- Email: [email protected]
Magnesium alloys (WE43 and ZE41) characterisation for laser applications
Applied Surface Science 233 (2004) 382–391
Magnesium alloys (WE43 and ZE41) characterisation for laser applications Sorin Ignata,b,*, Pierre Sallamanda, Dominique Greveya, Michel Lambertinb a
Laboratoire LTm, Universite´ de Bourgogne, 12 Rue de la Fonderie, 71200 Le Creusot, France Ecole Nationale Supe´rieure d’Arts et Me´tiers, LaBoMaP, Rue Porte de Paris, 71250 Cluny, France
b´
Received in revised form 1 April 2004; accepted 1 April 2004
Abstract One of the most important parameters in laser treatment is the quantity of beam energy absorbed by the substrate. Despite its important role played in laser processes, this factor is rarely available for the laser sources wavelengths and at high temperatures reached during such treatments. A series of experiments were carried out in order to characterise, from this point of view, two types of magnesium alloys, WE43 and ZE41, often used in laser applications (cladding, alloying, welding, etc.). The results represent an important step in order to understand Mg-alloys behaviour under laser beam action. # 2004 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Laser treatment; Microstructure; Coating; Absorption
1. Introduction The industry, as well as the Scientist, focused their attention on the development of the promising magnesium alloys, due to the increasing demand for lightweight materials in aerospace and automobile applications. Magnesium and magnesium alloys present a number of interesting advantages, such as [1,2]: high specific strength, good castability, can be turned or milled at high speed, good weldability under controlled atmosphere, etc. Despite these properties, magnesium alloys are used with many precautions, due to some poor properties, such as [1,2]: high chemical
*
Corresponding author. Tel.: þ33-3-85-59-53-15; fax: þ33-3-85-73-11-20. E-mail address: [email protected] (S. Ignat).
reactivity and, in consequence, limited corrosion resistance (in some applications), low elastic modulus, limited high strength and creep resistance (compared to some other materials) at elevated temperatures, etc. The fact that magnesium alloys are used in various conditions, from indoor environment (in electronic applications, for example) to intermittent salt splashes (in automotive applications, for example) results in an important interest manifested by the scientific community for the oxidation phenomenon [3]. Since the corrosion resistance of magnesium alloys is mainly influenced by its impurity content, two means are generally used to improve this resistance, namely alloy design and surface modification treatment [4,5]. The most part of protective coatings applied to magnesium alloys (such as electrochemical plating, conversion coatings, anodising or organic coatings [2,4,5,6]), in order to improve its behaviour
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.04.002
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
in an industrial environment, are realised at ambient temperature. However, in the last decade, laser applications have created and consolidated their place between the regular processes used to treat the magnesium alloys (cladding, alloying, welding, etc.). Such processes expose these alloys at high temperatures and detrimental contact with oxidising media, despite the gas protection that can be applied. The material behaviour during such treatments is very hard to be evaluated, followed and/or anticipated with precision. The first important parameter in any laser process is the coupling effect between the laser beam and the treated material. This effect depends on numerous and incontrollable parameters and, thus, it cannot be predicted with accuracy. The experiments that we carried out were conducted to explain, firstly, the coupling effect between magnesium alloys (with or without protective layers) and Nd:YAG laser beam and, secondly, to give an indication about the effect of laser energy for such materials. Since the absorbed energy is the only one that has an effect for the material transformation, it is very important to measure this energy. As for the magnesium alloys choice, ZE41 and WE43 were characterised, two types commonly employed in laser applications. ZE41 shows a good creep resistance, better than AZ91, for example [7]. On the other hand, the Mg alloys that contains yttrium and/or rare earths (such as WE43) appear to have the best combination of mechanical properties and corrosion resistance [8].
