured by the contact angle (θ) between the liquid and the solid.

Good wetting: θ < 90°.

Poor wetting: θ > 90°.

Why it’s important:

Ensures strong bonding between materials.

Poor wetting → voids, weak joints, or lack of adhesion.

How to ensure good wetting:

Clean surfaces (remove oxides, grease).

Proper temperature: keep filler molten but not overheated.

Use flux (in soldering): removes oxides, improves surface tension.

Match surface energies: adhesive must have lower surface tension than substrate.

Differences:

Question 7

Mention and shortly discuss some advantages of adhesive joining compared to other joining technologies.

Answer 7

Advantages:

Joins dissimilar materials (metal–plastic, composites, etc.).

Uniform stress distribution → less stress concentration than welding or riveting.

Lightweight structures → no need for fasteners.

Sealing function → protects against corrosion and leaks.

Vibration damping and noise reduction.

No thermal distortion (cold joining).

Question 8

Describe how two pieces of material are joined by soldering or brazing. What keeps the parts together?

Answer 8

Process:
1. Clean and flux the surfaces.
2. Heat the joint area (below base metal melting point).
3. Apply molten filler → wets surfaces and fills gap by capillary action.
4. Cool → filler solidifies, forming metallic bonds.

What keeps the parts together:

Metallurgical adhesion (atomic diffusion and bonding) and mechanical interlocking at the interface.

General steps:
1. Surface preparation.
2. Heating.
3. Filler application and flow.
4. Cooling and inspection.

Question 9

What role does the surface tension scale (or level) play? How can it be used for selecting adhesives?

Answer 9

Surface tension / surface energy:

Determines how well a liquid (adhesive or filler) spreads on a solid.

For good adhesion:
Surfaceenergyofsolid > Surfacetensionofliquid
Surfaceenergyofsolid>Surfacetensionofliquid

High-energy surfaces (metals, glass) are easier to bond than low-energy ones (plastics, PTFE).

Used for selection:

Choose adhesives with lower surface tension than the substrate’s surface energy to ensure good wetting and bond strength.

Surface treatments (plasma, sanding, chemical etching) can increase surface energy.