Rigidity (or mechanical coherency) temperature is a major input parameter in simulation and optimization of casting and welding processes, as it dictates the temperature below which thermal straining starts. With respect to hot tearing, this temperature is also a key parameter. In metallic alloys, it falls in the solidification interval at a high solid volume fraction. In situ neutron diffraction measurements had been performed by the authors in 2013 at ILL on the instrument SALSA (Grenoble) to study mechanical coherency and onset of stress generation in an Al-13wt.% Cu alloy. The feasibility of such a technique was proven by using the 3 times larger detector and 4 mm focus collimator's at SALSA. With the present proposal, we aim to cast different Cu contents (0.5 wt. pct to 4 wt. pct, maximum susceptibility to hot tearing is 1 wt. pct) in both cracking and non cracking conditions to determine the optimum composition to avoid hot tearing. In the absence of hot tearing, the “free to contract” configuration will allow us to determine the CTE of the alloy up to room temperature. Then the stress generation within the alloy will allow us to determine the as-cast rheology of the alloy.