Fabrication of Thin Walls with and without Close Loop Control as a Function of Scan Strategy Via Direct Energy Deposition

A RAPID code from ABB robotics was developed to fabricate the thin-walled structures. For creating simpler geometries, it was deemed to be more feasible, to use manual programming rather than a slicing software. The former statement holds as with manual programming there is more control over the deposition pattern.

The specimens were built with and without the closed loop control. All the parameters were kept constant except laser power which was controlled by CLAMIR in case of closed loop-controlled samples. Constant and gradual reduction in laser power were employed. Since the constant power mode resulted in inconsistent wall therefore manual reduction of power was employed to clearly examine the effect of scan strategy among the samples. In constant power mode the power was fed at constant rate via the controller whereas in manual power reduction mode the power was reduced via 100 W with each successive layer. This value was selected on previous handling of same powder and from viewing the data of CLAMIR but was mainly based on hit and trial approach. Two different scan strategies were used and were named as Unidirectional and Bidirectional as depicted in the Fig. 2. With unidirectional scan strategy the laser was turned off for the return path of the clad and vice versa for the bidirectional. A total of 70 layers were deposited for all the samples.

The geometric consistency is one of the most important features for any deposition. Visual inspection shows a similar trend for walls fabricated with and without CALMIR (Fig. 8). It is interesting to note that in both samples unidirectional scan strategy was employed. i.e., laser was kept on only on the start of the layer and was switched off on return. The use of closed loop control/manual power reduction does not show any substantial effect in keeping the geometry of wall.

When constant power mode was used with the same number of layers the wall only reached in height almost half as compared to controlled power (for both manual power reduction mode and for CLAMIR controlled) even though all the other parameters were kept constant. This phenomenon is well discussed in literature. With constant power for the initial layers more and more heat gets dissipated through the substrate. But as the layer advances more radial heat dissipation start to occur and hence the outwards velocity of melt pool flow increases due to outward Marangoni’s Effect [27]. This causes an uneven width of the layers and in this case an increase of clad width which caused a decrease of overall wall height. Also, gradually the layers were seen to be bent through the middle. Due to this effect gradual reduction of laser power is advised by many researchers [6, 10, 28, 29]. By manually reducing power by 100 W with each successive layer (Fig. 9(b)) an overall geometric consistency like ClAMIR controlled samples (Fig. 12) was achieved.

However, from table of results (Table 4) it is evident that the width in case of manual power reduction could not be controlled with much precision, in fact, the width increased with height having a maximum value of 3.2 mm (Fig. 10).

The clad width in case of closed-loop control (CLAMIR) with bidirectional scan strategy was constant and the wall height was also like what was predicted. Figure 12 shows these fabricated samples. When the melt pool width was forced to be controlled at 2 mm via CLAMIR the width of clad decreases with wall height. The un-melted powder was seen to be stuck to the wall Fig. 12 (a). The reason is that on inspection of CLAMIR data it was found that to keep the melt pool width at 2 mm the laser power was reduced to almost 250 W. The graph from CALMIR data supports this as shown in Fig. 11.

This reduction occurred gradually but since the powder feed rate was constant therefore this low laser power was unable to melt enough powder. Thus, a system coupled with close loop control for power as well as for feed rate adjustment would bring a near ideal system. This is also the reason of reduced clad width for the topmost layers where the powder is seen to be stuck to the outer surface of the wall.

The CALMIR was set to auto calculate the melt pool during the first three layers and then implement the control from the fourth layer in case of sample 5. A good geometric consistency for both the clad widths, over all height and uniform clad length was achieved. Since the melt pool width depends greatly on laser power and powder feed rate, therefore, when control was operated from system given values, better results were seen. The obtained graph for sample 5 is given in Fig. 13. The auto calculation of width for sample 5 for the first three layers, reveals that the reference width changes from 2 mm to 2.31 mm which can be seen in graph from Fig. 13 and then CLAMIR uses this reference width to control the power till the end of fabrication.

