Melting range
The temperature range between the solidus and liquidus lines in alloys where the material exists in a semi-solid state.
When metal melts, it doesn't become liquid instantly. Between solid and liquid, there's a range called the melting interval. In that range, the material is mushy and has little strength. As the weld pool cools, it contracts. In this pasty state, this can cause the weld to crack – these are hot cracks. The wider this melting interval, the longer the weld remains in this critical state, and the greater the risk of cracking. Impurities in the material often make things worse because they widen the interval further. So: keep the material clean and weld with as little heat as possible so the weld passes through the critical range quickly. Also, make sure the filler material matches the base material in terms of melting behavior.