II. Applied Laser technologies for industries

II-1 Laser quenching

II-2 CFRTP (Carbon fiber reinforced thermo-plastic) cutting process using pico-second pulse laser

II-3 Laser rust-resistance

II-4 Computer analysis of laser penetration plastic-resin joining

The laser quenching has the following strong-points, compared with other methods such as an induction quenching.

- No necessary of water or oil cooling

- Precise irradiation by laser beam accurate operation

- Short time quenching

In the laser quenching processing, laser moving heating changes a material surface to the austenite structure, after laser beam passing through, the heat of the metal surface immediately diffuses into the metal internal area, due to metal high thermal conductivity. Consequently, the metal surface temperature decreases at enough cooling rate, which changes the surface austenite area to the martensite structure to attain hardening(quenching).

The laser quenching depth is, however, required to be increased for higher durability. For this purpose, the JAEA computer code has been used to uncover the temperature distribution and its transition during the laser quenching processing and to determine appropriate laser irradiation conditions.

Figures II-1(1)[a] and [b] show a Galvano laser scanner system and a laser quenching experiment, respectively. The surface temperature transition of the test piece was observed by a thermo-monitor camera, as shown in Fig. II-1(1)[c].

Figure II-1(2) indicates the analytical results of the internal temperature transition of a test piece under two different cases. One is calculated under the high power and high sweep speed condition ([a] 5.0kW, 30mm/sec.), and the other is under the low power and low sweep speed condition ([b] 1.5kW, 1.5mm/sec.)

The comparison of these two cases has suggested that the lower laser power and the lower sweep speed condition (Fig. II-1(2)[b]) achieves deeper heat diffusion in the metal, expecting increased quenching depth.

Fig. II-1(1) Laser quenching experiment

Fig. II-1(2) 3D analitical results of temperature transition under two different laser irradiation cases

Shown in Figs. II-1(3)[a] and [b] are a micro-vickers hardening measurement result and a computer calculational temperature contour, both of which indicate the distributions at the perpendicular cross-section to the laser beam sweeping direction.

The comparison of both the Figs. [a] and [b] has revealed that the upper area is hardened by the sufficient temperature rise, whereas the lower area remains original hardness due to insufficient temperature rise.

To logically and precisely predict the laser quenching region, the CCT-curve diagram (Continuous cooling transformation) has been installed in the computer code, and the code classifies the metallurgical structure into Austenite, Martensite and Pearilte from the temperature history on the CCT-curve diagram.

Two cases of the metallurgical transformation calculational results are shown in Fig. II-1(4), where laser beams sweep from the right to the left side at different conditions. The blue, green and red regions stand for austenite, martensite and pearlite metalogical structure, respectively. The upper movie [a] clarifies the metal changes from austenite structure to martensite, which leads to successful hardening. The lower movie [b] indicates the metal changes from austenite structure to pearlite, which causes imperfect hardening, because of low cooling rate.

Thus, the JAEA computer code provides both the temperature distributions / transitions and the metallurgical transformation to clarify the hardening region, which helps the determination of appropriate laser quenching conditions such as laser power and sweep rate.

Fig. II-1(4) Analytical results of metallographic structure transformation during Laser quenching processing

Carbon fiber reinforced plastics (CFRP) are composite materials composed of carbon fiber and resin. CFRP combines the lightweight and high molding flexibility of resin with the high rigidity and strength of carbon fiber, offering a unique combination of properties. In recent years, CFRP have been utilized in the components of aircraft and automotive, and are now being employed in energy and infrastructure applications.

CFRP is classified into two categories. The first category comprises CFRP using thermosetting resin, which exhibits greater strength and rigidity than metal but is associated with prolonged molding times and non-recyclability. This type of CFRP is utilized in aerospace applications where mass production is not a priority. The second category encompasses CFRTP using thermoplastic resin, which displays reduced strength compared to CFRP but has a shorter molding time, enhanced economic viability, and recyclability due to the resin's melting point upon heating.

The objective of our research is to investigate the potential of laser processing to achieve minimal thermal effects on CFRTP (compact fiber reinforced thermoplastic resin), which is gaining interest in a number of fields, including automotive. Due to the low melting point of the thermoplastic resin utilized in CFRTP, processing must be conducted with minimal thermal effects. To this end, we are employing non-thermal processing techniques utilizing pulsed lasers with shorter pulse widths. We are conducting processing tests using a picosecond (10-12 seconds) pulse laser and performing preliminary experiments, including observations of the cutting of carbon fibers.

