An investigation of quality in CO₂ laser cutting of aluminum

Abstract

Aluminum alloys are widely used in the aeronautics industry and nowadays have become of importance to the automotive industry as well. This paper investigates experimentally the quality of laser cutting for the aluminum alloy AA5083, with the use of a pulsed CO₂ 1.8 kW laser cutting system. The quality of the cut has been monitored by measuring the kerf width, the edge roughness and the size of the heat-affected zone (HAZ). This work aims at evaluating processing parameters, such as the laser power, the scanning speed, the pulsing frequency and the gas pressure, for the laser cutting of aluminum alloys. A statistical analysis of the results has been performed in order for the effect of each parameter on the cutting quality to be determined. The regression analysis has been used for the development of empirical models able to describe the effect of the process parameters on the quality of laser cutting.

Introduction

Aluminum and its alloys are among the most versatile engineering and construction materials because of their unique characteristics. They have become the world's second most used metal after steel. Annual primary production of aluminum in 2006 was around 34 million tonnes and recycled production around 16 million tonnes. The total of some 50 million tonnes compares with 17 million tonnes of copper, 8 million tonnes of lead and 0.4 million tonnes of tin [1].

Aluminum alloys are characterized by their light weight, high strength and increased high resistance to corrosion compared to steels. They have become an essential metal in many industries such as the automotive, construction, and aeronautics industry. For maritime applications, Al-5-xxx alloys are used for boat building and shipbuilding due to their high corrosion resistance. This alloy series is enriched with magnesium and derive most of their strength from work hardening. The 5-xxx series are suitable for cryogenic applications and low temperature work.

Aluminum alloy 5083 presents exceptional performance in extreme environments; it is highly resistant to an attack by both seawater and industrial chemical environments and retains exceptional strength after welding. Typical uses of AA5083 include shipbuilding, rail cars, mine skips and cages as well as pressure vessels.

Aluminum can be machined with a number of different material removal processes. For cutting complex geometries in two dimensions (profiling) the most common way of processing is using laser beam or water-jet cutting. Laser cutting process is a thermal process that severs the material by locally melting and/or vaporizing it with the use of a focused laser beam. A kerf is created through the relative motion between the laser beam and the workpiece surface [2]. Water-jet cutting on the other hand is also a non-contact process, however since it is not a thermal one it does not present any heat-affected zones on the workpiece. The process uses a jet of water at high velocity and pressure, or a mixture of water and an abrasive substance for “eroding” the workpiece surface and thus generating the cutting kerf. Zheng et al. [3] compared the two processes in terms of cost and cut quality. In general, laser cutting is considered to be a faster way of cutting thin sheets; however, the water-jet cutting is a non-thermal process and as such, there is no heat-affected zone developed. Furthermore, the water-jet cutting allows the processing of thicker materials. From the cost point of view, it has been shown by Zheng et al. that laser cutting is a more cost efficient processing method.

Laser-based processes are increasingly adopted by industry [2], [4], [5]. The laser cutting process is the most established one and has a great number of applications in the automotive, aerospace, shipbuilding and material processing industries. The laser cutting process can be used for severing a wide range of materials, such as metals, plastics, ceramics and composites. However, the matching of the most suitable laser source with the material to be processed assures the best results. Each type of laser creates a laser beam at a given wavelength; CO₂ at 10.6 μm and Nd:YAG at 1.06 μm. As an example, for the laser cutting of ceramic materials, Toenshoff et al. [6] investigated the use of CO₂, Nd:YAG and excimer laser sources and came to the conclusion that each laser source had its own advantages and disadvantages for such industrial applications.

The Nd:YAG lasers wavelength is better absorbed by most of the materials (copper, aluminum, precious metals, etc.). Steel however, exhibits acceptable absorption levels under the CO₂ laser irradiation. Further to this, the CO₂ laser generators can achieve higher powers at a lower cost. Therefore, for the laser cutting of flat sheet metals, the CO₂ laser sources are the most established ones. There are a great number of publications investigating the effect of the process parameters, after having used the CO₂ laser cutting of various steel grades [7], [8], [9], [10], [11], [12].

