The characteristic of molten pool during laser gas nitriding of titanium alloy surface

doi:10.21203/rs.3.rs-4919761/v1

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The characteristic of molten pool during laser gas nitriding of titanium alloy surface

https://doi.org/10.21203/rs.3.rs-4919761/v1

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Poor surface mechanical properties are an important factor restricting the wide applications of titanium alloys, while the laser gas nitriding process can effectively improve the surface mechanical properties. The convection behavior on molten pool surface in the nitriding process was captured using a high-speed camera system, the images of the molten pool surface were processed, the special convection behavior was analyzed, and the convection velocity on the molten pool surface was calculated. The forming characteristics of the nitrided layer section were observed, and the relationship between the special convection behavior of the molten pool and the section forming was analyzed. The experimental results show that the nitriding molten pool convection has the characteristics of discontinuity, locality and randomness; the alternating change of the relationship between the melting point and the temperature of the molten pool leads to the discontinuity of the convection , and the segregation of the molten pool composition and the non-uniformity of the parameters lead to the local and random nature of the convection; the special convection behavior of the nitriding molten pool results in the irregular section of the nitrided layer. According to these characteristics, a scheme of controlling the quality of nitrided layer is put forward by using non-uniform heat source.

Physical sciences/Materials science/Structural materials/Metals and alloys

Physical sciences/Engineering/Mechanical engineering

Nitriding

Laser processing

Titanium alloy

Molten pool convection

Image processing

The laser gas nitriding process for the titanium alloy surface is to irradiate the titanium alloy with laser in the nitrogen environment and melt its surface to form a molten pool, and the nitrogen reacts with the molten pool to form a nitride modified layer^1,2,3,4. This process can effectively improve the surface hardness and wear resistance of titanium alloys, which can potentially expand the application range of titanium alloys.

Extensively researches on the surface nitriding process for titanium alloys have been carried out, and fruitful results have been achieved. Shirazi et al.⁵ carried out nitriding tests on three different titanium alloys, and found that the microhardness of the nitrided layer was higher than that of the substrate, and the microhardness of the commercially pure Ti nitrided layer increased more significantly than that of the titanium alloy. Chan et al. ⁶ found that the wear resistance of the nitrided layer was significantly higher than that of the substrate, and the grain size of the nitrided layer was reduced by nanometer. Zeng et al.⁷ found that the nitrided layer was more corrosive than the substrate. Senthilselvan et al.⁸ found that the nitrided layer was composed of dendritic TiN and acicular martensite, and the smaller the grain size was, the better the mechanical properties of the nitrided layer. In the work of Chan et al.⁹, the cracks on the nitrided layer were reduced by optimizing the action time of laser, gas and material. Li et al.¹⁰ diluted N₂ with Ar to prevent cracks from being formed on the nitrided layer. Nassar et al.¹¹ and Kamat et al.¹² studied the coupling behavior of laser and nitrogen, and found that the CO₂ laser can ionize nitrogen. Kamat et al.^13,14 proposed and developed a two-step nitridation method. Although many research results have been achieved in this field, the nitriding process is rarely applied in industrial practice. The fundamental reason is that we have no clear understanding of the mechanism of the nitriding process and cannot stably control the quality of the nitrided layer. Therefore, at present, it is necessary to study the laser gas nitriding process for the titanium alloy surface from a new perspective, such as from the perspective of molten pool convection.

In the processes of welding, cladding and laser selective melting, the molten pool experiences rapid convection under various forces, and the convection behavior significantly affects the formation and quality of the solidified layer. Limmaneevichitr et al.¹⁵ studied the flow behavior of laser molten pool of transparent material NaNO₃, and they obtained he convection image of molten pool, which directly proved the convection behavior of laser molten pool. Zhao et al.¹⁶ found that the maximum convection velocity of laser molten pool was 1m/s. According to the findings of Song et al.¹⁷, the convection behavior dominated the energy transfer of the molten pool, Shi et al.¹⁸ found that the convection behavior dominated the mass transfer of the molten pool, and Xiong et al.¹⁹ found that the molten pool convection affected the nucleation and growth direction of grains. Höche et al.²⁰ studied the molten pool convection behavior on the titanium alloy surface produced by laser gas nitriding by using the numerical simulation method, and found that the convection behavior of the molten pool in the nitriding process was similar to that in other laser processing processes, but the simulation of nitrogen element distribution in the nitrided layer was inconsistent with the experimental results. So far, there have been no other reports on the molten pool convection behavior on the titanium alloy surface produced by the laser nitriding process.

