Material Joining Processes

Joining is the bringing together of two or more parts to make a single assembly, either permanently or so that it can be taken apart.

Classification

Welded and mechanical connections

Brazing and soldering

They are preferred when parts are thin or heat-sensitive, when dissimilar metals (steel to copper, carbide tip to steel shank) or very small assemblies are joined, when low distortion and a neat appearance are needed, and for electrical joints (solder). Welding is preferred where high strength and heavy sections are required.

Metallurgy of a fusion weld

During welding the parent metal and filler melt in the weld pool and then freeze. The freezing begins at the unmelted grains of the parent metal (epitaxial growth) and columnar dendrites grow toward the weld centre, in the direction of heat flow. The rapid cooling gives a fine, cast structure with segregation of solute.

 Weld bead
  _______________________
 | fusion zone (weld metal)   cast, columnar grains
 |------ fusion boundary ----
 | HAZ: coarse grain (near the boundary)
 |      fine grain (normalised)
 |      partly transformed
 |------------------------------
 | unaffected parent metal

Zones (from the weld centre outward):

1. Fusion zone - melted and resolidified metal, with the composition of filler and parent.
2. Fusion (partially melted) boundary.
3. Heat-affected zone (HAZ) - not melted, but heated high enough to change the structure: coarse-grained region next to the fusion line (hard, brittle, with martensite in hardenable steels), then a fine-grain normalised region, then a partially transformed region.
4. Parent metal - unaffected.

The HAZ properties depend on peak temperature, time at temperature and cooling rate. High carbon equivalent steels form martensite in the HAZ, giving hydrogen-induced cold cracks; this is avoided by preheating, low-hydrogen electrodes, and controlled heat input.

Common defects

• Porosity (gas trapped), slag inclusion
• Lack of fusion and incomplete penetration
• Undercut and overlap
• Cracks - hot cracks (solidification), cold cracks (hydrogen)
• Distortion and residual stress

Energy sources compared

A higher heat intensity gives deeper and narrower welds, a smaller HAZ, less distortion and higher speed.

(a) Oxyacetylene flames

Acetylene burns with oxygen in a torch. Flame has an inner cone (about 3100 degree C) and an outer envelope. The ratio of oxygen to acetylene decides the type:

(b) SMAW

A consumable flux-coated electrode is held in a holder and an arc (typically 20-30 V, 50-300 A) is struck between the electrode tip and the work. Arc heat melts the electrode core and the base metal, and the molten core wire is transferred to the pool as droplets. The coating burns and forms slag, which covers the weld. Current is AC or DC (electrode negative or positive).

Functions of the coating:

1. Produces shielding gas (CO$_2$, H$_2$O) that protects the metal from oxygen and nitrogen.
2. Forms slag that protects the cooling weld and slows the cooling.
3. Stabilises the arc (potassium, sodium compounds).
4. Adds alloying elements and iron powder (increasing deposition).
5. Acts as a deoxidiser and refines the metal; shapes the bead.

(c) Heat input and melted area

Arc power:

$$P = VI = 24\times200 = 4800\ \text{W}$$

Heat input per unit length of weld, with transfer efficiency $f_1 = 0.8$:

$$H = \frac{f_1\,VI}{v} = \frac{0.8\times4800}{5} = 768\ \text{J/mm}$$

Heat used in melting is $f_2 = 0.5$ of the transferred heat. Energy balance for melting:

$$f_2\,H = U_m\,A_w$$ $$A_w = \frac{0.5\times768}{10} = 38.4\ \text{mm}^2$$

Answer: heat input $= 768$ J/mm (0.768 kJ/mm); melted weld area $= 38.4$ mm$^2$.