Laser quenching equipment

The iLAM-D-1004 laser quenching equipment was used for the experiment. The equipment is shown in Figure 1. Its main configuration is as follows: 6-axis KUKA robot, 3000W Laserline semiconductor laser, laser quenching head spot size adjustment range of 3.5mm3.5mm-21mm21mm, in addition, the system is equipped with a two-color infrared thermometer and 900-1500℃ closed-loop temperature control software.

Fiber head LLK-C Fiber connection module Collimation unit Bidirectional adjustment shaping module Two-color infrared thermometer Thermometer connector 90° beam splitter module Focusing mirror module Protective mirror module

Part 5 of Figure 2 is a two-color thermometer, Laser Line AutoZoom bidirectional adjustment shaping module, which is suitable for quenching heads for medium and high power industrial applications. The adjustable and uniformly distributed rectangular spot can be applied to the surface treatment of parts of various sizes. The variable spot lens group can adjust the length of the homogenized square spot by driving the motor. The built-in motor drive can select standard 0-10V analog control, power supply requirement: 24V/3A; the lens can be equipped with an infrared pyrometer to achieve closed-loop temperature control.

Temperature closed loop control

Temperature closed-loop control LASCON® is a laser controller software for temperature-controlled laser processing. The processing temperature is detected by a two-color pyrometer. The main applications include laser quenching, micro-hardening and laser welding (especially laser welding of plastics) as well as any process that causes the workpiece temperature to rise, such as induction heating. LASCON® controls, optimizes and supervises laser processes. Using a simple laser process script programming language, LASCON® is able to determine good and bad laser processing and can easily sort out bad parts in laser-supported production. The software supports specially developed hardware components such as the LPC04 controller, integrating high-speed infrared pyrometers, laser processing heads, calibration units and adapters. Easy integration into machines and factory equipment. The entire software package is divided into different units and communicates via TCP/IP protocol.

The principle of material quenching is to heat the material to a certain temperature above the critical temperature Ac1 (738°C) or Ac3 (912°C), and keep it warm for a period of time to make the material structure fully or partially austenitized, and then quickly cool to room temperature at a rate greater than the critical cooling rate for martensitic transformation. It can be seen that the laser quenching temperature has an obvious and important influence on the quenching quality, and the quenching temperature is determined by the quenching process parameters. Different quenching process parameters determine different quenching temperatures. The quenching temperature is also a more intuitive parameter for repeated quenching processes. Therefore, during the laser quenching process, infrared monitoring is usually used to measure the material quenching temperature in real time, and the quenching temperature is controlled within a reasonable range. The temperature closed-loop feedback system is used to adjust the laser quenching process parameters (mainly adjusting the laser power) in real time to stabilize the quenching temperature at an appropriate value.

Process test

The experimental material is pre-hardened plastic mold steel 2738, with a pre-hardening hardness of 29-33HRC. This material is mainly used in large plastic molds and mold frames, such as automobile bumpers, TV shell molds, etc.

According to the research on relevant literature on laser quenching in the early stage, a preliminary experimental design was carried out. The quenching form was single-pass quenching. The laser spot size was 10mm*10mm and the spot scanning speed was 10mm/s, which were set to fixed values. The influence of the size of the experimental substrate on the quenching quality was ignored. The absorption rate of the laser by the experimental material was assumed to be 100%. By setting different quenching temperatures to detect the quenching hardness of the material and the change in the depth of the hardened layer, the actual light output power of the laser can be calculated and displayed in real time through the temperature control software conversion.

Figure 5 shows the quenching surface conditions under 6 groups of parameters. After quenching, the surface oxidation of 1# and 2# is shallow, the quenching temperature is low, the traces left by the substrate grinding can be clearly seen, and the single-pass quenching width of 1# is relatively small; after quenching, the surface oxidation of 3#, 4# and 5# is moderate, basically covering the traces left by the substrate grinding, and the quenching temperature is moderate; after quenching, the surface oxidation of 6# is serious, there is surface peeling, and the quenching temperature is relatively high.

Surface hardness test

The quenching hardness was measured using a Leeb hardness tester. The specific measurement values ​​are shown in Table 2. From the hardness value, we can know that the quenching temperature of 1# is obviously insufficient, the quenching hardness is low and fluctuates greatly; the average quenching hardness of 2# is 52.9HRC and the hardness value fluctuates slightly, but according to the material properties of pre-hardened plastic mold steel 2738, the quenching hardness can reach above 55HRC. Obviously, the quenching temperature of 2# is slightly insufficient; the quenching hardness values ​​of 3#, 4#, 5#, and 6# are all above 55HRC, and the quenching hardness of 5# is close to 60HRC, and the hardness fluctuation range is about 1HRC; based on the quenching surface condition and quenching hardness, it is preliminarily believed that under the conditions of 10mm*10mm spot and 10mm/s spot scanning speed, the reasonable range of quenching temperature is 1200℃-1400℃, the corresponding laser power density range is 1680-1980W/cm2, and the laser surface energy density range is 840-990J/cm2, among which the quenching process parameters of 5# are the optimal parameters for this experiment.

Depth of hardened layer

A cross section of a single quenching process was cut off to prepare a metallographic specimen. The cross section was corroded with a 4% nitric acid alcohol solution. Vickers hardness tester test showed that the effective depth of the hardened layer was from the quenching surface to the boundary of the single quenching heat affected zone. The effective hardening depth of the 4# specimen was 0.9 mm, as shown in Figure 6. The hardness values ​​from the substrate to the hardened layer surface are shown in Table 3.

Figure 6 4# sample Vickers hardness test 50X	Table 3 Vickers hardness value of 4# sample