“…Fundamentally being a mechanical surface treatment, shot peening is a complicated process, in which many parameters affect the process performance [ 20 , 49 , 50 , 51 ]. These can be classified into four main groups [ 52 ]: (1) shot characteristics (size [ 20 , 33 , 34 , 53 , 54 , 55 ], type [ 34 , 55 ], shape, density [ 34 ], hardness [ 34 , 53 ], and yield strength [ 20 , 33 , 34 , 52 , 53 , 54 , 56 , 57 ]); (2) process parameters [ 34 ] (shot flow rate, shot pressure [ 20 , 54 , 57 , 58 ], shot speed [ 59 ], impact angle [ 59 , 60 ], nozzle-to-target distance [ 60 ], nozzle diameter, and shot peening duration [ 20 , 32 , 60 , 61 ]); (3) target material properties (mechanical properties [ 34 ] such as hardness, yield strength, toughness, chemical composition and crystal structure [ 20 , 62 ]); and (4) environmental factors (temperature and humidity) [ 53 , 63 ]). Yildiran et al reported that the surface roughness and hardness of Ti6Al4V alloy might significantly vary depending on shot size and peening pressure [ 5 ].…”