Designing lightweight materials for use in the automotive industry requires carefully joining together different types of materials like metals and polymers, which can drive up manufacturing costs. Recognizing this, a team of engineers at the Fraunhofer Institute for Material and Beam Technology (Fraunhofer IWS), the Leibniz Institute for Polymer Research, and Technical University Dresden (all in Dresden, Germany) has demonstrated a laser joining technique for binding plastic to aluminum by pretreating sheets of aluminum with infrared (IR) lasers.
In their paper describing the work, the researchers show that roughening the surface of aluminum with continuous-wave (CW) IR laser beams created a mechanical interlocking with thermoplastic polyamide and led to significantly strong adhesion.
"In other joining methods, you have a plastic part you want to fit together with a metal part. In the injection-molding process, we generate a plastic part on top of the metal part in a cavity of the machine," says Jana Gebauer, an author on the paper. "As a consequence, it is very difficult compared to thermal pressing or other joining technologies because of the specific thermal conditions."
To tackle these issues, Gebauer and her colleagues used both a CW laser and a pulsed laser for 20 ps at a time to make the surface of aluminum sheets more adhesive for a polyamide layer to be molded over it. They then placed the sheets in an injection mold and overmolded them with thermoplastic polyamide, a polymer related to nylon that is used in mechanical parts like power tool casings, machine screws, and gears.
"Following that, we analyzed the surface topography and conducted mechanical tests of the bonding behavior to find out which parameters led to maximum bonding strength," Gebauer says.
Tests using optical 3D confocal microscopy and scanning electron microscopy (SEM) revealed that the aluminum sheets treated with pulsed lasers enjoyed much smoother line patterns in the trenches on their surfaces than those pretreated with continuous laser radiation. Aluminum sheets treated with IR lasers also exhibited stronger bonding, but these properties diminished in tests with increasing levels of moisture.
This image shows SEM images of (a) aluminum swarfes at the edges of the continuous wave laser structure and (b) remaining aluminum in the trenches of the molded polymer surface after tensile shear test. (Image credit: Matthieu Fischer)
Despite the team's success, Gebauer says that much work lies ahead to understand how pretreatments of the metal's surface can be optimized to make the process more economical for manufacturers. Now, she and her colleagues look to take on studying how molded thermoplastics shrink when cooled.
"The thermal contraction leads to mechanical stresses and can separate both parts. The current challenge is to generate a structure that compensates for the stresses during shrinkage without softening the aluminum by the laser treatment," Gebauer says. "Now we want to produce a reliable bonding under usage of ultrashort pulsed laser to reduce thermal damage in the metal component."
Full details of the work appear in the Journal of Laser Applications.