Final answer:
The thermite reaction is used in welding to join two metal pieces together by generating molten iron which fills gaps between the parts and cools to form a strong weld.
Step-by-step explanation:
The thermite reaction is used in welding primarily for joining two metal pieces together. This highly exothermic chemical reaction involves aluminum metal and iron(III) oxide as reactants, producing aluminum oxide and molten iron as products.
The heat generated from the thermite reaction is sufficient to melt the iron, which when carefully directed, can fill gaps between metal parts and upon cooling, forms a strong weld. This technique is commonly applied in situations where it's difficult to remove parts for repair, such as in welding railroad tracks and in repairing broken locomotive axles.