Latent heat of fusion
Enthalpy of fusion is the energy required to bring a solid to a liquid state without raising temperature. In welding, it determines the energy needed to melt the joint partners.
Physical-Chemical Mechanism of Action
Heat of fusion is the energy released when a solid turns into a liquid. This happens without a temperature rise because the energy goes entirely into breaking the bonds between atoms.
Metallurgical Relevance & Welding Risks
In welding, heat of fusion determines how much energy you need to melt the parts. Too little energy gives incomplete fusion and weak joints; too much energy can cause the weld to run and lead to phase changes.
Shop Practice & How-To
To get a good weld, you have to match the energy input to the heat of fusion of the metals. That means carefully controlling the temperature and the time you're melting. For steel, the heat of fusion is about 350 kJ/kg. If you don't put in enough energy, you'll get incomplete melting, which means weak joints and cracks. Too much energy, and you might get a runny weld and phase changes like austenite in steel.