How are metals joined permanently and temporarily, and how do you choose between arc welding, oxy welding, brazing, soldering and mechanical fasteners?
Describe the welding and joining processes used in metal fabrication, including arc, MIG and TIG welding, oxy-acetylene welding, brazing, soldering and mechanical fastening, and select appropriate joining methods
A focused guide to welding and joining for HSC Industrial Technology Metal and Engineering. Arc, MIG and TIG welding, oxy-acetylene welding, brazing and soldering, mechanical fasteners, weld defects and welding safety, and how to select a joining method.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
Joining is how metal components become a finished fabrication, and the method you choose decides the joint's strength, appearance and whether it can be undone. NESA expects you to describe the main welding and joining processes, from arc and gas welding to brazing, soldering and mechanical fasteners, and to select the right one for a task. This is central fabrication knowledge for the focus area, tested in the written paper and demonstrated in the joints of your Major Project.
Fusion welding
Welding fuses metals by melting their edges together, usually with a filler, to form a joint as strong as the parent metal. The main arc processes use an electric arc as the heat source:
- Manual metal arc (stick) welding: a coated electrode strikes an arc and melts into the joint while its coating shields the weld; rugged and portable, used on heavier steel.
- MIG (gas metal arc) welding: a continuous wire feeds through a gun under a shielding gas; fast, easy to learn and well suited to thinner steel and production.
- TIG (gas tungsten arc) welding: a non-consumable tungsten electrode and separate filler under shielding gas; precise and clean, used for thin material, aluminium and stainless steel where appearance and quality matter.
Oxy-acetylene welding burns a fuel gas with oxygen to produce a flame hot enough to melt steel, and the same equipment cuts metal. It is versatile but slower and adds more heat to the work.
Brazing and soldering
Brazing and soldering join metals with a molten filler that bonds to the surfaces without melting the base metal:
- Brazing uses a filler that melts at a higher temperature than solder, giving a strong joint suitable for steel, brass and dissimilar metals.
- Soldering uses a low-melting filler for light, often electrical joints; it is weaker than brazing but easy and does not distort the work.
Because the base metal is not melted, these processes suit thin, delicate or dissimilar parts.
Mechanical fasteners
Mechanical fasteners join without heat and many can be undone:
- Bolts and nuts: strong, removable joints for assemblies that may need servicing.
- Rivets: permanent joints in sheet metal, including pop rivets for access from one side.
- Self-tapping and machine screws: quick fixings for sheet and threaded components.
These are chosen where heat would distort the work, where dissimilar metals must join, or where the joint must come apart.
Weld defects and quality
Welds can fail through poor penetration, porosity from trapped gas, undercut, cracking or distortion from heat. Good joint preparation, correct current or gas settings, steady technique and control of heat input reduce these, and welds are inspected before a fabrication is trusted.
Selecting a joining method and welding safely
Choose by the metal, the strength and the permanence required: MIG for fast production steel work, TIG for clean welds on aluminium and stainless, brazing for dissimilar or thin metals, and bolts where the joint must be demountable. Welding safety is non-negotiable: wear a welding mask with the correct shade and protective clothing against arc rays and spatter, ventilate fumes or use extraction, keep flammables clear, and check gas equipment for leaks. State the method and its safety needs when you justify a joint in your folio.
Exam-style practice questions
Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2021 HSC1 marksHow is a neutral flame obtained during oxyacetylene welding? A. Using equal amounts of oxygen and acetylene B. Mixing argon gas with oxygen and acetylene C. Mixing nitrogen gas with oxygen and acetylene D. Using a greater volume of oxygen and a lesser volume of acetyleneShow worked answer →
The correct answer is A: using equal amounts of oxygen and acetylene.
A neutral oxyacetylene flame is produced when oxygen and acetylene are supplied in roughly equal (balanced) proportions. This burns cleanly so the weld is neither oxidised nor carburised, which is why the neutral flame is used for general steel welding. It is recognised by a clearly defined, rounded inner cone.
Excess oxygen (D) gives an oxidising flame and excess acetylene gives a carburising flame; argon and nitrogen (B, C) are not part of the oxyacetylene gas mix. So A is correct.
2021 HSC5 marksHow can welding defects be identified and corrected in a fabricated article?Show worked answer →
A five-mark answer should name common defects, explain how each is detected, and describe how it is corrected.
Identifying defects. Begin with visual inspection for surface faults such as porosity (small holes), undercut (a groove along the weld toe), cracks, lack of fusion and poor profile or spatter. Internal defects can be found with non-destructive testing such as dye penetrant for surface cracks, magnetic particle testing, ultrasonic testing or radiography (X-ray) for hidden porosity and inclusions.
Causes. Link defects to their cause, for example porosity from contamination or loss of gas shielding, undercut from excess current or travel speed, and lack of fusion from too low a current or poor technique.
Correcting defects. The faulty section is ground or gouged out back to sound metal and rewelded with the corrected settings (current, travel speed, cleaning and shielding). Pre-cleaning the joint, correct edge preparation and controlling heat input prevent recurrence. The repaired weld is then re-inspected and retested to confirm it is sound.
Marks reward naming specific defects, a detection method and a clear correction or prevention step.
2019 HSC2 marksThe diagram shows two mild-steel plates to be joined. Describe the procedure for arc welding a joint on the mild-steel plates.Show worked answer →
Award up to two marks for a correct, ordered procedure.
Set up. Clean the plates, position and clamp them with the correct gap, then clamp the earth (work) lead to the work and select a suitable electrode and amperage for the plate thickness.
Weld. Strike the arc by lightly scratching the electrode on the metal, then hold a short, steady arc at the correct angle and travel along the joint at an even speed to deposit the weld. Chip off the slag and inspect the bead.
A response that includes safe set-up, striking and maintaining the arc, and running the bead at a steady speed earns full marks.