Preparation and application of the hottest laser l

2022-08-08
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Laser light absorbing coating preparation and process optimization application

Abstract: laser processing technology realizes the combination of optical, mechanical and electrical technology. It is an advanced manufacturing technology. At present, it is in the stage of actively penetrating into many technological processes in traditional manufacturing technology. The reflective surface of metal substrate is serious, which causes a lot of light energy loss in the process of laser processing. In laser phase transformation hardening or melting treatment, light absorbing coatings play an important role in improving the utilization of light energy. This paper introduces the performance requirements, composition selection and process selection of light absorbing coatings; The orthogonal test method was used to optimize the coating ratio and process, and the optimal coating was obtained. The test results were verified by infrared absorption spectrum, and the optimal coating was finally determined

key words: light absorbing coating; Orthogonal test; Absorption spectrum; Laser

introduction

because CO2 laser has the advantages of high output power, stability, continuous adjustment, long service life and so on, laser surface treatment of metal materials mostly uses high-power CO2 laser to heat metal [1]. Most metals have high reflectivity to CO2 laser, generally higher than 70%, while bright metals can even reach 80% - 90% [2]. This is a major obstacle to laser surface modification of metal materials [3]. This not only causes the loss of most valuable laser energy, but also seriously threatens the health of operators. Therefore, in laser heat treatment, how to improve the absorption rate of laser irradiation energy on the metal surface, so as to make full use of the valuable laser energy and reduce the production cost of laser heat treatment is a very important problem. Pre coating on the metal surface can significantly improve the laser absorption []. The phosphating and blackening methods of coating carbon black or graphite used at home and abroad have a series of problems that can also reduce the amount of raw materials, such as low absorption rate, poor thermal stability, decreased absorption rate with the increase of temperature, difficult removal, pollution and other shortcomings. Several domestic coatings are not suitable for industrial production because of their immature technology and relatively expensive price. Therefore, the development of a new type of laser surface treatment absorbing coating with good performance and low price is undoubtedly of great practical significance and economic value to promote the industrialization of laser treatment

1 coating composition and process selection

1.1 determination of coating composition

the components of laser light absorbing coatings are mainly divided into: base materials (including diluents and binders), aggregates, thickeners, additives, etc. according to their functions. Among them, the base material mainly plays the role of dilution and bonding, and the aggregate is the main component of light absorption; Thickener can adjust the spraying process performance of the coating, and its combination with the base material can meet the requirements of coating surface film formation, and different additives have different functions. In the water-based coating prone to flash rust, selecting appropriate additives can avoid the generation of flash rust, and in the coating system that does not produce flash rust, you can also select appropriate additives to improve the light absorption performance of the coating. In general, the absorbing coating for laser surface treatment should have: good pre coating processability, flat surface, uniform thickness, good adhesion between the coating and the substrate material, pre coating and__ Laser treatment does not pollute the environment, does not corrode the base metal, and the coating is easy to remove after laser treatment, with low price

1.1.1 selection of base material

after the coating is pre coated, the coating should be dried in a short time, that is, the diluent is required to have good volatility. At the same time, in order to have better adhesion between the coating and the matrix material, the coating should have a certain viscosity. Therefore, industrial alcohol is selected as the diluent, and shellac sold in the market is used as the binder of absorbing coating. Choosing industrial alcohol as diluent can avoid the appearance of flash rust. Therefore, it is unnecessary to consider anti flash rust agent in the selection of additives

1.1.2 selection of aggregate

in the coating system, aggregate is an important part, and it is also the part with the largest amount of effective light absorbing ingredients. It is very important for the absorption coating used for laser surface treatment. In addition to high absorption rate of laser, SiO2 coating is considered to have the advantages of becoming liquid under laser radiation, evenly covering the material surface, forming a solid film after cooling [6], and easy removal after heat treatment. In addition, SiO2 as coating aggregate also has the characteristics of wide source, low price and no reverse spray when heated. Therefore, SiO2 is selected as the aggregate of light absorbing coating in this test

1.1.3 selection of thickening system

in order to meet the requirements of coating processability and storage stability of absorption coating, appropriate thickener should be added to the absorption coating. In this paper, bentonite is selected as thickener

1.1.4 selection of additives

rare earth can improve the surface activity of coating particles and increase the absorption of light [7]. Therefore, rare earth oxides are selected as coating additives

1.2 selection of treatment process

during the test of light absorbing coating, it was found that the coating prepared by directly mixing various components without any treatment had poor spraying performance, fluffy coating, uneven thickness and poor molding. Due to the fluffy surface, there must be more voids in the coating, which will greatly hinder the heat transfer. Therefore, ball milling is selected to treat the coating in order to improve the molding and light absorption properties of the coating. Ball milling can significantly refine the powder. With the ultra refinement of the material, its surface molecular arrangement, electronic distribution structure and crystal structure have changed, resulting in peculiar surface effect, small size effect, quantum effect and macro quantum tunneling effect, which makes the ultra-fine powder have a series of excellent physical, chemical, surface and interface properties compared with conventional materials [8]. Before ball milling, all components should be mixed in proportion, with a ball material ratio (mass) of 15 ∶ 1, and then a lower ball milling speed of 300r/min should be selected for ball milling, so as to prevent the "anti crushing" phenomenon of ultra-fine powder prone to occur during ball milling under high energy. Fig. 1 (a) is the micrograph without ball milling. It can be seen that the size of powder particles without ball milling is very uneven, and large particles visible to the naked eye are scattered on the agglomeration layer of small particles, and the maximum diameter of particles can reach 2 ~ 3 μ m。 And the thickener reduces the fluidity of the coating, which is an important reason for the poor surface molding of the coating