The sample selected was a CFRTP comprising carbon fiber and PPS with a thickness of 1 mm, which is a commonly utilized commercially available material (Fig. II-2(1)). Figure II-2(2)[a] depicts the surface of the sample following laser processing, exhibiting a molten region on the surface. In addition, as shown in Fig. II-2(2)[b], when the cross section was measured at a low magnification of 20 times (see the image in the light blue frame), a heat-affected zone (HAZ) was observed where the resin had vaporized approximately 1 mm horizontally in the direction of the laser beam. When measured at a higher magnification of 50x (see image in brown frame), it was confirmed that the carbon fiber was cut with a small kerf width.

As a future plan, we will explore laser irradiation conditions to further suppress the HAZ.

JAEA Tsuruga comprehensive R&D center has conducted the research on the laser rust-resistance heat treatment in cooperation with the Nuclear Science Research Institute of JAEA (Radioactive waste management section 1). Laser heating on the metal surface forms the magnetite oxide layer that prevents red-rust development. Figure II-3(1) shows the magnetite oxide layers formation test by laser scanning irradiation, and the formation was investigated under various irradiation conditions.

Fig. II-3(1) Laser rust-resistance experiment(Magnetite layer formation test)

After the laser magnetite formation, its test pieces were examined by the corrosion test and the X-ray diffraction analysis, which are shown in Fig. II-3(2) with photos of the test pieces.

The photos (a) and (b) show a test piece before the laser irradiation and a magnetite formed test piece by the laser irradiation, respectively.

The corrosion test results, where the test pieces were subjected to salt-water spray for one month, are indicated in the photo (c), (d). The test piece with no magnetite was all covered with red-rust (c), whereas the red-rust was limitedly formed on the laser irradiated test piece (d). This corrosion test has suggested that the laser heating magnetite layer provides the resistance against red-rust developing.

The left two graphs(e), (f) of Fig. II-3(2) show the composition of each the test piece by the X-ray diffraction analysis. The laser heating forms the magnetite compositions such as Fe3O4, which supports the corrosion test result indicated in the photos (c), (d).

The magnetite layer requires to be uniform structure in order to enhance the rust-resistance performance and its durability, because lack of the uniformity develops cracks on the magnetite surface and deteriorate the rust-resistance property. One of the method to attain the uniformity is to flatten the temperature distribution on the surface during laser scanning irradiation. For this purpose, the temperature history has been investigated by the thermal conductance analysis of the computer code.

Figure II-3(3) presents the transition of the temperature distribution in the test piece during the laser scanning operation, in which the red-arrows of the test piece indicate the image of the laser scanning direction. The two central contours show the surface temperature transition, where the dotted square area of the lower contour is enlarged into the upper one. The vertical temperature distribution is shown in the right contour. These three contour movies synchronize for understanding the 3-dementional temperature transition.

This analysis has revealed the temperature history on the surface, which is useful to flatten the temperature distribution by adjusting laser conditions such as beam profile and scanning beam interval.

This study has suggested the prospects of the rust-resistance by laser heating, however, it's at an early R&D stage and necessitates future work to detrmine appropreate laser irradiation. The demonstration experiments are also required for actual use.

Fig. II-3(3) Temperature transition on metal surface by 3D thermal conduction computer analysis

The computer code has been developed to be applied for the joining of laser penetration plastic-resin materials.

In metal thermal processing, laser beam turns into heat at a metal surface, and the heat diffuses into the metal inside by thermal conduction. In laser penetration plastic-resin processing, however, laser beam travels into the material inside, and the beam changes to heat gradually here. With the progress of this plastic-resin heating, the interface of two plastic-resin materials are melted to be joined, as illustrated in Fig. II-4(1)[a].

The laser absorption model of the plastic-resin has been installed in the computer code to reveal the temperature distribution and its transition and to optimize laser conditions. Figure II-4(1)[b] indicates an analytical results of the changing process from the laser beam to the heat, leading to the temperature rising and the interface melting junction.

Fig. II-4(1) Analytical results of temperature distribution in Laser penetration plastic-resin

Actually, the interface of two materials is joined by laser sweeping operation. Figure II-4(2) shows an analytical result (movie) of the plastic-resin melting junction by the moving heat source. This movie clarifies the 3-dimensional temperature distribution and its transition with the laser moving from the left side to the right.

This calculational approach is believed to be applied for determining the suitable laser conditions for plastic-resin melting junction products. To confirm its accuracy, the analytical results will be compared with experimental data in the near future work.

Fig. II-4(2) Analytical results of temperature transition under Laser sweep irradiation