Keywords: laser strengthening; Mould; Wear/life
With the rapid development of China's automobile and home appliance industries, higher requirements are put forward for the mold industry. How to improve the processing quality and service life of molds has always been a subject that people constantly explore. Using surface strengthening treatment is an important way to improve the quality and service life of the mold, which is of great significance to improve the comprehensive performance of the mold, greatly reduce the cost and give full play to the potential of the traditional mold. Commonly used die surface strengthening processes include chemical heat treatment * * * such as carburizing and carbonitriding * * * *, surface cladding treatment * * * such as surfacing, thermal spraying, electric spark surface strengthening, PVD and CVD * * *, and surface treatment strengthening treatment * * * such as shot peening. Most of these methods have complex processes, long treatment period and large deformation after treatment. In recent years, with the appearance of high-power laser and the increasingly extensive and mature application of laser processing technology in industry, it provides a new technical way for die surface strengthening.
1 laser surface strengthening treatment method
There are many methods of laser surface treatment, including: laser transformation hardening ***LTH***, laser surface melting treatment *** LSM * *, laser surface coating and alloying ***LSC/LSA***, laser surface chemical vapor deposition * * * LCVD * *, laser physical vapor deposition * * *. Among them, laser phase transformation hardening, laser surface cladding and alloying are studied to improve the service life of the die. This paper mainly discusses the mechanism and method of improving die life by using laser transformation hardening technology.
Laser transformation hardening * * * laser quenching * * is to irradiate the metal surface with laser, so that the surface quickly reaches the transformation temperature at a very high heating rate and forms austenite. When the laser beam leaves, it uses the heat conduction of the metal itself to carry out "self-quenching", which makes the surface of the metal undergo martensite transformation. Compared with the traditional quenching method, laser quenching is carried out in the process of rapid heating and rapid cooling, and the temperature gradient is high, thus forming a layer of special quenching structure with extremely high hardness, such as grain refinement, dislocation density and so on. The hardness of quenched layer is higher than that of ordinary quenching 15% ~ 20%. The depth of hardened layer can reach 0. 1 ~ 2.5 mm, which greatly improves the wear resistance of the die and prolongs its service life.
2. Composition of laser hardening processing system
Figure 1 is a schematic diagram of the working principle of the multi-axis linkage laser strengthening processing system. It consists of three parts: the first part is the laser system, which consists of a laser head, an excitation power supply, a cooling system and a resonator refractive index conversion device; The second part is the beam transmission and transformation device, which directs the laser beam to the surface of the part to be machined according to the machining requirements, and transforms the spatial intensity distribution of the laser beam at the same time, thus effectively strengthening the different stress parts on the die surface. After the beam transformation, the required strengthening unit can be generated on the surface of the mold, and the three-dimensional surface of the mold can be controlled, quickly and effectively strengthened through the multi-axis linkage numerical control system. The third part is the computer numerical control system, which controls the multi-axis movement of the laser working head and the numerical control worktable, and the trajectory of the laser beam relative to the workpiece determines the shape of the strengthening belt, thus realizing the laser strengthening treatment of the complex die surface.
3 laser strengthening treatment process
3. 1 Pretreatment coating on workpiece surface
When the laser device is determined, the laser absorption capacity of metal materials mainly depends on its surface state. Generally, the surface of metal materials that need laser treatment is machined, and the surface roughness is very small, and its reflectivity can reach 80% ~ 90%, so that most of the laser energy is reflected. In order to improve the absorption rate of laser on the metal surface, the surface of the material should be treated before laser heat treatment, which is usually called blackening treatment, that is, a coating with high laser absorption ability is coated on the metal surface that needs laser treatment.
The methods of surface pretreatment include phosphating, improving surface roughness, oxidation, spraying paint and coating, among which phosphating and spraying paint are commonly used. Commonly used coating aggregates include graphite, carbon black, manganese phosphate, zinc phosphate, water glass and so on. There are also those that directly use carbon ink and matte paint as pretreatment coatings. For some low carbon steel materials, carbon black powder treatment on their surfaces can play a carburizing role in laser quenching. We use the blackening liquid ***86- 1 developed by Shanghai Institute of Optics and Mechanics. The treatment method is simple, it can be directly sprayed on the surface of the workpiece, and the laser absorption rate is over 90%.
