How is metal joined using heat, and what is the difference between welding, brazing and soldering?
Thermal joining: joining metal with heat by welding (e.g. MIG/arc), brazing and soft soldering, the difference between them (melting the parent metal versus a filler), and the safety needed for hot work.
How SQA National 5 Practical Metalworking covers joining metal with heat: welding (MIG or arc), brazing and soft soldering, the difference between melting the parent metal and using a filler, the relative strength of each, and the safety needed for hot work.
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What this dot point is asking
Fabrication joins separate pieces of metal into one product. The SQA expects you to know the thermal (heat) joining methods - welding, brazing and soft soldering - the difference between them (whether the parent metal is melted), their relative strength, and the safety needed for hot work. The skill is applied in the practical activity and sampled by the case study.
The key difference: parent metal or filler
Because welding actually fuses the metals, a welded joint is as strong as the metal itself, which is why it is the strongest of the three. The trade-off is that the intense, very local heat of welding can distort thin sheet and needs the most skill and protective equipment, whereas brazing and soldering work at lower temperatures and are gentler on thin material.
The three methods compared
- Welding (MIG / arc): an electric arc (or a gas flame) supplies intense heat that melts the joint edges and a filler wire, fusing the parts. Strongest joint; used for steel frames and structures.
- Brazing: the joint is heated and a brazing rod (brass/bronze filler) is melted into it at a lower temperature; the parent metal stays solid. Strong, neat joint; joins steel and dissimilar metals.
- Soft soldering: the lowest heat; a soft solder is melted with flux to join thin sheet, tinplate and electrical connections. Weakest joint, but quick and good for light work.
A useful way to remember the order is by temperature and strength together: welding is hottest and strongest, brazing is in the middle, and soft soldering is coolest and weakest. You choose the method to suit the job - welding for a load-bearing steel frame, brazing for a strong but neat joint, soldering for delicate or electrical work where high heat would damage the parts.
Joining safely
Try this
Q1. Name the thermal joining method that melts the parent metal to fuse the parts. [1 mark]
- Cue. Welding (for example MIG or arc welding).
Q2. State which gives the stronger joint: brazing or soft soldering. [1 mark]
- Cue. Brazing.
Q3. Explain why flux is needed when soldering. [2 marks]
- Cue. It cleans the metal and stops oxide forming, so the molten solder flows and bonds to the surface instead of balling up.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA-style Describe4 marksDescribe the difference between welding, brazing and soft soldering as methods of joining metal.Show worked answer →
Award up to 4 marks: up to 2 for welding, 1 for brazing and 1 for soldering. Welding joins metal by melting the edges of the parent metal itself so they fuse together, usually with a filler of the same metal added, giving the strongest joint (1); in MIG or arc welding an electric arc supplies the heat (1). Brazing does not melt the parent metal; it is heated and a brazing rod (a brass or bronze filler) is melted into the joint, bonding the parts at a lower temperature than welding, giving a strong but slightly weaker joint (1). Soft soldering uses the lowest heat and a soft solder filler with flux to join the parts, giving the weakest joint, used for light or electrical work (1). A strong answer makes clear that welding melts the parent metal while brazing and soldering use a filler.
SQA-style Explain3 marksExplain why flux is used when soldering or brazing, and state two safety precautions for hot work.Show worked answer →
Award up to 2 marks for flux and 1 for safety, to a maximum of 3. Flux cleans the metal and stops a layer of oxide forming as it is heated, so the molten solder or brazing filler flows freely and bonds (wets) to the surface instead of sitting in a ball (1); without flux the joint does not take (1). Two creditable safety precautions for hot work: wear the correct eye protection (a welding mask or goggles) and protective gloves and apron; work in a well-ventilated area away from flammable materials; let the hot metal cool before handling; have suitable fire precautions to hand (any two for 1). A clear answer links flux to clean wetting of the joint.
Related dot points
- Sheet metalwork: cutting sheet with tin snips and a guillotine, bending and folding on folding bars or a bending machine, and forming joints such as a folded seam, with allowance made for the bend.
How SQA National 5 Practical Metalworking covers routine sheet metalwork: cutting sheet with tin snips and a guillotine, bending and folding on folding bars or a bending machine, forming folded seams, and allowing for the bend so the finished size is correct.
- Mechanical joining and forming: joining metal with rivets (including pop rivets) and threaded fasteners (nuts, bolts and machine screws), choosing temporary or permanent joints, and bending and forming bar and rod to shape.
How SQA National 5 Practical Metalworking covers joining metal without heat: riveting (including pop rivets) and threaded fasteners (nuts, bolts, machine screws), the difference between temporary and permanent joints, and bending and forming bar and rod to shape.
- Finishing processes: removing sharp edges and burrs (deburring), cleaning and preparing the surface (emery cloth/abrasives), and applying a finish such as paint, lacquer or plating to protect the metal and improve its appearance.
How SQA National 5 Practical Metalworking covers finishing a metal product: removing sharp edges and burrs (deburring), cleaning and preparing the surface with abrasives, and applying a finish such as paint, lacquer or plating to protect the metal and improve its appearance.
- Metals and their properties: ferrous metals (mild steel) and non-ferrous metals (aluminium, copper, brass), and the properties that decide their use - strength, hardness, toughness, malleability, ductility, conductivity and corrosion resistance.
How SQA National 5 Practical Metalworking groups metals into ferrous (mild steel) and non-ferrous (aluminium, copper, brass) and the properties - strength, hardness, malleability, ductility, conductivity and corrosion resistance - that decide which metal suits a workshop job.
- Health and safety in the workshop: identifying hazards, using personal protective equipment (safety glasses, apron, gloves where appropriate), guarding machines, keeping a tidy area, and following safe working practices for hot, sharp and rotating processes.
How SQA National 5 Practical Metalworking expects you to work safely: spotting hazards, using personal protective equipment such as safety glasses and aprons, guarding machines, keeping a tidy workspace, and following safe practices for hot, sharp and rotating processes.