**5. Results and discussion**

#### **5.1 Characterization of wire batches**

The chemical compositions of S Ni 7718 wire batched are listed in **Table 1**. The AM batch has lower contents of Cu and Co. Furthermore, it has slightly higher contents of C, Nb, and Fe than the standard variant. These elements influence the precipitation of the Laves phase, MC-type carbides, and TN in the weld metal [24–26]. Furthermore, the solidification cracking of austenitic materials is strongly dependent on the solidification temperature range (STR) and especially the solidification at the grain boundaries and interdendritic regions. The Nb/C ratio affects the amount and distribution of the γ + NbC eutectic and γ + Laves eutectic, which influence the STR. Addition of Nb at higher C levels promoted γ/NbC eutectic type constituent and at higher Si and Fe


**Table 1.**

*Chemical composition of different wire batches of S Ni 7718 (wt%).*


#### **Table 2.**

*Mechanical properties at RT and roughness of solid wire electrodes.*

levels promoted γ/laves [27]. When Nb/C ratio was increasing, the solidification temperature range (STR) was reduced [28].

A basic requirement of a stable gas-metal arc welding process is a permanently constant feed of the wire electrode, even with cable assembly of different lengths. The main factors influencing this are the surface condition of the wire electrode, mechanical properties, and also technological characteristics. Basically, the wire electrode for GMAW should have a high tensile strength (UTS) with sufficient elongation at rupture (A) to ensure high stiffness even in long cable assemblies [29]. Both wire variants guarantee these requirements (**Table 2**). For the sliding behavior in long hose assemblies, the basic rule is that with lower roughness (Rz) and greater pre-bending, the sliding ability of wire electrodes in the hose assembly increases [30]. The AM variant has a lower average roughness values, which also correlate with the appearance of the wire surface. Despite the different properties and characteristics, no significant influence on the wire feeding and thus on the process behavior was found under the selected test conditions.

#### **5.2 Heat input and deposition rate**

An arithmetic mean value for all weld beads of the wall or block was formed from the mean value of the respective stringer bead read off the power sources. The tolerance given is the average of the positive and negative error indicators. The heat input (E) is the quotient of arc power (Parc = I x U) and travel speed (TS). The deposition rate (DR) is the amount of wire melted per unit time. Despite the same synergic line and setting parameters, differences in the measured values for current (I) and voltage (U) occurred for both current sources and consequently in the heat input (E) and deposition rate (DR) when producing the walls (**Table 3**). The large blocks were welded only with a high wire feed speed and a low welding speed to generate a high deposition rate (**Table 4**).

Moreover, this combination represents the critical case, since the heat input is comparatively high. Fourteen layers of nine stringer beads per layer each were made. Due to the long production time of 10.5 hours resulting from the interpass temperature, only one block was welded at a time. The welding time was only 70 minutes.

**Tables 5** and **6** show the weld parameters of walls and blocks welds with the varied shielding gases. It can be seen that the shielding gas has a slight influence on these

*Properties of Additively Manufactured Deposits of Alloy 718 Using CMT Process Depending… DOI: http://dx.doi.org/10.5772/intechopen.102455*


#### **Table 3.**

*Welding parameters for CMT-WAAM® of walls, welded with ArHeHC.*


#### **Table 4.**

*Welding parameters for CMT-WAAM® of blocks, welded with ArHeHC.*


#### **Table 5.**

*Mean values for CMT-WAAM® welds (standard batch, PS1, WFS: 9.0 m/min, TS: 0.6 m/min).*


**Table 6.**

*Mean values for CMT-WAAM® welds (AM batch, PS1, WFS: 9.5 m/min, TS: 0.6 m/min).*

#### **Figure 4.**

*Cross sections of stringer beads depending on shielding gas: (a) standard batch (b) AM batch.*

parameters. The lowest heat input is generated when ArHeO is used, while ArHeHC results in the highest values.

Cross sections of stringer beads as a function of shielding gas show the lowest wetting angles θ when ArHeHC is used (**Figure 4**). This gas caused the highest heat input. The wetting angle increases with all other shielding gases. A low wetting angle has a positive effect on reducing lack of fusion in multi-pass welds.

