What must be done to a heat-treated aluminum part once welded repairs are finished?

Once welding repairs are completed on a heat-treated aluminum part, it is essential to re-heat treat the part. This is necessary because welding can alter the metallurgical properties of the aluminum, affecting its strength and overall integrity. Heat treatment restores the part's original mechanical properties that may have been compromised during the welding process.

Re-heat treating allows the aluminum to regain its designed strength and hardness, ensuring it meets the necessary specifications for its application. If this step is skipped, there is a risk of reduced load-bearing capacity and potential failure in service. This understanding is critical for maintaining the safety and performance standards required for aircraft components.

The other choices are not appropriate in this context. Simply painting the part would not address the changes in structural properties due to the welding process. Leaving the part untreated could lead to compromised integrity, while cooling it down slowly is not part of the necessary post-weld process for heat-treated aluminum.