Metal Joining Process

Types of joints

• Temporary joints can be taken apart without damaging the parts: bolts and nuts, screws, studs, keys, cotters, pins, couplings.
• Permanent joints cannot be separated without damage: riveting, welding, brazing, soldering, adhesive bonding.

Soldering, brazing and welding

Brazing works by capillary action like soldering, with a gap of 0.05 to 0.25 mm. It allows joining of different metals (steel to cast iron or copper) and uses less heat, thus less distortion than welding.

Types of riveted joints

• Lap joint: plates overlap and are riveted (single, double or triple riveted; chain or zigzag arrangement).
• Butt joint: plates are placed edge to edge and covered by one or two cover plates (single or double strap).

Modes of failure

1. Tearing of the plate across the rivet hole line.
2. Shearing of the rivet (single shear in lap joint).
3. Crushing of the rivet or plate where they bear on one another.
4. Tearing of the plate at the edge (avoided by margin of 1.5d).
5. Shearing of the plate in front of the rivet (avoided by margin).

Calculation (per pitch length)

Given: t = 10 mm, d = 18 mm, p = 50 mm, $\sigma_t$ = 100 MPa, $\tau$ = 60 MPa, $\sigma_c$ = 150 MPa; one rivet per pitch.

Tearing of plate:

$$P_t = (p - d)\,t\,\sigma_t = (50-18)\times 10\times 100 = 32{,}000\ \text{N}$$

Shearing of rivet (single shear):

$$P_s = \frac{\pi}{4}d^2\tau = \frac{\pi}{4}(18)^2\times 60 = 15{,}268\ \text{N}$$

Crushing:

$$P_c = d\,t\,\sigma_c = 18\times 10\times 150 = 27{,}000\ \text{N}$$

Strength of the joint is the least of the three: $P = 15{,}268$ N (shear failure).

Strength of the solid plate:

$$P_{solid} = p\,t\,\sigma_t = 50\times 10\times 100 = 50{,}000\ \text{N}$$ $$\eta = \frac{P}{P_{solid}} = \frac{15{,}268}{50{,}000} = 0.305$$

Answer: joint strength = 15.27 kN per pitch (rivet shearing governs); efficiency = 30.5%.

Gas welding joins metals by melting the edges with a flame produced by burning a fuel gas (acetylene) in oxygen; a filler rod is added to the molten pool.

Set-up

 O2 cylinder      C2H2 cylinder
   |regulator       |regulator
   +------hose----------hose-----+
                    |             |
                 [ blow pipe / torch ]
                          \__ nozzle -> flame
                  work + filler rod

• Cylinders of oxygen (about 125 to 150 bar) and dissolved acetylene (about 15 bar) with regulators and pressure gauges.
• Hoses, flashback arrestors, blow pipe (torch) with mixing chamber and tip.
• Goggles, spark lighter, filler rods and flux (for non-ferrous metals).

Types of flame

The neutral flame has a bright inner cone and a pale outer envelope; the carburising flame has a feather beyond the inner cone.

Welding technique

• Leftward (forward) method for thin plates; rightward method for thick plates (more economical, better strength).

Advantages

• Cheap, portable equipment; flexible; no electricity needed; good control of heat; can weld, braze, solder and cut.

Limitations

• Slow, wide heat affected zone and distortion; not suited for thick plates; fire and explosion hazard of gases; lower strength than arc welds.

Shielded metal arc welding (SMAW)

An electric arc (about 3500 to 5000 °C) is struck between a coated consumable electrode and the work. The arc melts the electrode tip and the base metal into a weld pool, which solidifies as the electrode moves.

   electrode holder
        |\ coated electrode
        | \
 ~~~~~~~|~~~~~ arc ~~~ slag over weld bead
 ===== work =================
 transformer/rectifier: 20-40 V, 50-400 A

• Supply: AC transformer or DC rectifier. With DC straight polarity (electrode negative) more heat goes to the work, giving deeper penetration for thick plates; with reverse polarity (electrode positive) less heat goes to the work, suiting thin sheets and non-ferrous metals.

Functions of the coating (flux):

• Gives a shielding gas to protect the pool from oxygen and nitrogen.
• Forms a slag that covers the weld and slows cooling.
• Stabilises the arc (ionising elements), and adds alloying elements.
• Cleans the molten metal (deoxidises).

TIG versus MIG

Principle

Two overlapping sheets are clamped between copper alloy electrodes, and a large current passes for a short time. The joint resistance generates heat by the Joule effect, $H = I^2 R t$, which melts a small nugget at the interface. The pressure is held while the nugget solidifies, forming a spot weld.

   electrode (Cu)   <- force
        |  |
  ======XXXX======  sheets (nugget X)
        |  |
   electrode (Cu)   <- force

Steps: squeeze, weld (current on), hold, release.

Applications: car bodies, metal furniture, utensils, electric boxes, wire mesh, thin sheet steel assemblies; high production and easily automated.

Heat generated

$$\begin{aligned} H &= I^2 R\,t \\ &= (9000)^2\times 120\times 10^{-6}\times 0.25 \\ &= 8.1\times 10^{7}\times 1.2\times 10^{-4}\times 0.25 = 2430\ \text{J} \end{aligned}$$

Heat needed for the nugget

Heat to raise 1 g by 1500 K and melt it:

$$q = c\,\Delta T + L = 0.5\times 1500 + 270 = 1020\ \text{J/g}$$ $$H_{nugget} = 0.25\times 1020 = 255\ \text{J}$$

Efficiency

$$\eta = \frac{255}{2430}\times 100 = 10.5\%$$

The rest of the heat is lost by conduction into the water-cooled electrodes and the surrounding sheet.

Answer: heat generated = 2430 J; heat used to melt the nugget = 255 J; efficiency = 10.5%.