in a scientific paper, $0\mathrm{.}55$ wt$.\%$ and $0\mathrm{.}45$ wt$.\%$ stearic acid were added in the two stages of ball milling, respec$-$ tively, to facilitate the generation of cold$-$welded Al$-4$ wt$.\%$ Mg$-5$ wt$.\%$ CuO $($Al$-4$Mg$-5$CuO$)$ composite powders. and when adding more amount of stearic acid $(0\mathrm{.}75$ wt$.\%$ and $1\mathrm{.}0$ wt$.\%$ respectively$)$ it prevents cold welding please explain