To weld metals together, high temperature is required. Such a high temperature is obtained by burning:

To weld metals, a high temperature is required to melt the base metals and achieve fusion. This is most effectively achieved by burning acetylene in oxygen, a process known as oxy-acetylene welding [2]. Acetylene is the preferred fuel gas because it produces the highest flame temperature, reaching approximately 3100°C to 3316°C when mixed with oxygen [1]. While other fuel gases like LPG (propane) or methane can be burned with oxygen, they produce lower flame temperatures (around 2800°C for propane) and lack the specific reducing zone required for high-quality steel welding. Oxygen is used instead of air because the nitrogen in air does not contribute to combustion and significantly lowers the flame temperature below the fusion point of most metals [1]. Consequently, the oxy-acetylene flame provides the necessary heat and propagation rate for efficient metal joining.

Sources

1. [1] https://msvs-dei.vlabs.ac.in/mem103/Unit4lesson2.html
2. [2] https://www.sciencedirect.com/science/article/abs/pii/S2214785320341237