Selecting Material and Process for a Welded Cantilever Bracket
A workshop has an order for 40 identical cantilever brackets. Each bracket is a flat bar welded to an MS frame and projects 150 mm; the service load is 4 kN at the free end, static and indoor. The supervisor proposes 50 mm × 6 mm MS flat, mounted on edge, with a 6 mm fillet weld all round, and says 'mild steel is cheap and easy to weld'. The customer asks whether aluminium 6061-T6 would be better because it is lighter and needs no painting. The workshop has a manual metal arc welding set, a horizontal band saw, a drill press and an angle grinder, but no heat-treatment furnace. Delivery is 10 working days. The allowable bending stress for MS is 120 N/mm²; for 6061-T6 use 110 N/mm². You are asked to check the design and recommend material and process.
1. Calculate the bending moment and required section modulus for one bracket. Check whether 50 × 6 mm flat on edge is adequate.
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M = 4 kN × 0.150 m = 0.600 kN·m = 600,000 N·mm. Required Z = M/σ = 600,000/120 = 5,000 mm³. For a flat on edge, Z = b h²/6. For 50 × 6: Z = 6 × 50²/6 = 2,500 mm³. Stress = 600,000/2,500 = 240 N/mm², double the allowable. So 50 × 6 is not adequate. A 75 × 6 flat gives Z = 6 × 75²/6 = 5,625 mm³, stress about 107 N/mm²; 75 × 8 gives 7,500 mm³. Specify at least 75 × 6, preferably 75 × 8 for weld and stiffness margin, then check deflection and weld size separately. The proposed 6 mm fillet is not automatically adequate; weld throat must be checked.
2. Compare MS and aluminium for this bracket, including stiffness, welding, corrosion and cost.
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MS 75 × 8 is the better choice here. It is stiffer, with E about 200 GPa against about 69 GPa for aluminium, so for the same bracket it deflects about one-third as much. To match stiffness in 6061 you need a much deeper section, which eats the weight saving. MS is easy to weld with the available MMA set, cheap and forgiving. Aluminium 6061-T6 needs TIG/AC or MIG with the correct filler and cleaning; it also has a lower modulus and forms a galvanic couple with the steel frame if bolted, so it needs insulating washers. Aluminium saves painting, but the workshop would have to buy filler, gas and skill. Choose MS, prime and paint; use aluminium only if weight is dominant and the joint is redesigned.
3. Outline the manufacturing and inspection steps, including how you would control weld distortion and verify the 150 mm projection.
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Process: cut 75 × 8 flat to length, deburr, mark the 150 mm projection from the frame face, tack-weld in a fixture, check squareness, then weld 6 mm fillets in a balanced sequence, opposite sides and back-step, to limit distortion. Let it cool naturally; do not quench. Clean slag, prime and paint. Inspection: projection 150 ±1 mm, squareness within 1 mm over the bracket, weld size with a fillet gauge, visual check for undercut, porosity and cracks, and a first-off check. For 40 off, use a drilling and welding jig and record the first and last piece. If a bracket distorts, straighten cold with controlled pressure rather than heating unknown steel.