Abstract : The laser hardening principle, process and computer control method of the engine block and crankshaft in key parts of automobile overhaul are analyzed and studied.
Keywords: laser hardening cylinder crankshaftâ–²
1 Introduction
Laser surface modification technology is an optical, mechanical, and electrical integration high-tech that has rapidly developed after the emergence of high-power lasers in the 1970s. This process has a series of advantages such as good hardenability, high degree of automation, simple operation, low energy consumption, high efficiency, minimal workpiece deformation, and almost no waste. The use of lasers to process the surface of the part can change the physical structure, chemical composition and metallographic structure of the part surface, thereby changing the physical, chemical and mechanical properties of the part surface, such as wear resistance, corrosion resistance and fatigue resistance, effectively solving the problem. Many conventional heat treatments are still difficult to solve or solve the problem of poor part surface properties. Therefore, rapid progress has been made in various countries of the world. Such as AVCOEVETT Institute of Ford's gray iron camshaft surface hardening treatment, the maximum deformation of <0.13mm. At the same time, the Institute also heat-treated the crankshaft journals and necks of diesel ductile iron crankshafts. The hardness was 55-62HRC. The crankshafts after treatment did not need to be straightened. As a result, the cost of heat treatment of crankshafts was reduced by 20%, and good results were achieved. effect.
China's laser surface hardening technology has only entered the use stage in recent years, but in some areas is still blank, this article describes the multi-function laser machine and its control system jointly developed by the author and Qingdao Zhongfa Laser Technology Co., Ltd. in the automotive engine block, Application of laser case hardening on cylinder liners and crankshafts.
2 laser surface hardening principle
Laser surface hardening is also called laser heat treatment. The principle is that the laser beam is used as a heat source to irradiate the surface of the engine to be strengthened, and the surface is rapidly heated to a temperature above the metal phase transition temperature. When the laser is left, the heat of the high temperature surface is quickly transferred to the metal matrix to form a self-quenching, so that after treatment, The surface layer forms a cryptocrystalline martensite with high hardness, thereby improving the surface hardness, wear resistance, and service life of the parts.
3 laser equipment and process
3.1 Laser equipment
Laser surface hardening equipment includes three major parts: 1.5kW CO2 laser generator, multi-function laser processing machine and computer control system.
3.2 General Methods and Defects of Vehicle Engine Overhaul
When the engine is overhauled, it is mainly processed by boring and grinding shafts. After the cylinder wall is ground several times, the surface hardness is reduced. Because the crankshaft journal loses roundness due to wear, it is generally necessary to re-shaft the shaft and replace the bearing. The hardness of the surface of the crankshaft journal is reduced after multiple grinding and cannot meet the requirements for use. Therefore, the engine overhaul period after the conventional method is processed becomes shorter.
3.3 Laser Treatment Process for Automotive Engine Overhaul
The laser treatment process for automotive engine overhaul is a conventional overhaul process plus laser processing. Laser heat treatment is generally arranged after semifinishing. Because the reflectivity of the CO2 laser with a wavelength of 10.6 μm is higher than 80% at room temperature, the smoothness of the surface processed by the grinder is higher and the reflection of the laser will cause a large amount of energy loss. Therefore, a pre-applying absorptive coating is required before the laser heat treatment. The layer method is to increase the absorptivity of the part surface to the laser. The coating can absorb the far-infrared part of the electromagnetic wave. Various metal or non-metal oxides, graphite, carbon ink, and phosphate are all good coating materials.
When the crankshaft is heat-treated, it is mounted on the machine tool table and clamped with two chucks. The diameter of the laser spot is adjusted to 2 to 3 mm, and the power density is uniform, so as to ensure uniform hardness on a hardened strip of a certain width.
The laser head is mounted on the bracket and the bracket is mounted on the crossbeam of the machine tool, so that the laser head can be moved vertically and horizontally. The laser beam is positioned above the crankshaft journal, the stepper motor on the worktable drives the crankshaft to rotate, and the laser head moves in the horizontal direction, and the bracket moves along the crossbeam so as to scan the shape of the scan along the width direction of the crankshaft surface. Line (line type controlled by microcomputer). The coverage (hardened area ratio) of the hardened belt on the journal is generally taken as 30% to 50%. The rotation of the crankshaft, the horizontal movement of the laser head, and the combined movement are automatically controlled by the program control system.
When the cylinder block is heat-treated, it is mounted on a rotary table at the bottom. The cylinder rotates and the laser head moves up and down.
The process parameters of the laser hardening process mainly include scanning speed, hardening area ratio, hardening depth and processing surface roughness. These parameters depend on the laser power density, scanning speed, type of material, and surface coating properties. The laser hardening zone has two to four layers of tissue characteristics, specifically how many layers are determined by the laser processing specifications and material composition. During high-speed heating and cooling, a "bright white layer" can generally be observed on the surface of the steel.
Because the laser beam scans on the inner wall of the cylinder according to a certain geometrical trajectory, the laser-treated hardened band constitutes a hard skeleton; in the place not treated by the laser, when the engine works, the first trace wears out to form an oil storage structure. This soft and hard combination of the inner wall of the cylinder not only improves the wear resistance, but also has a good tensile resistance. Piston rings work under good lubrication conditions and their service life is greatly improved. The same principle applies to crankshaft journals.
3.4 Experimental Comparison and Wear
(1) The comparison of untreated cylinders and laser-treated cylinders sports car data is shown in Fig. 1.
Figure 1 Comparison of sports car data
(2) The crankshaft wear before and after 600 hours of bench test is based on QC/T524-1999 "Automotive Engine Performance Test Method", QC/T525-1999 "Automotive Engine Reliability Test Method" and other standards, the laser heat-treated crankshaft is mounted on the 492QA gasoline engine, full-speed full load reliability test. After 600h bench test, precision measurement was carried out and the wear amount is shown in the table.
Wear amount of journal before and after 600h test
Maximum wear(mm) Minimum wear
(mm) Average wear
(mm) Spindle neck 0.020 0.010 0.017 Connecting rod journal 0.020 0.011 0.016
The test proves that the laser surface treated crankshaft has good wear resistance after 600 hours of bench testing, and its maximum wear amount is 0.02mm, which meets the requirements of the state regulations and can meet the 2×105km overhaul period standard.
4 Computer Control Principle
In this project, PC bus industrial controller was selected as the controller and CRT was used as the monitor. Therefore, a better user interface can be achieved and the control function is stronger. The principle is shown in Figure 2.
Figure 2 Engine Hardening Computer Control Schematic
For the reliability of the system, the strong and weak parts are joined by an opto-isolator. In the figure, the direction of the arrow indicates the flow of control information. Each trip switch is connected to the I/O board. Therefore, as long as there is a bit detected in one direction, you can Causes the machine to shut down.
The programming language adopts C++ and 80386 assembly language. The programming adopts the linear interpolation method after analogy. Due to space limitations, this article does not explain.
It must be emphasized that before heat-treating parts such as the engine block, the workpiece must be pre-rotated first, and after turning off the light, there must be a period of backward rotation. Otherwise, it will cause partial burn of the workpiece.
5 Conclusion
The engine is the heart of the car. After the laser surface hardened engine is used, the hardness is generally increased from the original 20HRC to 55-62HRC, and the reinforced depth is 0.2 to 1mm. The service life is increased to 3 to 5 times and the heat treatment cost is reduced. From 20% to 40%, the workpiece after laser heat treatment has almost no distortion. Therefore, it not only prolongs the overhaul cycle, but also saves the cost of repeated maintenance, has good social and economic benefits, and has broad application prospects.