3.2 Process parameter optimization
The parameters of laser phase transformation hardening process mainly include laser output power P, spot size D and scanning speed V. Under other conditions, the depth of laser hardened layer H has the following relationship with P, D and V: H = P/* * D.V * * *. In order to get the best process parameters, the basic method is to determine a range of process parameters according to the existing successful data, and then take three levels of P, D and V, make an orthogonal test table, and carry out experimental research on the specimen. Fig. 2 shows the relationship curve between laser power and hardened layer depth of Cr-Mo cast iron used for drawing die of automobile taillight bracket at different scanning speeds. Fig. 3 is a graph showing the relationship between scanning speed and hardened layer under different laser powers. The chart shows that: generally speaking, the higher the laser power, the deeper the hardened layer; The higher the scanning speed, the shallower the hardened layer. Fig. 4 shows the relationship between the hardness of the hardened layer and the depth of the hardened layer under the conditions of laser power P= 1200W, scanning speed v= 15mm/s and spot diameter d = 4.5 mm It can be seen that the hardness of the material surface has been obviously improved after laser treatment.
4 Residual stress and wear resistance of hardened layer
In the process of laser hardening, the change of surface structure of metal materials and the generation and disappearance of temperature difference between the surface and the inside of materials will inevitably produce residual stress. The magnitude and distribution of residual stress have great influence on the practical efficiency of the die. The distribution of residual stress produced by laser hardening along the depth of hardened layer is shown in Figure 5. As can be seen from Figure 5, laser transformation hardening has produced a large residual compressive stress on the surface of the die, which can effectively prevent the occurrence of fatigue cracks and improve the fatigue life of the die.
The wear resistance of the die surface is related to the microstructure, grain size, hardness and surface state of the material, and these factors are influenced by the process parameters, so the laser strengthening parameters directly affect the wear resistance of the die. Figs. 6 and 7 show the effects of laser power and scanning speed on the wear resistance of 35CrMn steel. As can be seen from the figure, in a certain range, when the scanning speed is constant, the wear resistance of improving power increases; When the power is constant, the increase of scanning speed is also helpful to improve the wear resistance. Fig. 8 shows the wear comparison of 42CrMo material * * * P = 1200 W, V = 55 mm/s and D = 3.5 mm * * * It can be seen that laser strengthening technology can greatly improve the wear resistance of materials.
5 conclusion
Through the laser strengthening treatment of several different die materials, and compared with the actual working situation, it is shown that the service life of the die can be greatly improved by using laser strengthening technology, and the strengthening effect of the cold die is more obvious. If the punch of T8A steel and the die of Cr 12Mo steel are laser hardened, the laser hardened layer is 0. 15mm and the hardness is 1200HV, and the service life is obviously improved, from 25,000 pieces to 65,438+10,000 pieces, that is, the service life is improved by 3-4 times. The advantages of laser strengthening technology are as follows:
*** 1*** can be carried out in the designated area according to the shape characteristics and use requirements of the mold without any damage to the surface quality. After laser treatment, the mold can be directly put into production without subsequent processing, which reduces the manufacturing cost of the mold.
* * * 2 * * By compiling special laser strengthening machining software, the computer automatic optimization of laser machining parameters, computer simulation and real-time monitoring of machining process and computer prediction of surface structure and efficiency after laser machining are realized, and the complex shape of mold and artificial intelligence surface treatment are realized.
* * * 3 * * Using laser cladding and alloying technology, an alloy with any composition and corresponding microstructure can be obtained on the surface of cheap metal materials, so as to obtain good comprehensive mechanical properties and improve and improve the wear resistance, corrosion resistance and heat resistance of the material surface. These technologies are used to repair and strengthen scrapped molds and have broad market prospects.
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