# Optimization of AZ91D Process and Corrosion Resistance Using Wire Arc Additive Manufacturing
## 3 Results and Discussion  
### Sample Preparation  
Figure 2 shows representative lines printed over a range of printing conditions. Low temperature conditions can be achieved by using fast moving speed and applying low arc current, such as in the case of condition A. In this case, it is noticeable that the wire did not completely melt. Traces of the original wire can be seen in many points of the line. When dealing with low currents (20-30 A) but low printing speeds, such as the case of condition B, mounds of fully molten metal can be seen. It is believed that the high amount of materials deposited due to the low speeds made created the partition of the original line. On the contrary cases, when high currents (>40 A) the metal wire becomes highly fluid. Thus, at printing speeds greater than 170 mm/min (condition D), the molten metal will flow and congregate on the beginning of the printed line, since it is the first solidified region. At lower speeds (<160 mm/min) the molten metal has more time for its solidification. For this reason, the molten metal will flow and conglomerate in those places where the liquid metal has become solid, as seen on conditions E. However, printing conditions in which mostly visually uniform lines was found which was the combination of medium arc currents (30-40 A) and relatively low printing speeds yielded the best results. C shows representative lines printed under conditions laying in this \"printing window\".  
Figure 2: Visual representation of the macroscopically appearance of printed lines created under different printing conditions.  
Through printing, a decrease in the contact angle as a function of the heat conditions was detected. For all the printing parameters the wire feed rate remained constant. The flow rate of both the pilot arc and the shielding gas was also kept constant. Thus, only two out of the four parameters affecting the contact angle of the printed specimens were varied. First, the size of the weld pool, which is in direct correlation with the current intensity. Second, as the supplied current increases, more heat is delivered to the substrate, increasing the size of the molten pool. The printing speed of the build is based on the movement of the melting pool and not on the wire feed rate. The wire feed rate must be greater to allow enough material to melt and fuse to the plate. Also, by increasing the size of the molten pool (increasing current) the contact angle would decrease since the surface energy differential between the molten substrate and the depositing metal would be lower than that between a solid substrate and a liquid filler. Table 3 shows the heat input into the system depending on the printing conditions. Noticeable is the fact that the amount of heat delivered into the system is directly proportional to the arc current and inversely proportional to the printing speed, to calculate the heat input the following formula was used:  
\[Heat\ Input=\frac{Arc\ Voltage*Arc\ Current*Thermal\ Efficiency}{Travel\ speed}\]  
where arc voltage is 25 volts (constant for all the cases), thermal efficiency is 60% for PAW, arc current (Ampere) and travel speed (mm/s) are the variables [21].  
Due to the relatively low melting point of magnesium and its alloys, remelting is a common problem when printing these materials [22]. Thus, as the temperature of the substrate increases because of the increasing number of lines printed over the same plate, the lines become more susceptible to remelting. It is believed that, along the increased size of the molten pool, remelting is a cause of the reduction of contact angle between the substrate and the printed line as more lines are printed over the same plate. Figure 3 shows two examples of lines presenting remelting. A change in contact angle is noticeable on the areas closest to the substrate. This indicates that heat coming from the printing plate was enough to melt part of the printed line.