The innovation of this study lies in detection of the special convection behavior of molten pool in the nitriding process through experiment. A method to optimize the quality of the nitrided layer by regulating the convection behavior is proposed.

2.1. Experiments and samples

Figure 1 shows the schematic diagram of the experimental system. The nitriding experiment was carried out using a fiber laser (YLS-4000, IPG Laser Corp, Oxford, US) with a maximum output power of 4 kW. The laser wavelength is 1064 nm, and the laser spot diameter is 3 mm. The laser head was mounted on the arm of a six-axis industrial robot (KR30HA, KUKA, Augsburg, Germany). A high-speed camera system (Ispeed 3, Olympus Corporation, Tokyo, Japan) was combined with the narrow-band filter, and used to photograph the convection behavior on the molten pool surface. In the experiment, the frame rate of high-speed camera system was 20000 Hz. The high-purity nitrogen (99.9%) and laser beam were output coaxially from the laser head.

Nitrogen and titanium have metallurgical reaction in the molten pool to generate TiN, and the molten pool solidifies to form a nitrided layer. The surface of the Ti-6Al-4V sample with a size of 100 mm (L) × 80 mm (W) × 8 mm (H) was nitrided. Before the experiment, the sample surface was polished with an 800 # SiC abrasive paper and wiped with acetone to remove impurities. After the experiment, the nitrided layer was cut transversely and longitudinally, and the section was processed using the standard crystalline techniques (sectioning, mounting, manual grinding, and automatic polishing). The cross section was observed using a confocal laser scanning microscope (OLS5000, Olympius Corporation, Tokyo, Japan).

We performed more than 50 experiments and obtained similar results. This paper presents the most representative experimental results. Table 1 shows the process parameters.

Table 1

Process parameters.
Power (kW)	Scanning speed (mm/s)	Off-focal distance (mm)	Gas flow rate (L/min)	Nozzle Distance (mm)
1.5	10	3	10	10

2.2 Image processing

The video captured by the high-speed camera system is processed into frames to obtain the photos of the molten pool surface. Figure 2 is a typical picture of the molten pool surface. Because the narrowband filter can filter out the radiation of other wavelengths, the molten pool surface presents a blue color in general. There is a certain angle between the shooting direction of the high-speed camera system in Fig. 1 and the molten pool surface, so the molten pool in Fig. 2 shows an elliptical shape.

In Fig. 2, most areas on the molten pool surface are bright blue, and only a few areas are dark blue. The bright blue and dark blue areas can also be seen in the video. There is no convection in the bright blue area, like it is solidified; there is rapid convection in the dark blue area. Sometimes, the dark blue area will flow to the edge of the molten pool in bursts, just like the waves.

To further understand the convection behavior, the image of the molten pool surface was processed. The image processing process is as follows: Fig. 3 (a) is the original photo, and Fig. 3 (b) is the photo after cutting. To facilitate image processing, the true color image is converted into a gray image. Figure 3 (c) shows the gray image of the molten pool.

First, the edge of the molten pool is detected, then the convection area is identified, and finally the two are combined for display. The method for detecting the molten pool edge is as follows. The molten pool edge in Fig. 3 (c) is blurry, so Fig. 3 (c) is sharpened and binarized to get Fig. 3 (d). In order to reduce the noise in Fig. 3 (d), the expansion and corrosion processing is performed for multiple times to obtain Fig. 3 (e). The edge detection technology is used to detect the edge of the molten pool. Figure 3 (f) shows the edge of the molten pool.

The method to identify the convection zone is as follows: sharpen and binarize Fig. 3 (c) to obtain Fig. 3 (g). The threshold value of this binarization is higher than the threshold value of binarization processing for detection of the molten pool edge. Because the convection area in Fig. 3 (g) is connected with the molten pool edge, the closed function operation is performed to obtain Fig. 3 (h). Figure 3 (i) is obtained after noise reduction, and Fig. 3 (j) is obtained after filling. Figure 3 (k) is obtained by subtracting Fig. 3 (i) from Fig. 3 (j). Figure 3 (k) shows the convection area.

Combine the molten pool edge in Fig. 3 (f) with the convection area in the molten pool in Fig. 3 (k) to get Fig. 4. In Fig. 4, the white curve is the molten pool edge, and the white area is the convection area.

Figure 5 shows the process of image processing, which only represents a typical image processing process. Because the photos have their own characteristics, the processing flow and specific parameters should be flexibly adjusted during image processing.

