## 6 Conclusions and Future Work  
A hybrid manufacturing process combining WAAM and milling (HWMP) is proposed in this paper, which provides an entirely new method to fabricate stiffened panels in contrast to existing joining or machining methods. The comprehensive effects of the deposition parameters (including wire-feed rate and travel speed) and the milling parameters (including spindle speed and tool-feed rate) on surface quality, material utilization, and efficiency are investigated systematically. Only good matching of the deposition and the milling parameters could maximize the performances of HMWP. Through the optimization, it is obtained that a high travel speed, a target width dependent wire-feed rate, a high spindle speed, and a moderate tool-feed rate are required to obtain a good balance between surface quality, material utilization, and efficiency. This provides a guide to select approximate process parameters for hybrid manufacturing processes. With the optimal process parameters, significant reduction in material consumption and improvement in efficiency can be realized without any loss of accuracy for the fabrication of stiffened panels, compared against the traditional machining. Therefore, HWMP that accords with the concept of sustainable development would be attractive to the manufacturers.  
Despite the above advantages, the WAAM process based on welding may introduce undesirable residual stresses and, therefore, distortions due to the large heat input, especially for thin structures. Several measures could be taken to solve this problem. For example, some new welding techniques with lower heat input could be used instead of the conventional GMAW, such as cold metal transfer (CMT), dual-bypass GMAW and hot wire GMAW [32]. Some preheating approaches such as induction heating could also be used to regulate the thermal gradient. Additionally, well-designed fixtures are necessary to reduce the distortions. These works will be considered in the future.  
This paper was supported by the National Natural Science Foundation of China (No. 51475009) and a China Postdoctoral Science Foundation funded project (No. 2017M610726).  
[PERSON] performed the modeling and wrote the manuscript; [PERSON] contributed to the optimization and the analysis of the data; [PERSON] designed and performed the deposition experiments; [PERSON] designed and performed the milling experiments; and [PERSON] helped perform the experiments and analyze the data.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**Traditional Machining**} & \multicolumn{2}{c}{**HWMP**} \\ \hline Thick plate’s mass & 5.35 kg & Thin plate’s mass & 1.46 kg \\  & & Metal wire’s mass & 0.55 kg \\ \hline Final part’s mass & 1.82 kg & Final part’s mass & 1.82 kg \\ Material utilization & 34\% & Material utilization & 91\% \\ \hline  & & Deposition time & 24 min \\ \hline Roughing time & 144 min & & \\ Finishing time & 22 min & Milling time & 38.7 min \\ \hline  & & Cooling, etc. & 39.3 min \\ Construction time & 166 min & Construction time & 102 min \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison between HWMP and the traditional machining for the stiffened panel.