# The Influence of Heat Input on the Surface Quality of Wire and Arc Additive Manufacturing
## 1 Introduction  
Wire and arc additive manufacturing (WAAM) is a new manufacturing method that uses electric arc as a heat source and wire as raw materials to form parts in a layered manner [1]. Because aluminum alloy has the characteristics of light weight, high specific strength and good corrosion resistance, it has been widely used in automobiles, ships, aerospace, and other fields [2, 3]. Al-Mg alloy has good welding performance, high strength and good thermal conductivity. In recent years, it has attracted more and more attention in the field of additive manufacturing [4]. Compared with the laser-based additive manufacturing process, WAAM has high deposition efficiency and low manufacturing cost [5]. Additive manufacturing is a powerful tool for the aerospace industry [6, 7, 8, 9, 10]. In WAAM, the high deposition rate is the main advantage of the process, and the cost of this advantage is a large amount of heat input [11]. Heat input affects the thermal gradient, nucleation rate, and grain growth rate during WAAM, and has a direct impact on the mechanical properties and surface quality of WAAM parts [12]. The heat transfer during WAAM is mainly heat radiation and heat convection in the molten pool, and the cooling process after WAAM is mainly heat conduction to the base material and air [13]. Controlling the transfer and emission of heat has a greater impact on the quality of parts, which illustrates the importance of process temperature monitoring and control.  
A mass of heat input will produce extensive residual stress and deformation, and the residual stress and deformation will reduce the mechanical properties and shape accuracyof the parts. How to improve the surface quality of WAAM parts has become a research hotspot [14,15]. [PERSON] observed that, in the WAAM Al-Cu-Sn alloy, as the heat input increases, the size and number of pores also increase, which not only leads to a decrease in its mechanical properties, but also a decrease in the surface quality of the parts [16]. [PERSON] studied the influence of the preheating and cooling of the substrate on the weld bead profile in WAAM [17]. In order to solve the problem caused by heat input, [PERSON] used roll pressure to control the residual stress of the manufactured part in the WAAM process, which not only refined the crystal structure, but also reduced the deformation of the part [18]. In addition, [PERSON] proposed a new process planning method, while maintaining high manufacturing efficiency, by applying different heat input at different positions to control the transformation of the crystal structure, so as to minimize defects in the parts [19]. [PERSON] found through experiments that the surface quality can be improved by optimizing process parameters to reduce heat input [20]. However, the current research focuses on the methods of improving the surface quality of the parts manufactured by WAAM, and the systematic evaluation of the surface quality is less researched. There are large subjective errors in the evaluation of the surface quality, and the surface quality cannot be quantified. The characterization is not conducive to further improving the surface quality of the parts manufactured by WAAM [21].  
In this paper, a single-layer single-channel aluminum alloy sample was fabricated by WAAM under different heat inputs by changing process parameters, and temperature changes during the forming process were monitored by thermocouple, and an evaluation method was established based on 3D scanning data. Using this quantitative evaluation method, it is found that heat input has a higher degree of influence on the surface quality of arc additive manufacturing samples, and higher or lower heat input has an adverse effect.