**3. Results**

## *3.1. Test Rig*

The experiments with the initial material combination of the aluminum alloys EN AW-1050 and EN AW-6060 in T4 temper revealed a welding window for collision angles β between 4.5◦ and 6.6◦ at a constant impact velocity *<sup>v</sup>*imp of 262 m/s. Increasing the roughness of the parent surface to 15 μm narrowed and shifted the range for successful welds to a collision angle of 9.1◦. The amount of ejected material for both roughness conditions within their specific welding windows was similar, see Figure 6c,d, while the intensity of the process glare was reduced, see Figure 6f,g. Furthermore, polished cross sections revealed that the waviness of the joining zone increased and exhibited pockets for the laser structured surface (b) instead of a continuous layer (a) along the interface. A roughness of 30 μm inhibited the CoP formation, the process glare and welding completely at the given impact velocity, see Figure 6e,h, respectively.

Replacing the parent material with the copper alloy Cu-OFHC enabled welding for collision angles between 6.6◦ and 7.5◦, while welding was not achievable in this range with the ultrafine grained copper as parent material. Although the shadows in the joining gap and, thus, the CoP densities seemed to be comparable, the process glare was significantly reduced, see Figure 7

**Figure 6.** Influence of the surface structure on (**<sup>a</sup>**,**b**) the joining zone in polished cross sections, (**<sup>c</sup>**–**<sup>e</sup>**) the formation of a cloud of particles, and (**f**–**h**) the process glare at *<sup>v</sup>*imp = 262 m/s. Welding direction to the left from the initial collision point.

**Figure 7.** Influence of the parent properties on (**a**) the joining zone in a polished cross section, (**b**,**<sup>c</sup>**) the formation of a cloud of particles and (**d**,**<sup>e</sup>**) the process glare. Welding direction to the left from the initial collision point.

## *3.2. MPW Process for Sheets*

MPW experiments with the same material EN AW-1050 for both the flyer and the parent sheet were performed at ambient pressure first. Due to missing PDV measurements, the non-welded samples processed with 18 kJ charging energy are not listed in Table 4. Nevertheless, they enabled the definition of the lower process boundary at 19 kJ charging energy since welding occurred at this energy level with an impact velocity of 224 m/s. Reducing the ambient pressure in the joining gap to approx. 100 Pa enabled welding even with lower impact velocities of 203 m/s. At this point it should be mentioned that due to the design of the vacuum chamber, the distance between the coil wire and the flyer sheet in the active zone increased during the evacuation process of the vacuum chamber, leading to a reduction of the acceleration distance between the joining partners. This effect was compensated by adjusting the charging energy *E* in 30 preliminary tests to ensure the targeted level of impact velocity *<sup>v</sup>*imp and monitor it via PDV.

The reduced ambient pressure in the joining gap not only decreased the lower process boundary, but also influenced the process glare and the interfacial microstructures. Compared with the experiment in ambient atmosphere at an impact velocity of 243 m/s, the process glare under reduced pressure decreased significantly and appeared in an orange to red color, see Figure 8. In addition, the welded area increased, see Table 5. The interface did not contain a porous layer close to the central gap, which resulted in a smooth transition from the non-welded zone in the middle to the adjacent welded regions in Figure 9.



1 measured by PDV, 2 checked with manual peel test.

**Figure 8.** (**a**) Bright process glare at 100,000 Pa ambient pressure and (**b**) reduced process glare at 100 Pa ambient pressure in the joining gap at constant impact velocity *<sup>v</sup>*imp ≈ 243 m/s. Welding directions to the left and right from the central initial collision point.

**Table 5.** Influence of the ambient pressure in the joining gap on the length of the weld seams at constant impact velocity *<sup>v</sup>*imp ≈ 243 m/s.


The combined inverse pole figure (IPF) and Band Contrast/Image Quality (IQ) maps in Figure 9 reveal some interesting differences of the weld interfaces caused by the change in ambient pressure. Most notably, the central gap at 100,000 Pa is wider close to the initial weld compared to the sample welded at 100 Pa ambient pressure. The detail map at 100,000 Pa ambient pressure in (b) reveals a slightly porous layer in the weld interface with many very small grains, although to a smaller extend compared to previous reports [22,25]. In comparison, the initial weld interface produced at 100 Pa ambient pressure in (d) is smoother and without a nano-crystalline interlayer.

The high-speed images reveal a CoP that dispersed in the joining gap and glowed brightly in normal ambient atmosphere. If the ambient pressure was reduced, the appearance of the CoP changed to a dark and tongue-like shape, see Figure 10. In both cases no band pass filter was used. The CoP velocity was estimated by dividing the propagation distance between two time steps. It was approx. 4 to 6 km/s in normal ambient atmosphere and about 10 km/s at reduced ambient pressure.

