Captain reported the failure of two elevator power control units and expressed concern over the quality of the components being acquired for installation on company aircraft.

Narrative

Elevator PCUs (Power Control Units) - I have had two recent maintenance events regarding PCUs in the last 45 days. The first occurred in ZZZ on the Date; Aircraft X. The airplane was coming out of a light C check. Two elevator PCUs had been replaced. During our before taxi check/flight control check I noticed a vibration/bang through out the air-frame when pulling back on the yoke. This noise/vibration was confirmed after shutdown and Maintenance was brought back onto the plane. It was later determined that a bad PCU out of the box had been installed. The second occurred on the leg ZZZ1-ZZZ2. After de-icing I noticed the elevator felt heavy during the flight control check; brief the FO (First Officer); he was the PF (Pilot Flying). I attributed the heaviness to the resent de-icing and application of type 4 fluid. During approach into ZZZ2 the FO commented that the elevator felt even heaver than before leaving ZZZ1. I did another flight control check on the ground and I agreed the elevator did feel heavier. It also would not return to the neutral position on the flight control display. I wrote it up; explained to maintenance the issue. The following morning I inquired about the write-up and was told that they found two B nuts loose on different PCUs. The system (s) had been sucking air and leaking HYD fluid. The elevator was full of HYD fluid. To have two primary flight control PCUs events 45 days apart is alarming. Where is the QC (Quality Control)?