383
2. Experimental details 2.1. Equipment The laser–matter interaction energetic efficiency is defined by: Z¼
Ea Ei
where Ei is the incident energy of the laser beam and Ea the energy absorbed by the irradiated material. The efficiency defined this way can also be called absorption coefficient. If we suppose that the absorbed photon energy is entirely transformed into calorific energy within the material, Ea energies can be obtained using a calorimeter, in our case an isothermal differential Calvet micro-calorimeter (SETARAM C80D) [9]. The first step in this kind of experiments is the laser ‘‘real energy’’ determination Ei. By real energy, we understand the energy that physically arrives at the impact zone, after passing by the optical transmission and focalisation system. Ei energy, which corresponds to one laser pulse, can be measured by trapping the beam at a hollow sphere made in an absorbing material. The laser beam comes from a pulsed Nd:YAG laser (l ¼ 1:06 mm) and the sphere is machined in a graphite cylinder (Fig. 1). At the end of the laser pulse, the electric jack controlled by a microprocessor rapidly plunges into the calorimeter cells carrying silica rods, having inside its bottom part the graphite
Fig. 1. CALVET micro-calorimeter.
384
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
cylinder. Since this movement is very fast, the lost amount of energy (during the cell transportation to the bottom) can be neglected. After laser energy calibration, absorption measurements for all the substrate types were conducted. The absorbed energy Ea, provided each time by one laser pulse, is established using a similar principle as for the calibration: the thermal flows difference is measured between the two ‘‘cells’’ of the micro-calorimeter, the laboratory cell giving the thermal energy to measure (Ea), while the other cell is the reference one. The laser employed being the same as above, at the end of the laser pulse, two electric jacks controlled by a microprocessor rapidly plunge into the calorimeter cells, carrying two silica rods having two absolutely identical (in dimensions and mass) steel cells. As described previously, the fast movement of the two cells makes that the lost energy (during the cell transportation to the bottom) can be neglected. The steel cells carry the studied sample and, respectively, the witness one (reference cell), as shown in Fig. 1. The absorbed energy measured this way does not include that of the plume formed occasionally, when laser beam arrives at samples surface, which can be considered insignificant compared to Ea. This method can be applied in a lot of experimental set-up, which is an important consequence of our experimental configuration. However, it is a very sensitive method because the laser source used during the experiments must be stable in time and all the pulses delivered by the laser source identical. The main advantage of this technique is the fact that the measurements can be conducted even when the sample material passes to the melted state. Each measure takes between one and two hours, as the impact energy is low or high. To obtain coherent and reproducible values for absorption coefficient, each measure was replicated at least three times. The laser beam was focused on a 2 mm circular spot, at the sample surface. The spot dimension is, thus, in concordance with the real treatment conditions, applied to this kind of alloys in our laboratory, for the welding process. The sample dimensions were imposed by the measuring device, having 10 mm 10 mm 2 mm. Since the witness and the lab cell must have the same mass, each sample has 300 mg. For a better reproducibility, the surface of B-type (see Section 2.2) samples was polished and cleaned. The polishing process was
Table 1 Substrate additional elements content [7] WE43 (wt.%)
ZE41 (wt.%)
Y (4.11) Nd (2.28) Dy (0.27) Gd (0.19) Yb (0.09) Zr (0.45)
Zn (4.37) La (0.32) Ce (0.78)
conducted to have a plane surface but not a mirror one, because such a surface has a negative influence for laser beam–material initial interaction. 2.2. Materials Two magnesium alloys are investigated, WE43 and ZE41, whose standard composition are reported in Table 1 [7]. Besides the as-cast materials from the two types of magnesium alloys, two types of coated substrates were used, the coatings being realised by a HAE treatment (anodising process) and a conversion treatment (mordancage process), respectively. These standard coating processes are generally applied in order to improve the corrosion resistance. Chemical analysis of these coatings is provided in the third paragraph of this paper. Further in the paper, the first treatment will be noted T1 and the second T2. The substrate without treatment will be referred as the B letter. Thus, practically six types of materials/surfaces are studied: WE43-B, WE43-T1, WE43-T2, ZE41-B, ZE41-T1 and ZE41-T2.
