In　fusion　welding,　cast　magnesium　alloys　typically　present　higher　sensitivity　to　liquation　cracking　than　wrought　alloys.　In　this　study,　a　fluid-mechanics-based　analytical　model　was　established　to　investigate　an　unusual　phenomenon:　a　lack　of　cracking　in　cast　magnesium　alloy　while　cracking　took　place　in　wrought　magnesium　alloy.　This　was　observed　in　resistance　spot　welding,　in　which　the　pressure　within　the　molten　nugget　allows　the　liquid　to　feed　through　the　interface　between　the　fusion　zone　and　the　partial　melting　zone,　an　aspect　which　is　not　generally　considered　in　the　typical　criteria　for　liquation　cracking.　The　analytical　model　shows　that　the　liquid　is　easy　to　feed　into　the　inter-grain　zone　of　a　cast　coarse-grained　metal　but　difficult　to　feed　into　wrought　metal.　The　liquid　feeding　effect　changes　the　composition　of　the　inter-grain　region,　causing　the　semi-solid　grains　to　bond　to　each　other　tightly　in　the　cast　magnesium　alloy.　Electro-probe　microanalysis　illustrates　the　mass　transfer　pattern　of　liquid　feeding　at　the　interface　between　the　fusion　zone　and　the　partial　melting　zone.　Magnesium　alloys　AZ31,　AZ91,　and　ZK61　were　selected　to　carry　out　resistance　spot　welding　tests　of　which　results　match　the　theory　raised　in　this　work　well.　(c)　2021　The　Author(s).　Published　by　Elsevier　B.V.　This　is　an　open　access　article　under　the　CC　BY-NC-ND　license　(http://creativecommons.org/licenses/by-nc-nd/4.0/).