#### **5.3 Cooling time**

Infrared (IR) pyrometers were used to measure the temperatures directly on the seam surface for every second layer at the wall structures. Since there is no specific temperature interval for the cooling time for Ni-based materials, this was calculated for cooling from 1000–600°C. **Figure 5** shows the cooling times t10/6 depending on heat input and layer for both batches when welding the walls. In the first layers, shorter cooling times occur due to the still possible heat conduction into the substrate. From about the 10th layer, the 3D changes to a 2D-heat conduction, which is why the cooling time remains almost constant. As expected, the cooling times increase with increasing heat input when welding the walls. At the highest heat energy of 414 J/mm, the cooling time is about 21 s. And at the lowest heat input of 105 J/mm, the cooling time is about 7 s.

The blocks cool significantly faster at comparable setting values and similar heat input due to the 2D-heat conduction. This is shown as an example in **Figure 6a** for the S Ni 7718 AM. The average t10/6 times are approximately between 4 s and 5 s. The further layers then cool down somewhat more slowly. In addition, it can be observed that the outer beads of each layer in the upper layers also usually cool down somewhat slower than the inner beads (**Figure 6b**).

#### **Figure 5.**

*t10/6 cooling times of S Ni 7718 walls: (a) standard batch (b) AM batch depending on number of layers and heat input (shielding gas: ArHeHC).*

*Properties of Additively Manufactured Deposits of Alloy 718 Using CMT Process Depending… DOI: http://dx.doi.org/10.5772/intechopen.102455*

**Figure 6.**

*Mean t10/6 cooling times of S Ni 7718 AM blocks depending on (a) layer and (b) bead (shielding gas: ArHeHC).*

#### **Figure 7.**

*t10/6 cooling times in walls of S Ni 7718: (a) standard batch and (b) AM batch depending on number of layers and shielding gas.*

#### **Figure 8.**

*Mean t10/6 cooling times in blocks of S Ni 7718 depending on shielding gas: (a) standard batch (b) AM batch.*

The shielding gas exerts only a minor influence on the cooling time, since the amounts for the heat input do not differ significantly (**Figures 7** and **8**).

#### **5.4 Nondestructive testing**

As a result of the visual inspection, no external defects such as cracks, lack of fusion or pores were observed. Only some welds of the standard charge of S Ni 7718 with shielding gases containing active gas components (CO2 and O2) showed dark particles on the weld surfaces (**Figure 9**).

#### **Figure 9.**

*Surfaces of S Ni 7718 blocks depending on shielding gas: (a) standard batch (b) AM batch.*

It is assumed that metallurgical reactions with the oxygen-affine elements Al or Cr occur due to the size of the molten pool and slow cooling. On the stringer beads' surfaces of welded walls fabricated with higher heat input, these particles also occurred, but in both wire batches. However, no deterioration of the CMT process stability was observed.

The penetration tests on walls and blocks resulted in some red indications independent of wire and welding speed (**Figure 10**).

On the cross sections of walls, it was determined that these defects always occur in the last welded layer of walls and in some cases extend to the surface (**Figure 11**). Due to their structure and dimensions, these phenomena are not classified as solidification cracks or pores but as micro blowholes. Since the components are subsequently machined, these defects are not significant.

The X-ray examinations showed no or very small porosity of less than 0.001%. The porosity is the quotient of the sum of the areas of all pores and the X-rayed weld metal area. **Figure 12** shows an X-ray image of a wall with one pore. No radiographic tests were performed on the blocks due to the low image quality number, as they did not provide sufficient information.

#### **Figure 10.**

*Example of penetration test of last layer surface with red indications.*

#### **Figure 11.**

*Cross sections (Y-Z plane) with examples for micro blowholes: (a) 100x magnification (b) and (c) 1000fach.*

*Properties of Additively Manufactured Deposits of Alloy 718 Using CMT Process Depending… DOI: http://dx.doi.org/10.5772/intechopen.102455*

**Figure 12.** *X-ray test on the example of S N 7718 AM wall welded with ArH.*

### **5.5 Chemical analysis**

Since a shielding gas with low active and reducing shielding gas components was used and no brushing between the individual layers took place, the chemical composition of welds was determined. **Table 7** shows the values of the additive welded wall as a function of the different shielding gases in comparison with the element contents of wire batch.

