^{1}

^{1}

^{*}

Laser processing and laser surface texturing in multiple fields have become a popular topic of study in recent decades. Understanding the principles behind the laser irradiation mechanism is an essential step in choosing the most effective process parameters. Through this study, the effects of power and pulse duration on the structure and surface pattern of stainless steel type 304 were examined, and optimized laser parameters were introduced for desired laser penetration and heat-affected areas on the surface. The analyzed sample was prepared by using variations of pulse durations and different pulsed energies. Looking at the trend of change of non-dimensional temperature along the surface, thickness, and center of the sample, the effects of pulse duration and intensity (corresponding to energy) were observed. Upon considering all the aspects of the irradiated spots, such as heat-affected area diameter, surface patterns, and penetration depth, the advantages and disadvantages of short and long pulse durations are mapped out clearly. Also, a new method to obtain the ablation threshold of stainless steel is introduced, and a thorough analytical solution is obtained.

In recent decades, lasers involving technologies and modeling of laser material interactions have gained notable attention in micro/nano manufacturing fields. Some of the most important applications of lasers are treatment and processing of variety of metals and alloys. Laser material processing involves high concentrated heating of solid targets using lasers to enable several types of structural changes, such as amorphization, hardening, and surface patterning, in a fast and precise manner, in micro, sub-micro, and nano scales [

Laser processing can be applied in two categories based on the energy requirements: applications that require relatively low energy with limited structural and physical changes, such as annealing of semiconductors, and applications that call for high energy transforms for significant structural changes over a large volume, such as welding [

In order to fully understand the mechanism behind laser irradiation as well as to increase the efficiency of the process, understanding the effects and controlling the process parameters is of utmost importance; therefore, choosing the most optimal pulse duration is essential to the final quality of a particular application [

Thin sheets of stainless steel type 304, with thickness of 0.5 mm were used for surface treatment in this study. The sample was irradiated with microsecond pulses to create the predetermined bullet-point patterns across its surface. Experiments were conducted using a pulsed Nd:YAG laser system with a peak power of 6 kW, and beam diameter of 20 um. For a wavelength of 1064 nm provided by this laser system, a coefficient of absorption of 0.3 was used for stainless steel type 304 throughout the calculations [

The study was completed using variations of intensity of the laser system. The delivered intensity was controlled by reducing the voltage at 25 intervals starting at 400 V to 275 V. This trend was applied to three different pulse durations of 20 ms, 10 ms, 5 ms.

This experiment provided a very confined and controlled experimental environment for the required observations.

Upon completing the experiment, the effects of power and pulse duration on the surface structure, heat-af- fected zone on the surface, depth of penetration, and the created surface patterns were fully examined. After obtaining a thorough map of effects of process parameters, a series of optimal parameters are presented based on the observations.

Laser Setup | Surface of the Sample (Stainless Steel 304) | |||||
---|---|---|---|---|---|---|

Spot 1 400 V (J/µm^{2})_{ } | Spot 2 375 V (J/µm^{2}) | Spot 3 350 V (J/µm^{2}) | Spot 4 325 V (J/µm^{2}) | Spot 5 300 V (J/µm^{2}) | Spot 6 275 V (J/µm^{2}) | |

Pulse Width 20 ms | 0.244 | - | 0.186 | 0.159 | 0.132 | 0.107 |

Pulse Width 10 ms | 0.158 | 0138 | 0.119 | 0.100 | 0.081 | 0.064 |

Pulse Width 5 ms | 0.088 | 0.076 | 0.064 | 0.053 | 0.042 | - |

Since laser processing mainly involves transformation hardening (which is dominantly a heat transfer process), examining the temperature distribution and modeling the thermal process from the initial stages of heating is an effective method to investigate the connection of process parameters with the desired outcome results [

To proceed with this method, a cylindrical symmetric flow without internal heat generation, and constant, isotropic material properties was assumed; from there, a transient heat conduction equation and a proper boundary condition, as shown in the following, were defined to develop the temperature history of the process to estimate the temperature at each point [

where

where

Taking into account the vast range of scale variation between the required parameters (for example, while temperature changing is in orders of thousands of degrees, pulse duration only changes in orders of milliseconds), it is more convenient to define a set of non-dimensional groups, and to simplify the obtained equations in terms of temperature, radius, location of the spot, and the required constant values [

where,

These non-dimensional variables are then used to simplify Equations (1) and (2). Equation (3) displays the simplified boundary equation [

where Q is the non-dimensional power and is defined as:

Taking the Laplace transform of the redefined heat conduction equation, and solving the resultant differential equation, we can then calculate the integral form of the temperature. Hence, the maximum surface temperatures of various pulse durations can be obtained to show the non-dimensional temperature profiles under each condi- tion. As stated, the non-dimensional temperature in this study was tracked along the radius, thickness, and center of the work piece. Equations (4)-(6) define these temperature profiles [

Comparing the temperature profiles of each case can consequently lead to estimating the most optimal process parameters for more efficient laser processing applications.