The microhardness measurements were taken for substrate, dilution zone (depth of melt pool below the substrate level) and the wall for all the samples. Vickers Microhardness with 300 g load for a dwell time of 15 s was used. Since wall was also cut into half therefore the position (mm) mentioned in graphs was combined for all the three areas (wall start, mid and extreme) and therefore depicts only the increments between successive indents for each of three, but when combined this distance does not reflect the exact position of indent on the wall. For example, for sample 1 the position (mm) value is 5.6 mm but in fact it is showing the extreme end of the wall which reached a height of 17 mm as from Table 4. The three areas are therefore mentioned in the graphs for each of the samples. In all the cases regardless of closed loop control the hardness value was found to be high for dilution zone. Microhardness decreased as the wall progresses in height. The first layers also had higher values due to the dilution between 316L and the low carbon steel substrate as also shown by the Abdalla R. Nassar [9]. The middle layers of the wall had low values of hardness as compared to both start and the top layers. This attribute is typical for DED fabricated parts due to cyclic thermal history as the heat build-up in middle layers is higher as compared to both first and last layers. Due to this reason the central layers have less micro hardness values compared to first and last layers of DED fabricated parts [30]. This trend is most visible for sample 4 shown in Fig. 15. The graph for the hardness values for samples fabricated without closed loop control and with manual power reduction mode is shown in Fig. 14. The graph in Fig. 15 represents the microhardness values for samples fabricated via closed loop control. There was a scatter in the values within the three defined areas. Since sometimes the indent might be placed just on the interface between the two layers that returned higher values. This was evident after the etching of the samples with higher scatter values. In all the cases the laser power had a significant effect on the hardness values. Upon analysing samples which were geometrically inconsistent it was found that the microhardness values were lower as compared to the other samples. Since in this case the section was taken when the layer just started towards its length. While all the other samples were cut through the middle. This shows that the hardness values change along the height as well as along the layer length. This could be due to the different temperature gradient along the laser scan direction as compared to the temperature gradient along the height of thin wall as also demonstrated by the Wang et. al. [31]

The images of polished samples taken from Microscope were evaluated for density analysis by using ImageJ software by conversion to binary images. Overall, it was found that samples with excellent density were fabricated in case of all the samples. This attributed to the feasibility of process parameters for fabrication of such thin-walled structures. The highest density was recorded for the sample#1 (fabricated via constant power). The constant laser power induces almost fully dense part but compromised the geometrical features of the wall. The density results obtained for each sample are reported in Table 4. The porosity increased upon using closed loop control yet managed to fabricate thin wall with good geometrical features. This increase of porosity occurred due to the excessive reduction of laser power by closed loop control system.

The microstructure of DED manufactured parts cannot be predicted with much reliability due to complex thermal behaviour. Process parameters that influence the height cooling/heating rate, bulk temperature increment define the grain size and morphology of microstructure [32]. The microstructure of sample1 is shown in Fig. 16.

The microstructure for sample 4 for the 1^st and the last layer is shown below in Fig. 17.

On comparison between the samples it can be seen that the microstuctre of samples represent the columnar dendritic structure. This is typical for parts manufatured via DED. The layers were found to be free from defects. The samples manufactured via close loop control somewhat represents a more homogenous microstructure as compared to the ones that were built with constant power. This could be due to the fact of better cooling condions due to gradual reduction of laser power. The microstructure reulted coincides with the work of Bi et al. Saboori et al. [33] also shows that the microstructre differs in the middle and edges of the meltpool. For overall part however, the microstructre grows with columar dendritic structute in direction of maximum thermal gradient dominates for the first and middle layers. For the last layers cellular structures are reported [33]. Figure 18 shows the microstructure of substrate. The images taken for the measurements of depth of melt pool below the substrate level are also given as reference (Fig. 19, Fig. 20).

To achieve a metallurgical bond a minimum amount of bonding is required between the subsequent layers which is quantified by using a dimensionless parameter called dilution. This attribute is more common where clad layers are deposited to improve the tribology of substrates such as corossion and wear. Nevertheless the dilution among the layers impacts the final microstructure due to subsequent melting and remelting of sucessive layers [34]. However in this study the penetration depth between the melt pool below the substrate level is reported. A stable bond between the substrate and the clad can be seen from the Fig. 19 and Fig. 20. The low penetration depth of melt pool into substrate for sample 4 could be explained with the extreme decrease of laser power that was commanded by CLAMIR to keep the melt pool at 2 mm width.