3.1 Convection behavior on the molten pool surface

The video of the molten pool surface convection is framed. Figure 6 shows the photos of a typical molten pool surface. The photos are named in chronological order, and for example, Fig. 6 (a) represents the 25898th photo. The photos in Fig. 6 are processed to obtain Fig. 7. The convection area in Fig. 7 (a) is located in the center of the molten pool, which is very small. The size of the convection area in Fig. 7 (b) increases, and the size of the convection area in Fig. 7 (c) reaches the peak, accounting for about 30% of the molten pool surface. At this time, the convection area is located in the middle and rear of the molten pool, indicating that the liquid metal flows from the center of the molten pool to both sides and the rear of the molten pool. Starting from Fig. 7 (c), the size of the convection area no longer increases, the color of the convection area gradually changes from dark blue to light blue, and finally the convection area disappears, as shown in Fig. 7 (d). The molten pool surface maintains the state shown in Fig. 7 (d) for a long time, as if it is solidified, and there is no convection. After a while, the convection area will be formed again, and the process as shown in Fig. 7 will be repeated.

3.2 Convection velocity on molten pool surface

To calculate the convection velocity of the molten pool, small particles with high melting points can be added to the molten pool. As the particles convect synchronously with the molten pool, the convection velocity of particles can be used to characterize the convection velocity of the molten pool. In Fig. 7, the convection area gradually increases, and the movement of the convection area edge is similar to the movement of particles, which can roughly represent the convection velocity of the molten pool.

The blue curve, green curve and red curve in Fig. 8 represent the contours of the convection areas in Fig. 7 (a), Fig. 7 (b) and Fig. 7 (c), respectively. The yellow straight line intersects with the blue curve, the green curve and the red curve at point A, point B and point C. Assuming that point A flows to point B along the straight line, and then flows to point C along the straight line, the moving speed of this point can represent the convection velocity on the surface of the molten pool. The convection velocity of molten pool can be calculated by Eq. (1).

$$\:\text{v}=\frac{s}{t}$$

where, v is the surface convection velocity of the molten pool, s is the distance from point A to point C in Fig. 8, and t is the time taken to move from point A to point C in Fig. 8.

In this study, the shooting frequency of the high-speed camera system is 20000 Hz. The time interval between Fig. 7 (a) and Fig. 7 (c) can be calculated, which equals to the time t to move from point A to point C in Fig. 8.

To calculate the actual distance between point A and point C in Fig. 8, the number of pixels between point A and point C should be obtained firs; then, the relationship between the pixel points in the figure and the actual distance is calculated; finally, calculate the actual distance s between point A and point C is calculated proportionately. According to Eq. (11), the convection velocity of molten pool is 231 mm/s.

The moving speed of the convection area boundary is different from the actual convection speed of the molten pool, but they are very close. The moving speed of the convection area boundary can be used to roughly characterize the convection speed of the molten pool.

3.4 Morphology of nitrided layer

The cross section and longitudinal section of the nitrided layer were observed using the laser confocal system. Figure 9 (a) shows the cross section of the nitrided layer, in which, the white curve represents the boundary between the nitrided layer and the substrate, and the cross section of the nitrided layer is irregular. Figure 9 (b) shows the longitudinal section of the nitrided layer. The nitrided layer has the maximum depth of 782 µm and the minimum depth is 533 µm, with a difference of 46.7% between them. The longitudinal section of the nitrided layer is also irregular.

4.1 Characteristics of convection behavior

In the process of laser gas nitriding of the titanium alloy surface, the molten pool convection behavior presents obvious characteristics of discontinuity, locality and randomness.

(1) In the time dimension, the convection behavior presents the characteristics of discontinuity and randomness. Discontinuity refers to that the molten pool convection lasts for a while and then stops for a while; randomness refers to the uncertainty of both the convection duration and suspension.

In previous studies, the continuous convection of the molten pool in the processes of welding, cladding and selective laser melting processes has been investigated ^21,22,23, but there has been no report on the intermittent convection of the laser molten pool. Through numerical calculation, Höche et al.²⁰ found that the molten pool in the nitriding process had continuous convection, but the distribution characteristics of nitrogen elements in the nitrided layer obtained by simulation were not consistent with the experimental results. We believe that the research of Höche et al. needs further optimization.

After detailed analysis of the video and image processing photos of the molten pool convection, we found that the convection duration and suspension of the molten pool are both uncertain. Therefore, we analyze the convection behavior of molten pool from the perspectives of probability and statistics. The 3000 ms video is divided into various frames, 60000 photos are obtained, and statistical analysis is performed on these photos. A total of 20 convection events have occurred. Table 2 summarizes the relevant data of molten pool convection. Convection occurs once every 150 ms, and the average duration of convection was 14.8 ms. The duration of convection accounts for about 10% of one cycle, and there is no large convection in the rest of the time.

