The　dissimilar　joints　of　TC4　and　IN718　were　successfully　welded　by　using　a　fiber　laser　and　Cu　interlayer　with　varying　thicknesses.　This　research　focused　on　the　interface　on　the　TC4　side,　where　two　transition　layers　were　identified:　a　Ti-rich　layer　and　a　Ti-poor　layer.　The　Ti-rich　layer　primarily　consisted　of　Ti2Cu　and　Ti2Ni.　Meanwhile,　the　Ti-poor　layer　contained　a　range　of　harder　and　more　brittle　intermetallic　compounds　(IMCs),　such　as　TiNiCu,　TiCr2　and　TiFe2,　with　the　thickness　increasing　as　the　interlayer　thickness　increased.　The　fracture　of　all　the　joints　occurred　at　the　interface　on　the　TC4　side.　Insufficient　Cu　introduction　tended　to　cause　the　penetration　of　Ti　into　Layer　B,　reacting　with　elements　such　as　Fe　and　Cr　to　form　more　brittle　and　hard　IMCs.　Conversely,　excess　Cu　in　the　molten　pool,　resulting　from　the　introduction　of　a　0.4　mm　Cu　interlayer　could　lead　to　the　formation　of　much　(Cu,　Ni),　which　caused　softening　and　a　deterioration　in　the　tensile　strength　of　the　joint.　When　a　0.3　mm　Cu　interlayer　was　employed,　the　diffusion　of　Ti　and　Cu　reached　equilibrium,　resulting　in　an　average　tensile　strength　of　319　MPa　for　the　obtained　joint,　which　was　28　%　higher　than　that　of　the　joint　with　a　0.4　mm　interlayer.