The　workpiece　fabricated　by　wire　and　arc　additive　manufacturing　is　usually　made　up　of　hundreds　of　welds.　If　the　slag　is　not　cleaned　up　and　adherent　to　the　weld　surface,　it　will　be　stuck　in　welds　and　becomes　inclusions　and　other　defects　within　the　structures.　This　is　not　only　the　case　when　using　flux　cored　wire　as　the　consumable　material,　but　also　with　solid　wire.　Different　weld　morphologies　were　studied　by　controlling　the　slag　treatment　after　each　layer　deposition,　in　terms　of　non-destructive　test　(X-ray　radiographic　testing).　Further　investigation　of　the　microstructures　and　composition　of　slag　inclusions　were　developed　by　SEM　and　EDS　analysis.　Charpy　V-notch　impact　test　has　been　conducted　on　the　specimens　both　in　vertical　and　horizontal　direction　to　the　deposition　direction　and　tested　at　0°C,　−20°C　and　−60°C.　The　fracture　surfaces　of　the　specimen　made　without　any　slag　treatment　after　deposition　exhibit　large　number　of　welding　defects,　which　is　highly　responsible　for　lowering　the　impact　toughness.　On　the　other　hand,　completely　clear　away　of　the　slag　by　extensively　inter-pass　grinding　gives　rise　to　a　fine　grain　structure　and　relative　high　impact　toughness,　however,　a　huge　sacrifice　of　the　production　efficiency　and　high　material　waste　is　unavoidable.　©　2022　Elsevier　B.V.