An important theory to be considered when working with non-dimensional temperatures is the behaviour of the temperature profile in cases of constant power and constant energy. When looking at a constant power case, a shorter pulse duration results in a lower non-dimensional temperature [

Multiple bullet point patterns with stated properties are created upon the surface of the indicated stainless steel sample.

Each irradiated spot displays different layers and patterns across its surface. This creation of different zones on the surface is due to the keyhole generation phenomenon that takes place during any laser surface texturing process [

larger triangle signifies the secondary heat-affected zone. When processing materials using a laser, controlling the size of each of these layers is an essential factor in the precision and quality of the final results. Hence, it is essential to identify the effects of intensity and pulse duration on these layers [

The effects of power and pulse duration on the size of each indicated zone can be further examined by looking at the non-dimensional temperature profiles of various pulse durations across the corresponding outer diameters (primary heat-affected zone).

It is important to note that in this study intensity is obtained based on the energy delivered to the sample. As expected, the diameter of the heat-affected zone increases with an increase in intensity, hence an increase in the delivered energy. At a constant intensity (energy per area), the shorter pulse duration results in a larger outer diameter. A case of a constant intensity limits the amount of available energy; therefore, when using shorter pulse duration, the constant energy will be delivered in a more concentrated manner, since the process is not long enough for the energy to spread across the material. This in turn results in a more heat accumulation, and therefore a larger heat-affected zone diameter is achieved using shorter pulse duration [

After examining the behaviour of the outer diameter, in accordance to intensity and pulse duration, exploring it at various key points is essential in understanding this behaviour. The important zones at which significant changes occur in the behaviour of outer diameter are indicated in

threshold, and that the material is reaching the saturation state. Laser ablation is generally defined as material removal in macroscopic scale from the surface of the work piece, due to a significant change of state, where the ablated amount transforms into the gas or plasma phase [

In this work, a method to develop the ablation threshold of stainless steel along with a thorough analytical procedure is presented. This method can be further customized to accommodate the ablation thresholds of any other metals and alloys.

For lasers with Gaussian beam profile, if heat propagation effects are neglected, the feature size of the ablated area can be calculated by [

where, r_{O} is the radius of laser spot, φ_{th}(N_{s}) is the multi-pulse ablation threshold which depends both on the number of pulses and material and φ_{O} is the maximum laser flounce which is given by:

From Equations (7) and (8), the one pulse ablation threshold of the stainless steel can be calculated by:

_{th}(1) at different pulse durations based on the analytical solution. As shown in the figure, the ablation threshold for one pulse decreases with decrease in the pulse duration. Also, the threshold nearly reaches zero at diameter of 60 um which is in a good agreement with our experimental results shown in

Depending on the application required, the intensity and pulse duration could be varied and controlled in order to reach or avoid the ablation threshold and achieve the desired results in any of the presented zones.

The trend of change in heat-affected zones can be further explained by looking at the relation between the maximum non-dimensional temperature, intensity, and non-dimensional power used with each indicated pulse durations.

Looking closely at graph 1), at a constant non-dimensional power, shorter pulse duration results in a lower non-dimensional temperature. However, in graph 2) the opposite is observed: at a constant intensity, the shorter pulse duration develops a higher non-dimensional temperature. This corresponds with the behaviour of the outer diameter at different pulse durations and intensity values presented in

The next portion of the study mainly concentrated on the changes in non-dimensional temperature along the surface and depth of the work piece. According to

In

Surface pressure gradient could be an initiative for the formation of these structures. It is known that the pressure near the concave portion of a surface of a material is larger than on the convex parts. Upon melting of the surface material due to laser irradiation, this pressure difference could cause the molten liquid to move towards the convex regions of the surface, resulting in the creation of wave-like patterns [

To further understand the relation among the outer and inner diameter due to the variation of laser parameters effects, the non-dimensional temperature along the surface is examined.

As presented in the experimental results, at shorter pulse durations the secondary heat-affected zone (molten area) is significantly smaller than with the longer pulse durations; this can clearly be seen in

After examining the changes across the surface of the sample, it is essential to look at the effects of laser parameters, particularly pulse duration, on depth and penetration of metals.

In

A stainless steel type 304 sample was used to investigate the effects of pulse duration and power on the surface structure, patterns, and penetration, during laser irradiation. Also, a method for obtaining the ablation threshold of stainless steel for one pulse was introduced, which could be customized for any other metals and alloys. After a thorough analysis, it was observed that pulse duration was an essential factor in controlling the quality of the final product. In order to obtain a better surface resolution with minimum damage to the surrounding areas of the irradiated spot, shorter pulse durations should be used. For applications involving surfaces, larger pulse durations are recommended due to their larger heat-affected zone diameters. Adjusting the intensity, that is, the energy delivered to the material, can control the depth of the penetration. Further, the introduced method for the ablation calculation showed that the ablation threshold for one pulse decreased with decrease in the pulse duration, and that the results were in very close agreement to the obtained experimental results.

The authors declare that there is no conflict of interests regarding the publication of this paper.

MitraRadmanesh,AmirkianooshKiani, (2015) ND:YAG Laser Pulses Ablation Threshold of Stainless Steel 304. Materials Sciences and Applications,06,634-645. doi: 10.4236/msa.2015.67065