Mg alloy enjoys the advantages of high specific strength and stiffness, good machinability, good shock absorption, high thermal conductivity, etc. It is popularly used in national defense, aerospace, chemical industry, automotive, biomedical, electronics, and other fields [1, 2]. Because of its crystal structure, Mg alloy has low formability and forming limit at room temperature and usually works in ‘‘warm conditions’’ [3, 4]. The warm Single Point Incremental Forming (SPIF) process is a highly flexible technology that can remarkably improve the formability and forming limit of Mg alloys [5, 6]. However, the orange peel patterns are sometimes observed on the non-contact surfaces of formed parts of Mg alloy in the warm SPIF. Strong abrasive and adhesive wear are also seen on the contact surfaces (which contact with the tool). They seriously affect the surface quality of the formed parts.
Recently, researchers have shown interest in the orange peel of non-contact surfaces in the warm SPIF process for Mg alloy. Leonhardt et al. [7] reported a conspicuous orange peel on the surface of the formed parts through the warm SPIF experiment using hot air. Varying degrees of roughening of orange peel on the surface appeared at different temperatures. Liao et al. [8] evaluated the influence of the heating modes on the orange peel phenomenon and suggested that the occurrence of orange peel patterns was significantly affected by the temperature distribution. Davis [9] and Carter et al. [10] reported that oversized grains were more likely to cause orange peel on the surface of the part. Antonyswamy et al. [11] believed that the appropriate temperature and rapid strain rate play a vital role in the orange peel effect. However, several researchers have focused on the wear of contact surfaces in the warm SPIF process. Duflou et al. [12] noticed that the increase in temperature significantly increased the contact friction coefficient of the parts, thereby affecting the surface quality of the contact surface. Xu et al. [13] investigated the causes of rough surface finish and concluded that the high temperature and contact pressure at the tool-sheet interface could cause scratches, resulting in a rough surface finish. Zhang et al. [14] investigated the effect of the lubricating method on the surface quality between the Mg alloy sheet and tool in warm SPIF and found an association of the adhesive wear to temperature and lubrication method. Göttmann et al. [15] found that the strong abrasive wear of the tools and the worked metal sheet surfaces of titanium is a typical problem in the warm SPIF.
Surface quality is an important standard to determine product quality. More complicated factors affect the surface quality of SPIF parts than that of conventional sheets forming. Several scholars have examined the effect of various process parameters, such as feed rate, forming force, step size, tool diameter, tool shape, spindle speed, wall angle, and lubrication conditions on the surface roughness of different parts formed by SPIF. Mulay et al. [16] investigated the influence of the step depth, feed rate, and tool radius on the surface roughness of the formed parts. Ajay and Vishal [17], in their study of the influence of different tool shapes on surface roughness, highlighted that the surface quality of parts formed with hemispherical tool shape is better than other parts. Wang et al. [18] argued that a reasonable combination of the rotating speed of the tool and step depth could improve surface quality. Attanasio et al. [19] observed the best surface quality on an automotive component with the optimization of the tool path in two-point sheets incremental forming. Diabb et al. [20] and Xu et al. [21] observed an improvement in the surface quality of Mg alloy by changing the lubricating methods on the sheet in warm SPIF. Zhang et al. [14] improved the surface quality of Mg alloy in warm SPIF using a novel lubricating method in which the potassium titanium oxide (K2Ti4O9) whisker was used.
In other respects, Sisodia and Kumar [22] used a dummy sheet to avoid the direct contact between tool and target sheet and revealed that the existence of a dummy sheet significantly improved the final surface quality of the target sheet. Amini et al. [23] studied the SPIF process of AA 1050 aluminum sheets using ultrasonic vibration (UV) numerically and experimentally. The results showed that applying UV would increase sheet formability and decrease surface roughness. Long et al. [24] and Li et al. [25] studied the impact of ultrasonic vibration on force reduction and deformation behavior of Al alloy in SPIF at room temperature. They documented that the UV could decrease forming force and friction. Sakhtemanian et al. [26] investigated the effect of UV-assisted on the SPIF process of St/Ti bimetal sheet at room temperature and showed that ultrasonic vibration decreased the coefficient of friction and refined grains. The effectiveness of ultrasonic vibrations to improve the surface quality also occurs in other forming processes such as grinding [27], polishing [28], bending [29], and grinding [30], etc. The impacts of UV on the surface quality of the SPIF process in hot conditions, however, remain undefined.
In the current study, ultrasonic vibration was introduced into the warm SPIF process of Mg alloy. The roughness and surface topography of Mg alloy surfaces were measured. The effects of the ultrasonic vibration on surface quality, including orange peel and adhesive wear, were examined. The intrinsic relation between the orange peel and microstructure of Mg alloy is explored with the use of microstructure examination. Lastly, the effect of three kinds of lubricants on contact surface quality was analyzed.