Table 2

Statistics of molten pool convection data.
Run #	Convection initiation time (ms)	Convection duration (ms)	Run #	Convection initiation time (ms)	Convection duration (ms)
1	96.1	13.1	11	1195.5	12.8
2	163.3	13.5	12	1308.6	14.3
3	451.5	13.1	13	1465.7	17.2
4	509.3	13.7	14	1521.3	13.6
5	635.9	14.0	15	1554.3	16.4
6	707.9	13.8	16	1944.1	15.8
7	816.5	14.8	17	2158.7	15.4
8	862.5	15.7	18	2293.3	16.4
9	1025.3	14.9	19	2615.7	15.6
10	1099.5	14.8	20	2899.1	16.5

(2) In the spatial dimension, the convection behavior of molten pool is characterized by locality and randomness. Locality means that the convection area does not occupy all the molten pool, but only occurs in part of the molten pool. Randomness indicates that the location of the molten pool convection is uncertain.

In previous studies, it is believed that convection would occur in all laser molten pools in the processes of welding, cladding and selective laser melting ^24,25,26, but there has been no report on the local convection. Nassar et al.¹¹ believed that multiple convection areas would form in the molten pool of the nitriding process, but they did not find strong evidence to support this idea. We found that the convection behavior of molten pool in the nitriding process presents locality. Figure 10 shows the schematic diagram for the convection behavior of the molten pool. In Fig. 10 (a), the titanium alloy melts under the action of laser, and the molten pool forms the traditional Marangoni convection. In this case, all the molten pools present continuous convection. Then, the nitrogen enters the molten pool and reacts with the titanium alloy. The melting point of the generated TiN is higher than the temperature of the molten pool, so the molten pool solidifies, and there is no convection at this time, as shown in Fig. 10 (b). With the accumulation of heat, the temperature of some areas is higher than the melting point of TiN, and some areas are remelted and form the convection area, as shown in Fig. 10 (c). Convection transports the low-temperature metal at the bottom of the molten pool to the top of the molten pool. When the surface temperature is lower than the melting point, the molten pool solidifies again, and the convection area disappears, as shown in Fig. 10(d). The heat accumulation causes the temperature of local area in the molten pool to be higher than the melting point, so there is local melting in the molten pool, which presents the local convection behavior.

Table 3 summarizes the characteristics of molten pool convection behavior in the nitriding process.

Table 3

Characteristics of convection behavior of molten pool in the nitriding process.
Ratio of convection area to total area (%)	Average convection duration (ms)	Ratio of convection time to total time (%)	Velocity (mm/s)	Time dimension	Spatial dimension
30	14.77	10	231	Discontinuous and random	Local and random

4.2 Causes for special convection

The melting point of TiN is up to 3588 K, and that of Ti-6Al-4V is 1923 K. In the nitriding process, the laser heat causes the surface of Ti-6Al-4V to melt and form a molten pool. Then, the nitrogen and titanium react on the surface of the molten pool and generate a large amount of TiN, causing the melting point of the surface layer of the molten pool to be higher than its temperature, and this area solidifies and stops convection, which is the bright blue area in Fig. 2. We just habitually call this area the "molten pool", but in fact, this area has solidified, so the "laser irradiation area" is a more accurate term.

Due to heat accumulation, the temperature of some areas on the molten pool surface is higher than its melting point. These areas are in the melting state, and convection occurs under the effect of surface tension gradient. Convection causes the low-temperature liquid metal at the bottom of the molten pool to reach the surface of the molten pool, and forms the dark blue area in Fig. 2. The radiation of the low-temperature liquid metal is weaker²⁷, so these areas show a dark blue color.

The molten pool in nitriding process is different from that in the welding, cladding and selective laser melting processes. In the nitriding process, the chemical composition of the molten pool will change significantly, so the melting point and other thermophysical parameters will also change significantly. We believe that the alternating change of the relationship between the melting point and temperature of the molten pool leads to the alternating change of the melting and solidification states of the molten pool, so the convection of the molten pool is discontinuous.

The specific physical parameters of the molten pool can be obtained through numerical calculation, and the results of numerical calculation can verify the above conclusions.

The alternate changes between the solidification and melting states on the molten pool surface lead to discontinuous convection of the molten pool.

Due to some random interference factors, such as local segregation of material components and uneven thermophysical properties, convection shows the characteristics of locality and randomness. In the numerical simulation, it is assumed that all conditions are ideal conditions, and the composition and performance of all parts of the material are completely consistent, so the convection does not show locality and randomness.