An influence of shielding gas or component geometry and heat input on the chemical compositions cannot be proven. If deviations occur, they are more likely to be due to measurement inaccuracies resulting from calibration tolerances. Thus, the weld metals also show the same tendency with respect to the Nb/C ratio.

The O- and N-contents of part geometries of both batches welded with ArHeHC show a slight increase compared with the wire batches (**Figure 13**). A similar behavior is shown for the blocks produced with the different shielding gases (**Figure 14**).

However, the weld metal of the standard batch dissolves slightly more oxygen and nitrogen compared with the AM batch. In addition, the reducing effect is observed for the shielding gas with the highest H content (Ar with 2% H2). The low O2 and CO2 contents do not cause oxygen pickup by the weld metal.


**Table 7.**

*Chemical composition of wire batches of S Ni 7718 blocks depending on shielding gas (wt%).*

**Figure 13.**

*ON-contents of S Ni 7718 walls and blocks depending on wire batch (shielding gas: ArHeHC).*

**Figure 14.**

*ON contents of S Ni 7718 blocks depending on shielding gas.*

#### **5.6 Macrostructure and defects**

**Figure 15** shows the macro cross sections (Y-Z plane) of the walls of different wire batches. At constant wall height, the weld width increases with rising deposition rate and the number of layers reduces. As a result of the lower deposition rate when welding with the AM batch, more plies had to be welded to achieve equal weld heights. In addition, comparatively large thickness reductions are recorded in the lower wall structure area when welding with 6.0 m/min wire feed and 1.0 m/min travel speed. To avoid this effect, it would be necessary to adjust the wire feed speed or, if necessary, to preheat the substrate sheet, but this was not the focus of these investigations. In order to obtain an evaluation of the lateral surface waviness, the end contour proximity (ECP) was determined according to Eq. 3.

$$\text{ECP} = \frac{\text{W}\_{\text{inside}}}{\text{W}\_{\text{outside}}} \cdot \mathbf{100} \text{ [\%]} \tag{3}$$

Since the sections have been split, the lower value of the nominal wall width and the higher value of the actual wall width are relevant. During the evaluation, the upper rounded and the lower, partially constricted areas of the samples were not considered. All wall structures except one wall achieved an ECP of ≥75%, which is according to [31] a good value. Since the image quality indicator of the X-ray inspection is not sufficient to detect micro hot cracks, the cross sections of the walls and blocks were inspected for internal seam defect. With one exception, there were no hot cracks in the walls. Only in one cross section of the AM batch of S Ni 7718 hot cracks were separated and visible in the second and third layers (**Figure 16**).

*Properties of Additively Manufactured Deposits of Alloy 718 Using CMT Process Depending… DOI: http://dx.doi.org/10.5772/intechopen.102455*

**Figure 15.** *Cross sections (Y-Z plane) of S Ni 7718 walls: (a) standard (b) AM welded with ArHeNC.*

**Figure 16.**

*Hot cracks in the lower layers of S Ni 7718 AM wall (WFS:9.0 m/min,TS: 1.0 m/min, ArHeHC).*

For the blocks, the higher welding feed speed in a wider block with the same number of layers and beads (**Figure 17**) can also be seen. The machining allowances are comparable to those of the walls. However, the ECP is very high due to the large width of the block. When the blocks are welded, lateral tracking of the beads can be observed on each side in each position. Due to this, the process is more unstable and a lot of weld spatter occurs. In principle, the wire feed speed would have to be adjusted for these weld beads, but this has not been done.


#### **Figure 17.**

*Cross sections (Y-Z plane) of S Ni 7718 blocks: (a) standard, (b) AM welded with ArHeHC.*


#### **Table 8.**

*Defects in S Ni 7718 blocks, welded with ArHeHC.*

Due to the significantly larger number of weld beads in large blocks, the thermalmechanical reactions were higher than for the walls, so that hot cracks occurred in both blocks despite faster cooling. In addition to the hot cracks, also few lacks of fusion were also found. A quantitative evaluation of the number of cracks and the weld metal arearelated crack length (Eq. 4) shows a significantly higher hot crack sensitivity of the AM batch of S Ni 7718 (**Table 8**). This confirms the investigation results of [27, 28] that a higher Nb/C ratio results in greater susceptibility to hot cracking.

