# Study of the Mechanism of a Stable Deposited Height During GMAW-Based Additive Manufacturing
## 1 Introduction  
Additive manufacturing technology, is a novel technology for making complex parts layer by layer. There are several additive manufacturing approaches, such as directed light fabrication (DLF), laser-based additive manufacturing (LBAM), rapid direct metal deposition, selective laser sintering (SLS), and electron beam free forming (EBF) [1]. Compared with other methods, gas metal arc welding (GMAW)-based additive manufacturing has shown great potential due to several advantages, including high productivity, low cost, and a strong bonding strength of the parts [2]. During the multilayer deposition process of GMAW-based additive manufacturing, either the substrate moves to a low position or the torch rises to a high position with a predefined distance after the welding of a layer finished. The critical issue is that the height of the deposited bead is not always equal to the predefined layer height. The reason can be attributed to a slight change in the manufacturing parameters, such as arc current and voltage as well as deposition rate, and the GMAW process is sensitive to these parameters. Furthermore, the geometry of the layer is related to changes in the heat dissipation of the different layers, interlayer temperatures, the distance between the nozzle and the top surface of the parts, and the geometry and initial temperature of the substrate [3]. Furthermore, the accumulated error in the layer height increases as the multilayer deposition process continues. In other words, the distance between the nozzle and the top surface of the part is inevitably changeable. It is well known that a long distance makes the arc unstable, reduces gas protective effects, and leads to poor quality or formation of the layer. Conversely, a short distance causes welding spatters to adhere to the nozzle and the top surface of the parts even contact the nozzle. These two situations make the GMAW-based wire and arc additive manufacturing process unstable. [PERSON] et al. [4] developed a new method by integrating plasma arc deposition and the milling technology to improve the processing quality of the parts. [PERSON] et al. [5] used a neural network and a second-order regression analysis for predicting bead geometry; they then developed a passive vision sensor system to monitor the nozzle to the top surface distance, and the deviations in thedistance are compensated by the movement of the substrate [3]. However, an increased productivity is desirable in manufacturing processes. The disadvantage of the approach of combining plasma arc deposition and milling techniques is that the process time and cost are increased. The method of adding a passive vision sensor system increases the facilities cost. [PERSON] et al. [6] built a mathematical model that uses the values of the height and width of the first layer to predict the geometries of other layers. In addition, the model predicts the relationship between the geometry of bead and key process parameters [7]. The deposited height is probably affected by the integrated transport phenomenon in the molten pool. To revel this mechanism, it is necessary to make a comprehensive description of molten pool dynamics. The computational fluid dynamics (CFD) method can be used to simulated the situation in the molten pool. [PERSON] et al. [8] developed a three-dimensional weak-coupling modeling method for the arc and metal transport to simulate the arc, the molten pool dynamics, and the droplet impingement during arc welding-based additive manufacturing. This paper combines experiments with numerical simulation, and these experiments include producing thin walls with different parameters using GMAW-based additive manufacturing. The results of the experiments show that when keeping other manufacturing parameters constant, the total height of the thin wall produced by GMAW-based additive manufacturing is almost equal to the predefined layer height multiplied by the number of layers if the value of the predefined layer height changes in a certain range.