Figure 1 coating micro morphology after ball milling

Figure 1 (b) is the micrograph of powder particles after 3H ball milling. After milling for 3h, the overall volume of the powder decreased to 1/3 of that without milling. It can be seen from Figure 1 (b) that the uniformity of powder particles has been greatly improved, and the maximum visible particle size has been reduced to 1 μ M or so. The coating has dense spraying layer, uniform thickness and good molding. It is concluded that ball milling changes the properties of the coating and improves its surface forming ability. Figure 1 (c) is the micrograph of powder particles after ball milling for 12h. It can be seen from the photo that except for agglomeration, the particle morphology can not be seen, and the maximum particle size can not be judged. It shows that long-time low-energy ball milling has greatly refined the powder, and there is no "anti crushing" phenomenon. To sum up: ball milling can refine and homogenize the powder with uneven particle size, which greatly improves the spraying process performance and surface forming ability of the coating

2 orthogonal test

2.1 orthogonal test combination

laser parameter selection p=2100w, scanning speed f=1000mm/min, spot 10mm × 1mm rectangular light spot

under the above process parameters, the surface of the coating determined by orthogonal test will undergo phase transformation hardening after CO2 laser scanning. Due to the different light absorbing properties of different light absorbing coatings, the surface hardness of each sample will also be different. Due to the same cooling rate, it is mainly the amount of energy absorbed that affects the transformation hardening structure and improves the coordination mechanism of major technical equipment. Therefore, in the absence of melting, the higher the absorbance of the light absorbing coating, the higher the surface hardness of the phase change hardening layer. In this test, HT500 Rockwell hardness tester is used to test the surface hardness of the sample after laser treatment at an equal distance along the width direction. The test results are shown in Table 1

2.2 analysis of test data

data processing is carried out according to the variance method of orthogonal experiment, as shown in Table 2

Table 1 Orthogonal test combination and surface hardness (HRC) test

Table 2 average value of indicators of various factors and levels

note: range: d1=2.14; D2=1.80; D3=0.88。

calculated:

error column: se=s total - (s1+s2+s3) =4.1295,

variance ratio:

check the F distribution table:

for f1

f( α= 0.025,f1=3,f2=6)=6.5988>F1

F( α= 0.05, f1=3, f2=6) =4.7571

for f2

f( α= 0.05,f1=3,f2=6)=4.7571>F2

F( α= 0.10, f1=3, f2=6) =3.2888

for f3

f( α= 0.40,f1=3,f2=6)=1.1581>F3

F( α= 0.45, f1=3, f2=6) =1.0127

calculated from the above data:

(1) significance analysis

from the variance ratio, it can be seen that the influence of yttrium oxide content on the index surface hardness is significant at the level of 0.05; The influence of bentonite content on the index is significant at the level of 0.1; The influence of ball milling time on the index is significant at the level of 0.45, indicating that the reliability of this column is low

(2) change rule of indicators with various factors

Figure 2 is the change trend chart of test indicators with various factors drawn according to table 1 and table 2

Figure 2. The change trend of indicators with various factors

it can be seen from Figure 2 that with the increase of yttrium oxide content, the indicators first decreased and then increased; For bentonite, with the increase of its content, the index first increased and then decreased; The influence of ball milling time on the index is significant at the level of 0.45, so it has no credibility

(3) optimal proportion and process

from the range D, it can be seen that the influence of the above factors on the indicators is Y2O3 content, bentonite content and ball milling time in turn. At the same time, from the significance analysis, it can be seen that the influence of yttrium oxide and bentonite percentage content on the indicators has a high reliability. Therefore, the optimal proportion of this series of coatings can be determined as a4b3 by orthogonal test. For the ball milling process, because the ball milling time has the least impact on the index, and the variance calculation shows that the reliability of its impact law curve on the index is low, so the ball milling time can be unlimited in the process. The best coating process and proportion can be determined as: a4b3cx. However, for the molding process performance of the coating, the surface molding ability is improved after ball milling, and with the increase of ball milling time, the surface molding ability is better and the surface is flat. In industrial mechanical spraying, the surface forming ability has a great influence on the light absorption effect. Therefore, in the case of little influence on the light absorption performance, choose the 3H ball milling time with strong molding ability as the optimal process of the coating, that is, a4b3c4 is the optimal coating. This study was also verified by infrared absorption spectrum test. When the ratio is the same, the coating milled for 3h has stronger light absorption performance than the coating milled for 1H

3 infrared absorption spectrum test

in order to verify that a4b3c4 combination is better than a4b3c2 combination, infrared absorption spectrum test was carried out. Infrared absorption spectrum test can measure the transmittance of some characteristic peak light. The lower the transmittance, the better the absorption effect. For CO2 laser coating, it mainly depends on 10.6 μ M the transmittance of light at the peak

Fig. 3 infrared absorption spectrum curve of coating with different ball milling time

it can be seen from Fig. 3 that the two curves are completely consistent, and are at 10.6 μ M (corresponding to wave number 943cm-1) has low transmittance, indicating that this series of coatings have good light absorption performance. However, the infrared absorption spectrum curve of the coating after 3H ball milling is generally lower than 1h

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