## 1 Introduction  
Sheet metal forming has become an important manufacturing process for various industrial applications. Many components are obtained by stretching, shearing and bending, etc. However, each of these forming methods requires dedicated and highly specialized equipment to achieve. As a result, these traditional sheet metal forming processes are expensive and time consuming to manufacture components. In order to meet the modern industrial demand for low cost and low cycle time, SPIF is increasingly adapted in the engineering field. SPIF is a die-less, flexible and rapid forming process. During the forming process, a hemi-spherical tool head is incrementally moving into the desired shape along a pre-defined trajectory forming path. Compared with traditional forming processes, SPIF deforms the material under severe plastic strain as a result of local deformation and through-thickness shearing. SPIF is particularly suitable for the rapid making of prototype products and components in small quantities with high demand on geometry accuracy [1; 2; 3; 4; 5]. For example, recent reports on the making of Al-Cu alloy truncated pyramid component by SPIF have attracted a lot attention due to its advantages on improving geometry accuracy [6]. In terms of material selection for these components, Al-Li alloys with low density, high elastic modulus and high specific strength, are gradually replacing Al-Cu alloys for rocket launchers and many other aerospace components [7; 8; 9; 10; 11]. Hence, it is of great interest to investigate the compatibility of SPIF with Al-Li alloys. However, Al-Li alloy is famous for its complex deformation behaviors and is known to have low formability at room temperature. Compared to Al-Cu alloy, it is easy to produce high internal stresses and microstructural defects during deformation of Al-Li alloy, which cause geometry distortion and cracking of the components during subsequent quenching [12; 13; 14]. Increasing the forming temperature during SPIF is advantageous for material flow, which improves the forming limit and reduces residual stresses. However, to date there have been no detailed reports on optimal processing parameters for SPIF on Al-Li alloys. In die-less forming processes, geometric accuracy is one of the most important indicators to evaluate the formability and integrity of the components. In recent years, a large number of scholars have conducted a lot of research work on the formability aspects of SPIF. [PERSON] [15] concluded that sheet thickness is the most important influencing factor on formability. The possible reason is the s with different thickness can lead to different surface strains during forming. At the same time, feed rate and tool diameter have little influence on the overall surface roughness. [PERSON] [16] found that forming tool rotation and lubrication state have little effect on stress but have a significant effect on formability. [PERSON] [17] found that mineral oil can effectively improve surface roughness and can form forces and friction. [PERSON] [18] investigated the effect of feed rate and rotation speed of forming tool on roughness and developed a mathematical model to effectively predict creases. [PERSON] [14] concluded that good formability can be obtained to the maximum extent by designing the trajectory of the forming tool as a spiral trajectory route. Specifically, the forming angle is reported to be a key factor to achieve good geometry accuracy of the SPIFed component. Studies show that the larger the forming angle, the smaller the forming force [19; 20; 21]. This is because the increase in angle can reduce the stiffness of the sheet. The formed components have better formability. [PERSON] [6] reported the geometric accuracy and residual stresses of truncated pyramid component made of AA2219 age-hardened alloy. The yield strength of the initial sheet was found to have a significant effect on the geometric accuracy. Secondly, solution treatment, SPIF and age-hardening can improve very high geometric accuracy.