th s ▁of ▁a ▁millimeter . ▁This ▁allows ▁for ▁a ▁very ▁high ▁vol um et ric ▁power ▁density , ▁which ▁can ▁reach ▁values ▁of ▁the ▁order ▁ 1 0 5 ▁– ▁ 1 0 7 ▁W / mm 3 . ▁Con sequently , ▁the ▁temperature ▁in ▁this ▁volume ▁increases ▁extremely ▁rapidly , ▁ 1 0 8 ▁– ▁ 1 0 1 0   K / s . ▁ ▁The ▁effect iveness ▁of ▁the ▁electron ▁beam ▁depends ▁on ▁many ▁factors . ▁The ▁most ▁important ▁are ▁the ▁physical ▁properties ▁of ▁the ▁materials ▁to ▁be ▁w eld ed , ▁especially ▁the ▁ease ▁with ▁which ▁they ▁can ▁be ▁m elt ed ▁or ▁v apor ize ▁under ▁low - press ure ▁conditions . ▁Electron - be am ▁w eld ing ▁can ▁be ▁so ▁inten se ▁that ▁loss ▁of ▁material ▁due ▁to ▁ev ap oration ▁or ▁bo iling ▁during ▁the ▁process ▁must ▁be ▁taken ▁into ▁account ▁when ▁w eld ing . ▁At ▁lower ▁values ▁of ▁surface ▁power ▁density ▁( in ▁the ▁range ▁of ▁about ▁ 1 0 3   W / mm 2 ) ▁the ▁loss ▁of ▁material ▁by ▁ev ap oration ▁is ▁neg lig ible ▁for ▁most ▁met als , ▁which ▁is ▁favor able ▁for ▁w eld ing . ▁At ▁higher ▁power ▁density , ▁the ▁material ▁affected ▁by ▁the ▁beam ▁can ▁totally ▁ev apor ate ▁in ▁a ▁very ▁short ▁time ; ▁this ▁is ▁no ▁longer ▁electron - be am ▁w eld ing ; ▁it ▁is ▁electron - be am ▁mach ining . ▁ ▁Be am ▁formation ▁C ath