The　resulting　properties　of　parts　fabricated　by　wire　and　arc　additive　manufacturing　(WAAW)　processes　are　determined　by　their　microstructure,　local　composition,　and　porosity.　The　objective　of　this　work　is　to　compare　the　difference　of　the　solidification　behaviors　by　means　of　numerical　simulation　when　it　precisely　deposits　material　upon　the　single-　and　multiple-layer.　Here,　a　three-dimensional　(3D)　heat　transfer　and　fluid　flow　model　of　arc-based　additive　manufacturing　to　calculate　thermal-flow　fields,　deposit　shape　and　size,　cooling　rates　and　solidification　parameters　was　developed.　The　calculated　fusion　zone　geometries　for　the　single-　and　multiple-layer　deposition　processes　considering　convective　flow　of　melts　agreed　well　with　the　corresponding　experimental　data　for　AA2319　deposits.　It　was　found　that　under　the　same　process　parameters,　the　cooling　rate　and　the　solidification　rate　of　the　molten　pool　for　the　single-layer　deposition　process　are　always　greater　than　that　for　the　multiplelayer　deposition　process.　Specifically,　the　refined　equiaxed　grains　are　more　easily　obtained　at　a　cooling　rate　greater　than　50　K　ps-1　when　the　polarity　is　designated　as　direct　current　electrode　positive　(DCEP).　Width　of　the　deposited　bead　of　multi-layer　structures　is　significantly　greater　than　that　of　single-layer　structures　due　to　the　lack　of　material　at　the　lateral　sides　and　the　ambient　conditions　for　the　heat　loss.　©　2020　Japan　Welding　Society.　All　rights　reserved.