# Optimization of Laser-Assisted Polypropylene Aluminum Joining
## 2 Materials and Methods
### Joining Process  
The joining process was realized by the laser conduction joining, which can be used by applying the laser on the Al surface and having the heat dissipate to the PP. The laser should hit the back side of the engravement leading to the molten PP filling the grooves. For the joining process, different factors were important to be optimized such as temporal and spatial modulations of the laser beam that define the temperature of the PP surface and the occurrence of thermal degradation. For the joining process, a TruFiber 400 equipped with Scanlab HS20 2D f-\(\theta\) scanner head is used. Figure 2 shows the details of the overlap configuration between Al and PP before the joining process, in addition to the modeled interlocking. An overlap of 25 mm \begin{table}
\begin{tabular}{l c c} \hline \hline
**Property** & **1050-H24 Aluminum (Al)** & **Polypropylene (PP)** \\ \hline Dimensions & (60 \(\times\) 30 \(\times\) 0.5) mm & (75 \(\times\) 25 \(\times\) 4) mm \\ Density & 2.71 g/cm\({}^{3}\) & 0.92 g/cm\({}^{3}\) \\ Melting Point & 650 \({}^{\circ}\)C & 160 \({}^{\circ}\)C \\ Evaporation temperature & 2327 \({}^{\circ}\)C & 300 \({}^{\circ}\)C \\ Modulus of Elasticity & 71 GPa & 0.9 GPa \\ Tensile Strength & 100 MPa & 25 MPa \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Materials properties. Figure 1: Overview of laser engraving area and lines for Al samples including the corresponding parameters regarding the density of engraved lines. was selected, following ASTM D5868-01 (2014) [23]. The clamping force was kept constant for all samples, and the focal point of the laser beam was fixed on the surface of Al samples. Wobbling in a forward-moving circular motion (as described by Equation (1)) was selected (see Figure 3) for the beam's trajectory. Unlike a simple linear path, this allows for a wider weld while guaranteeing a uniform heat transfer from Al to the PP surface to examine the effect of mechanical interlocking. \[ \begin{pmatrix}x(t)\\ y(t)\end{pmatrix} = \begin{pmatrix}-a\times\cos(2\pi ft)+v\times t+a\\ a\times\sin(2\pi ft)\end{pmatrix}  \tag{1}\]  
\(a\) amplitude of the circular movement (radius) (mm)  
\(f\) repetition frequency (Hz) (fixed 500 Hz)  
\(v\) feed rate in x (horizontal) direction (mm/s). Based on [24], the circular wobbling was optimized when the overlap (as described by Equation (2)) was 0.75. This overlap led to the feed rate of 111.1 mm/s for the laser beam in the \(x\) (horizontal) direction. \[n=\frac{4\times a-3\frac{v}{f}}{4a-\frac{v}{f}} \tag{2}\]  
\(n\) overlap giving a ratio of the covered area by the wobbling. \(a\) amplitude of the circular movement (radius) (mm)  
\(f\) repetition frequency (Hz) (fixed 500 Hz)  
\(v\) feed rate in \(x\) (horizontal) direction (mm/s). To minimize heat accumulation, reduce the output intensity, and avoid degradation of PP, power modulation was used. Therefore, a more uniform heat transmission along  
Figure 3: Selected beam trajectory for the joining process. Figure 2: Setup and dimensions of the joined samples (**a,b**); zoom on the joint and modeled interlocking (c). the joining area was achieved. Equation (3) represents the power modulation. Preliminary experiments showed that approximately 290 W of power was appropriate for the joining process. Thus, to optimize the engraving parameters, the power of 290 W was considered for the experiments. In Section 3.3.2, the effect of modulated power on the joint quality will be addressed separately. \[P_{mod}=t_{mod}\times f_{mod}\times P_{P} \tag{3}\]  
\(P_{mod}\) modulated power obtained by pulsing a CW (W)  
\(t_{mod}\) duration of the pulse (s)  
\(f_{mod}\) frequency of the pulses (Hz)  
\(P_{p}\) max power of the CW (W)  
Evaluation of the mechanical performance of the joints was implemented by a tensile-shear test with a fixture to avoid the bending of the samples due to the offset (see Figure 2a) between the PP and Al. This fixture was made of a low-friction carbon fiber composite and only applies a pure tensile shear force on the sample.