**Figure 9.** Combined inverse pole figure (IPF) and image quality (IQ) maps of the interface region obtained at constant impact velocity *<sup>v</sup>*imp ≈ 243 m/s at (**<sup>a</sup>**,**b**) 100,000 Pa and (**<sup>c</sup>**,**d**) 100 Pa ambient pressure: the inset in (**a**) shows the sample positions where the maps were obtained using EBSD measurements. The white outlines in (**<sup>a</sup>**,**<sup>c</sup>**) mark the positions of the detailed maps in (**b**,**d**). Welding direction to the left from the central initial collision point.

**Figure 10.** High-speed images of the MPW process at (**a**) normal ambient pressure and (**b**) at reduced ambient pressure with *<sup>v</sup>*imp ≈ 243 m/s shortly after the initial collision. Welding directions to the left and right from the central initial collision point.

All optical spectra of the process glare in Figure 11 show a prominent peak at short wavelengths, which can be assigned to the intense characteristic aluminum emission line at approximately 396 nm [42]. Although the overall shape of the spectra is unchanged between the two different positions under reduced ambient pressure, the intensity is increased over the whole spectral range at normal ambient pressure. Due to the limited spectral resolution, it is impossible to distinguish bundles of individual emission lines and the continuous blackbody radiation. However, the different process glare colors in Figure 8 and the shift of the intensity maxima to longer wavelengths in normal ambient pressure from the position close to the initial collision point (Pos. 1) to the end of the joining gap (Pos.'2) allow for two main conclusions:


**Figure 11.** Relative spectral intensities obtained at constant impact velocity *<sup>v</sup>*imp ≈ 243 m/s at two different positions under (**a**) normal ambient pressure and (**b**) reduced pressure 100 Pa in comparison with the characteristic aluminum emission lines [42].

## *3.3. MPW Process for Tubes*

The collision angle during MPW can be controlled by the working length *l*w [34]. It influences both the appearance of the process glare and the welding result [26]. A working length of 4 mm led to an initial collision angle of approx. 9◦ where welding was not achievable in the given setup. Increasing the working length to 8 mm decreased the collision angle to approx. 3◦ and allowed for a weld formation between the aluminum flyer and the steel parent [34]. Moreover, an evaluation of the process glare in the joining gap under vacuum-like conditions revealed a significant temperature increase far above the vaporization temperature of the involved materials. Now, additional MPW experiments were performed at normal ambient pressure to gain deeper insights into the relation between the collision angle and the formation of the CoP as well as its interaction with the witness pins that consisted of three different materials, see Figure 12a. The witness pins were placed side by side at the 180◦ position of the tool coil. The pin diameter of 2 mm was small compared to the inner circumference of the flyer tube with 116 mm. Thus, the influence of the different radial positions can be neglected. The results are summarized in Table 6. The CoP penetrated into the soft graphite pin, see Figure 12b, while it was deposited on the surfaces of the tungsten and steel pins, as shown in Figures 13 and 14, respectively. In both figures, sections (a) and (b) show the pin surfaces after the cutting procedure before the MPW experiments. Obviously, the deposited layer in (e) and (f) consisted

mainly of the aluminum flyer material as well as the dominating iron parent material. Furthermore, the increased oxygen content indicated a partial oxidation of the CoP during MPW.

**Figure 12.** (**a**) Detailed sketch of the joining setup and witness pins to study (**b**) the influence of the collision angle on the indentation depth of the CoP into the graphite witness pin (0 μm-position of the pin is in contact with the inside of the flyer tube during MPW).

**Table 6.** Influence of the collision angle on the process glare and interaction between the CoP and the witness pins.


 by analogy with [34], checked with manual peel test, defined in [43], see Figure 12b.

The collision angle had a big impact on the penetration of the CoP in the graphite witness pin and the structure of the vaporized surfaces on the steel and tungsten witness pins, respectively. For high collision angles the penetration depth in the graphite witness pins was approx. 1 mm, see Figure 12b. There were many single particles deposited on the tungsten pin, leading to a coarse and ragged structure. Energy dispersive X-ray spectroscopy (EDX) revealed no copper, neither on the tungsten pin nor on the steel pin, see Figures 13f and 14f, respectively. In contrast, small collision angles led to a line-shaped penetration area in the graphite pin. Based on the distance of 15 mm to the initial collision point, the angle of ejection could be calculated. It is within a range of 1.9◦ to 5◦, which is in good agreemen<sup>t</sup> with the simulated collision angle of 3.4◦ [34]. At this small angle, a homogeneous aluminum layer with a copper content of ~2 weight percent from the tracer coating and with only a few single aluminum particles was deposited. Compared to large collision angles, the thickness of the layer was lower since the subjacent tungsten was detectable during EDX analysis, too.

**Figure 13.** Surface topography and chemical composition of the tungsten witness pins (**<sup>a</sup>**,**b**) before MPW and after MPW with (**<sup>c</sup>**,**d**) low collision angle and (**<sup>e</sup>**,**f**) large collision angle.

**Figure 14.** Surface topography and chemical composition of the steel witness pins (**<sup>a</sup>**,**b**) before MPW and after MPW with (**<sup>c</sup>**,**d**) low collision angle and (**<sup>e</sup>**,**f**) large collision angle.
