To develop ultrafine grains (UFG) in 6.35 mm thick magnesium alloy, stationary shoulder friction stir processing (SSFSP) with steel and copper backing plates was conducted. Steel backing plate produced uniform fine grains (FG) size of 4.98, 4.75, 4.12 μm in top, middle, bottom of the stir zone (SZ), respectively. In contrast, copper backing plate tailored microstructure from FG (4.1 μm) in the top to UFG (0.96 μm) in the bottom of SZ. SSFSP produced uniform and gradient microstructures, altering temperature gradient by placing steel and copper backing plates, respectively. It is worth to note that UFG microstructure achieved without usage of external cooling, owning to the copper backing plate. Most of the grains found under ∼2 μm size in UFG microstructure. FG and UFG microstructures contributed to the enhancement in the ductility and strength, respectively. UFG resulted in significant improvement in hardness and tensile strength by ∼80% and 24% of the base material, respectively. The intensity of strong basal texture throughout the thickness found independent of the backing plate type. Microstructure evolutions across the SZ thickness for both processing conditions are discussed using electron back scattered diffraction (EBSD).

为了在6.35毫米厚的镁合金中形成超细晶粒（UFG），进行了带有钢和铜衬板的固定肩部摩擦搅拌处理（SSFSP）。钢背板在搅拌区（SZ）的顶部，中部和底部分别产生了4.98、4.75、4.12μm的均匀细晶粒（FG）。相比之下，铜垫板调整了微观结构，从顶部的FG（4.1μm）到SZ底部的UFG（0.96μm）。SSFSP产生均匀的和梯度的微观结构，分别通过放置钢和铜背板来改变温度梯度。值得一提的是，UFG的微观结构归功于铜背板，无需使用外部冷却。UFG显微组织中发现的大多数晶粒都在约2μm以下。FG和UFG显微组织有助于延展性和强度的提高，分别。UFG分别使硬度和抗张强度分别提高了约80％和24％。在整个厚度范围内，强烈的基础纹理强度与背板类型无关。使用电子背散射衍射（EBSD）讨论了两种加工条件下SZ厚度的微观结构演变。