DRESDEN, Germany, Sept. 13, 2018 — As developers in the automotive and airline industries push to make more efficient vehicles, they are looking for new and innovative ways to design sturdy, lightweight machines. Designing lightweight materials, however, requires carefully joining together different types of materials like metals and polymers, and these additional steps drive up manufacturing costs. New work in laser technology recently increased the adhesion strength of metal-plastic hybrid materials.

---

This image shows SEM images of (a) aluminum swarfs 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. Courtesy of Matthieu Fischer.

---

A group of German engineers from the Fraunhofer Institute for Material and Beam Technology, the Leibniz Institute for Polymer Research, and the Technical University of Dresden recently demonstrated a technique for binding plastic to aluminum by pretreating sheets of aluminum with infrared lasers.

The researchers found that roughening the surface of aluminum with continuous 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," said researcher Jana Gebauer. "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 continuous laser and one pulsed for 20 picoseconds 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 said.

Tests using optical 3D confocal microscopy and scanning electron microscopy revealed that the aluminum sheets treated with pulsed lasers had much smoother line patterns in the trenches on their surfaces than those pretreated with continuous laser radiation. Aluminum sheets treated with infrared lasers also exhibited stronger bonding, but these properties diminished in tests with increasing levels of moisture.

Despite the team's success, Gebauer said 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.