4.3 Effect of special convection behavior on nitrided layer

In the process of material processing and surface modification, the energy on the material surface is transported to the bottom of the molten pool by convection and heat transfer ²⁸. Convection dominates the energy transfer process, and the effect of heat transfer is relatively weak ²⁹. In the processes of welding, cladding and selective laser melting, continuous convection occurs in all the molten pools, and the energy on the molten pool surface is uniformly transmitted to the bottom of the molten pool. The bottom of the molten pool is relatively uniform, and the cross section and longitudinal section of the solidification layer both present regular morphology ^30,31. In this study, we found that the molten pool convection in the nitriding process has the characteristics of discontinuity, locality and randomness, so the energy transfer is not uniform. In the nitriding process, when the molten pool has strong convection, a large amount of energy on the molten pool surface will be transferred to the bottom of the molten pool, and the depths of the molten pool and the nitrided layer increase, as shown by point E in Fig. 9 (a). When the convection of molten pool stops, only a small amount of energy is transferred to the bottom of the molten pool through heat transfer, and the depths of the molten pool and the nitrided layer are relatively shallow, as shown by point F in Fig. 9 (a). The discontinuity, localities and randomness of the convection behavior are the fundamental reasons for the irregularity of the cross-section and longitudinal section of the nitrided layer.

4.4 Quality control scheme for nitrided layer

The convection behavior of molten pool is conducive to its heat and mass transfer, impurity discharge and metallurgical reaction^32,33. According to our findings, the discontinuity and locality weaken the convection behavior of the molten pool in the nitriding process, which is detrimental to the formation and quality of the nitrided layer, and it needs to be controlled. The melting point of TiN is 3588 K, while the vaporization temperature of Ti-6Al-4V is 3533 K. If the laser power is increased to increase the temperature of all molten pools to above 3588 K, it can ensure that the molten pool is always in the melting state during laser irradiation, and all molten pools have continuous convection. However, in this case, the Ti-6Al-4V substrate in front of the molten pool will vaporize massively, and the steam recoil force will seriously deteriorate the formation of the nitrided layer.

In this study, the non-uniform heat source was used to perform the nitriding experiment. The laser energy density is low in the front of the laser scanning direction and high in the rear. The temperature field distribution of the molten pool is as shown in Fig. 11. The temperature of the low-temperature zone in front of the molten pool is higher than the Ti-6Al-4V melting point. In this zone, the Ti-6Al-4V substrate melts and reacts with nitrogen to generate TiN. The temperature of the high-temperature zone in the rear of the molten pool is higher than the TiN melting point. This zone is always in the melting state, where large-area continuous convection occurs, which can improve the quality of the nitrided layer. In the future, we will adopt the above scheme to control the quality of nitrided layer.

1. In the titanium alloy surface produced by the laser gas nitriding process, the convection behavior of molten pool presents the characteristics of discontinuity, locality and randomness.

2. The maximum convection velocity of molten pool in the nitriding process is about 231 mm/s, which is significantly lower than the convection velocity of laser molten pool in the processes of welding, cladding and selective laser melting.

3. The change of chemical composition in the molten pool leads to the change of its melting point. The alternating change of the relationship between melting point and temperature leads to the alternating change of the melting and solidification states of the molten pool, so the convection in the molten pool is discontinuous.

4. Some random interference factors, such as the local segregation of material components and uneven thermophysical properties, lead to the locality and randomness of convection.

5. The discontinuity, locality and randomness of convection lead to uneven energy transfer in the molten pool, so the cross section and longitudinal section of the nitrided layer present irregular morphology.

Data availability

The datasets used and generated during the current study are available from the corresponding author upon reasonable request.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 52075235), the Major Science and Technology Projects (Grant No. 22ZD6GA008).

Author information

State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China

Jinchang Guo & Yu Shi

School of Intelligent Manufacturing, Longdong University, Qingyang 74500, China

Jinchang Guo

Contributions

All authors share this paper’s activites equally.

Corresponding author

Correspondence to Yu Shi.

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No competing interests reported.

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The characteristic of molten pool during laser gas nitriding of titanium alloy surface

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Abstract

Figures

1. Introduction

2. Methods

2.1. Experiments and samples

2.2 Image processing

3. Results

3.1 Convection behavior on the molten pool surface

3.2 Convection velocity on molten pool surface

3.4 Morphology of nitrided layer

4. Discussion

4.1 Characteristics of convection behavior

4.2 Causes for special convection

4.3 Effect of special convection behavior on nitrided layer

4.4 Quality control scheme for nitrided layer

5. Conclusions

Declarations

References

Additional Declarations

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