## 1 Introduction  
Additive manufacturing (AM), also known as 3D printing, offers significant advantages in terms of reduced buy-to-fly ratios, improved design flexibility, and shortened supply cycle compared to traditional subtractive manufacturing [1]. A variety of materials are now available for AM applications including metal, ceramic, plastic, etc. [2]. For metal materials, the related AM techniques mainly include selective laser sintering (SLS), selective laser melting (SLM), laser engineered net shaping (LENS), electron beam melting (EBM), electron beam freeform fabrication (EBF3), wire arc additive manufacturing (WAAM), etc. [3, 4]. These techniques differ in terms of energy source (laser, electron beam, or welding arc) and feedstock (powder or wire). Nevertheless, no matter which energy source and which feedstock are adopted, it is still difficult to fabricate parts with the same level of geometric accuracy and surface quality as traditional subtractive manufacturing due to the stair-stepping effect and the liquidity of molten metal [5]. The emergence of hybrid manufacturing, integrating additive and subtractive processes into a single setup, has provided a fundamental solution to overcome the above obstacle [6; 7; 8]. Hybrid manufacturing is realized by alternating additive and subtractive processes every one or several layers, the former producing near-net-shape raw part, whereas the latter refining the raw part to achieve the desired geometric accuracy and surface quality. Hybrid manufacturing makes full use of the advantages of each individual process while minimizing their disadvantages. In recent years, various hybrid manufacturing techniques, such as hybrid layered manufacturing [9], hybrid plasma deposition and milling [10], 3D welding and milling [11], iAttractive [12], etc., have been developed. Parts with high buy-to-fly ratios or with internal and overhanging features that are difficult/expensive to fabricate with traditional manufacturing techniques will favor hybrid manufacturing. A hybrid WAM and milling process (HWMP, for short) is focused on in this paper. WAM, employing welding arc as the energy source and metal wire as the feedstock for additive manufacturing purposes, is especially well known for its high productivity, low cost, high material utilization, and high energy efficiency [13; 14]. Particularly, the deposition rate of WAM can reach up to 50-130 g/min with almost no limitation of the build volume, compared to 2-10 g/min in laser- or electron beam-based processes [15]. Thus, WAM is considered as a more economic and efficient option for fabricating medium to large-scale metal parts compared to other metal AM techniques. In recent years, WAM has drawn significant interests from both academia and industry covering various types of materials. [PERSON] studied the relationship between depositing mode and porosity, microstructure, and properties in WAM of Al-6.3%Cu alloy [16]. [PERSON] investigated the effects of heat accumulation on stability of deposition, oxidation, geometrical shape, arc characteristics, and metal transfer behavior in WAM of Ti6 Al4V alloy [17]. [PERSON] studied the oxide accumulation mechanisms and the influence of oxides on the subsequent deposition in WAM of maraging steel wall structure [18]. BAE systems have applied this technique to build large components, such as 1.2 m Ti6 Al4V wing spar [19]. A typical application of HWMP is to fabricate stiffened panels, which have wide applications in aviation, aerospace, and automotive industries, etc., due to the advantages of light-weight and strong load-bearing capability, as shown in Figure 1a,b [20]. Generally, a stiffened panel can be fabricated either by joining the stiffeners to a thin plate via fasteners (rivets or bolts, see Figure 1c) and welding (friction stir welding or laser beam welding, see Figure 1d), or by machining from a thick plate (see Figure 1e).