Aluminum/Graphite composites (Al/C composites) have the advantages of Al (light weight, easy machinability and good heat conduction) in combination with the advantages of C whiskers (High Young s Modulus, small to negative CTE, high tensile strength and high thermal conductivity). However, Metallographic analysis, EDX and microhardness profile revealed that fibers react with molten aluminum to form the detrimental Al4C3 phase in the HAZ and fusion zone. In order to gain information about temperature transformation and formation conditions, scanning electron microscopy, and termochemistry analysis were used. For experiments, were used power density (I), pulse width (t), frequency (F), travel speed (S) and filler material (M) as independent variables. Thereby, was found that the main process variables that influence the formation of Al4C3 are pulse duration and welding speed, and that the effect of these variables cannot be explained only by using a thermodynamic model, requiring a subsequent kinetic analysis.