On the other hand, it is a common practice to use Nd:YAG lasers for cutting aluminum alloys. Due to its shorter wavelength, it is reflected to a lesser extent by metallic surfaces and this higher absorptivity of the Nd:YAG laser enables its processing with relatively less laser power. Recent studies on the cutting quality of various aluminum alloys have been reported. Dubey et al. [13], [14], [15] investigated the effect of various process parameters on the kerf quality using the Taguchi methods.

The use of CO₂ lasers for cutting of aluminum alloys is not common. Recent review studies [16] have reported few investigations into this field. However, as already mentioned, from the industrial point of view, such a possibility would be very welcome. In spite of the low absorption of the aluminum by the laser radiation at 10.6 μm, the process of cutting is feasible because the optical constants of the material vary with the temperature and then the aluminum's absorption is increased with the temperature. Olsen [17] was one of the first to have investigated the use of CO₂ lasers in a pulsed mode for cutting pure aluminum. Araujo et al. in [18] have experimentally investigated the microstructure in a heat-affected zone during the laser cutting of AA2024 by a CW CO₂ laser beam. Riveiro et al. [19] have also studied the microstructural characterization, the grain morphology, the kerfs’ dimensions and the surface finish of the CO₂ laser cutting of the same alloy, for a novel gas jet, working in a supersonic regime. On the other hand, Carpio et al. [20] have studied the fatigue behavior of that alloy. The literature survey has revealed that there is a lack for research on the use of CO₂ lasers for such applications, and furthermore, there is no investigation reported on the 5-xxx series of aluminum alloys.

The laser cut quality does not only depend on the selection of the appropriate laser source for the cutting of a specific material but several other process parameters must be taken under consideration. Investigations into the laser cutting of either steel or aluminum alloys by either CO₂ or Nd:YAG laser sources have identified as dominant process parameters the following ones [8], [13], [14], [15], [21], [22]:

• The laser power generated.

• The relative speed of the spot movement along the steel sheet, also referred to as cutting speed.

• The nature, pressure and flow rate of the cutting assisting gas.

• The frequency of the laser beam.

A correct set of process parameters is one that will allow the part to be cut with the minimum amount of thermal damage. As the contribution of heat depends on both the cutting power and the speed, the speed should be maximized and the power minimized in order for this damage to be minimised.

The proper selection of the process parameters will also decrease the development of plasma plume in front of the cutting zone. When plasma is formed in the cutting front, it results in absorbing and scattering the incoming light, thus, limiting the depth of the laser beam's penetration [10], [23]. The literature survey has revealed some basic guidelines for limiting the plasma development; namely, the selection of relatively low frequencies that allow the disappearance of the plasma between each pulse, and the use of laser beam powers [12]. Furthermore, the formation of plasma seems to be increasing in the processing of thicker workpieces [10]. Additionally, the use of a compressed assisting gas can further blow away the plasma from the cutting area.

Furthermore, the laser light's plane of polarization affects its reflectivity from the melted surface [24]. It has been proven that CO₂ lasers show greater dependence on polarization than do the Nd:YAG ones [24]. In order to minimize this effect, modern lasers, as the one used for this paper's experiments, have circularly polarized beams.

The scope of the present paper is to investigate into the CO₂ laser cutting process of aluminum 5-xxx alloys. The motivation for such an investigation is the fact that the CO₂ laser sources are more frequently found in machine shops and therefore, more and more engineers are trying to use them for cutting aluminum. Although, cutting is possible, the quality of its cutting is not the best. However, a high cutting quality is important, especially if the parts are going to be further assembled with tight tolerances. The aluminum alloy 5083 has been selected for the present investigation, since it is known for its exceptional performance in extreme environments. The effect of the most important parameters on quality characteristics, such as kerf width, HAZ and cutting edge surface roughness, for cutting aluminum alloy 5083 was assessed. A statistical analysis of the results has been utilized for determining the contribution of each individual parameter to the cutting quality.