# Porosity Elimination in Modified Direct Laser Joining of Ti6 Al4V and Thermoplastics Composites
## 3 Results and Discussion  
### LAMP Method  
Groove structures with a depth of 350 \(\upmu\)m were fabricated on Ti6 Al4V surface, as shown in Figure 3a. The width of the grooves is around 300 \(\upmu\)m (Figure 3b). Grooves with structure density ranging from 25% to 75% have been fabricated on Ti6 Al4V by changing the pitch of grooves (Figure S1). Figure 3c shows the images of LAMP joined Ti6 Al4V-PA66-GF30 specimens. Cross-section morphology of the joint is exhibited in Figure 3d. It can be observed that the grooves have been completely filled by the melted PA66-GF30 during LAMP process and the interlock structure forms between Ti6 Al4V and PA66-GF30 after solidification of PA66-GF30 melts. In the joint, a large number of micro-pores have been observed in the interior of PA66-GF30 with sizes ranging from dozens to hundreds of microns. These micro-pores can be divided into two types on basis of morphology and distribution [15]. One type is generated close to the bonding interface or located in the interior of PA66-GF30 filled in the grooves, where the materials experience very high temperatures during laser joining process [13; 15]. These pores independently distribute with irregular shapes and the size of over 100 \(\upmu\)m, as indicated in Figure 3d. These pores most likely result from the pyrolysis of PA66 due to the high temperature [16]. The pyrolysis of PA66 produces a large number of gaseous products such as ammonia and carbon dioxide. The gaseous products inside the melts result in the generation of micro-pores in the interior of PA66-GF30 after solidification of the melts. Figure 1: Schematic diagrams of laser joining process. (**a**) Conventional direct laser joining process; (**b**) the modified laser joining process. Figure 2: Schematic representation of tensile shear test. The other type of pores distributes close to the boundary of melted zone and has a distance of around 200-500 \(\upmu\)m from Ti6 Al4V-PA66-GF30 bonding interface [15], as indicated in Figure 3d and Figure S2. These pores exhibit rough morphologies and arbitrary shapes, and distribute throughout the cross-section regardless of the structure density, as can be seen in Figure S2. The size of these pores ranges from dozens to hundreds of microns, as shown in Figure 3d. These pores are mainly caused by the shrinkage of PA66-GF30 melts during solidification. Shrinkage porosity is commonly formed in the manufacture of composite polymeric materials due to solidification shrinkage [17; 18]. Ti6 Al4V has much higher thermal conductivity (7 W/(m K)) than PA66-GF30 (0.30 W/(m K)) [19; 20]. After the laser joining process, the heat quickly conducts by Ti6 Al4V, and the PA66-GF30 close to the bonding interface solidifies first due to the higher thermal conductivity of Ti6 Al4V compared to that of PA66-GF30. The PA66-GF30 melts near the boundary of melted zone solidify last, and solidification shrinkage of PA66-GF30 results in the formation of porosity near the melted zone boundary [15]. Structure density is a key parameter that determines the effective bearing area of the joint interface during tensile shear test, and it thus significantly influences the joints strength [21]. The grooves' structure, with depths of around 350 \(\upmu\)m, was fabricated on the Ti6 Al4V surface to investigate the bonding strength of the Ti6 Al4V-PA66-GF30 joints produced through the laser joining process. The results of the tensile shear tests (Figure 4a) indicate that the fracture strength of the joints increases from around 8.8 MPa to 13.8 MPa as the structure density increases from 25% to 58%, which is ascribed to increasing the effective bearing area in joint interface. When the structure density reaches 58%, the fracture strength of the joints does not increase anymore. Figure 4b-d shows the surface morphologies and cross-sections of Ti6 Al4V-PA66-GF30 joints with different structure density after tensile shear tests.