$$\text{CL} = \frac{\text{Total length of cracks}}{\text{well metal area}} \ \left[ \mu \text{m/mm2} \right] \tag{4}$$

**Figure 18** shows the blocks produced with different shielding gases. Eq. 1 and Eq. 2 (**Figure 3**) were used here to determine the center distance d between the beads. At the side block edges, there is always a lateral flow of the melt, which leads to an uneven deposit. While the calculated center distance was set for the blocks of the AM batch, a fixed spacing of 5.5 mm was set for welding with the standard batch. This, together with the higher wire feed speed of 9.5 m/min, resulted in wider blocks with lower heights for the same number of beads per layer in the AM batch.

The evaluation of the macro sections showed for the blocks from the AM batch not only many hot cracks but also a large number of lacks of fusion (**Figure 19**, **Table 9**). But these defects also occurred in the blocks of the standard batch. No internal defects were visible on the walls of S Ni 7718.

#### **5.7 Mechanical properties at room temperature and hardness**

**Tables 10** and **11** show the mechanical properties at room temperature of S Ni 7718 walls and blocks welded with ArHeHC.

*Properties of Additively Manufactured Deposits of Alloy 718 Using CMT Process Depending… DOI: http://dx.doi.org/10.5772/intechopen.102455*


#### **Figure 18.**

*Cross sections (Y-Z plane) of the block structures of S Ni 7718 batches: (a) standard (b) AM.*

#### **Figure 19.**

*Example of unacceptable defects in S Ni 7718 blocks of AM batch, welded with ArHeHC.*


#### **Table 9.**

*Defects in S Ni 7718 blocks, welded with different shielding gases.*


**Table 10.** *Summary of tensile properties and hardness measurements of S Ni 7718 walls (shielding gas: ArHeHC).*

*Properties of Additively Manufactured Deposits of Alloy 718 Using CMT Process Depending… DOI: http://dx.doi.org/10.5772/intechopen.102455*


**Table 11.**

*Results of tensile and charpy tests as well as hardness measurements of S Ni 7718 blocks (shielding gas: ArHeHC).*

**Figure 20.**

*Mean values of tensile properties of S Ni 7718 walls (a) and blocks (b), welded with ArHeHC.*

If the influence of wire batch is considered independently of the geometry, the AM batch leads to a lower elongation at rupture (A) compared with the standard batch (**Figure 20**). The values for the 0.2% yield strength, on the other hand, behave divergently, while no significant difference occurs for the tensile strength. Considering the influence of the geometry, the blocks show a slightly higher tensile strength, 0.2% yield strength and hardness, while the elongation at rupture is significantly lower. The reasons for the higher strengths of the blocks compared with the walls are probably due to the faster cooling.

An influence of the shielding gases on the strength is not detectable (**Table 12**). The impact energy values determined scatter slightly, but a correlation with the shielding gases cannot be demonstrated (**Table 13**).

For the identification of an aging effect on the deposit hardness, local hardness maps (HV0.2) were performed over 2–3 layers in the center of the walls and blocks welded with the highest heat input (**Figure 21**). The results of the walls show no significant changes in hardness, indicating an age hardening effect.

#### **5.8 Microstructure of deposit weld metals**

Already on the polished cross sections of walls and blocks, a large number of irregularly distributed cubic particles are visible, which are either TiN or NbC (**Figure 22**). The nitrides containing a certain amount of Ti are already formed in the melt at a later stage of solidification [24].

The Laves phase is already clearly visible in the etched sections at the light microscope at sufficiently high magnification (**Figure 23**).

The scanning electron microscope (SEM) images demonstrate the precipitated phases in the microstructure of samples, such as Laves Phase and complexe NbC, Ti N-particles (**Figure 24**).

The precipitation of these phases appeared for all walls and blocks. The brittle Laves phase is generally considered to be the major microstructural segregation


**Table 12.**

*Summary of tensile properties and hardness measurements of S Ni 7718 walls depending on shielding gas.* *Properties of Additively Manufactured Deposits of Alloy 718 Using CMT Process Depending… DOI: http://dx.doi.org/10.5772/intechopen.102455*

