Oxyhydrogen Welding is a Gas Welding process using a combustion mixture of Hydrogen (H2) and oxygen (O2) for producing gas welding flame.

Oxyacetylene flame has a temperature of about 4500°F (2500°C).
Combustion reaction is as follows:

2H2 + O2 = 2H2O

Oxyhydrogen Welding is used for joining metals with low melting points, like aluminum, magnesium, etc.

Oxyhydrogen is a mixture of hydrogen (H2) and oxygen (O2) gases, typically in a 2:1 molar ratio, the same proportion as water. This gaseous mixture is used for torches for the processing of refractory materials and was the first gaseous mixture used for welding. In practice a ratio of 4:1 or 5:1 hydrogen:oxygen is required to avoid an oxidizing flame.

Oxyhydrogen will combust when brought to its autoignition temperature. For a stoichiometric mixture at normal atmospheric pressure, autoignition occurs at about 570 °C (1065 °F). The minimum energy required to ignite such a mixture with a spark is about 20 microjoules. At normal temperature and pressure, oxyhydrogen can burn when it is between about 4% and 95% hydrogen by volume.

When ignited, the gas mixture converts to water vapor and releases energy, which sustains the reaction: 241.8 kJ of energy (LHV) for every mole of H2 burned. The amount of heat energy released is independent of the mode of combustion, but the temperature of the flame varies. The maximum temperature of about 2800 °C is achieved with a pure stoichiometric mixture, about 700 degrees hotter than a hydrogen flame in air. When either of the gases are mixed in excess of this ratio, or when mixed with an inert gas like nitrogen, the heat must spread throughout a greater quantity of matter and the temperature will be lower