3. Results and discussions 3.1. WE43 and ZE41 chemical composition analysis Before any study carried out on a material, its chemical composition and structure must be known. Previous experiments made for the two types of magnesium alloys (WE43 and ZE41) permitted to have an indication concerning the chemical composition, described in Table 1 [7]. SEM images of the two materials are presented in Fig. 2 (for ZE41) and Fig. 3 (for WE43), along with an SEM–EDS profile, in order to obtain the distribution of the main addition
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
385
Fig. 2. ZE41 microstructure and composition profile (on 245.1 mm).
elements. In the both cases, the addition elements are concentrated at grain boundaries. 3.2. T1 layer (anodising technique) The anodising treatments applied on the two types of magnesium alloys are quite similar. Both coatings look alike and can hardly be distinguished visually by their aspect or colour. Some compositional differences can be observed using SEM images (Fig. 4 for WE43 and Fig. 5 for ZE41) and especially SEM quantitative analysis (Table 2). However, the main layer’s constitutive elements are the same and in similar proportions, as we can see in Table 2, looking at the O, Al and
Mn percentages. It must be noted that Nd was also detected (present in the magnesium alloys), but it was not quantified since our data base does not contain references on it. 3.3. T2 layer (mordancage process) Despite the presence of the same main constitutive elements (Table 3), the protective layers that cover the two types of magnesium alloys present some differences, as it can be seen when comparing Figs. 6 and 7. It must be said that even visually, the two coatings are very different, WE43-T2 covering layers being more brilliant than those of ZE41-T2.
Fig. 3. WE43 microstructure and composition profile (on 281.6 mm).
386
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
Fig. 4. WE43-T1 surface layer SEM images.
Fig. 5. ZE41-T1 surface layer SEM images.
3.4. Laser beam absorption (at 1.06 mm)
decreases, in general, with the temperature because the apparent electron-lattice collision time shortens. In addition, hot metals are reactive and irreversible changes in reflectance due to chemical reactions at the surface (oxidation, roughness, etc.) tend to occur, except under high vacuum conditions. The total absorptance of a metal can be thought of as composed of three contributions, due to free electrons, interband transitions and surface effects. The reduced reflection
As explained in literature [10], metals are not immune to beam-induced changes in their optical properties: their reflectance usually decreases under irradiation. The effect is thermal in nature and makes metals susceptible to thermal runaway, particularly in the infrared domain where reflectances tend to be high. Moreover, the bulk reflectance of metals Table 2 SEM quantitative analysis—T1 coat layers Analysed zone
WE43: Fig. 4-X200 ZE41: Fig. 5-X200
Element Mg (wt.%)
Al (wt.%)
Mn (wt.%)
K (wt.%)
O (wt.%)
Y (wt.%)
Zn (wt.%)
Si (wt.%)
Ca (wt.%)
P (wt.%)
Ce (wt.%)
46.35 46.52
10.71 9.59
5.06 4.65
0.50 0.66
36.48 35.23
0.89 –
– 1.01
– 0.14
– 0.14
– 1.48
– 0.58
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
387
Table 3 SEM quantitative analysis—T2 coat layers Analysed zone
WE43: Fig. 6-X200 ZE41: Fig. 7-X200
Element Mg (wt.%)
Cr (wt.%)
Zr (wt.%)
K (wt.%)
O (wt.%)
Y (wt.%)
Zn (wt.%)
Si (wt.%)
Ca (wt.%)
S (wt.%)
Ce (wt.%)
69.93 46.33
8.08 16.31
– 1.06
– 2.10
17.84 24.71
4.04 –
– 7.24
– 0.29
0.12 0.16
– 0.71
– 1.10
under laser irradiation (which may or may not indicate enhanced absorption) can be the result of the two following reasons: mechanical deformation of the surface, possible at moderate irradiance, or plasma effects, which require irradiances sufficient for strong evaporation. Another encountered effect is that above room temperature the conductivities of typical metals decrease roughly linearly with temperature, and the same holds for liquid metals at too high temperatures.
Upon melting, most metals show an additional drop in conductivity [10]. The above considerations can also be employed in order to explain the absorption coefficient variation in the cases studied below. 3.4.1. Absorption for B-type samples The energy absorption measured in this case (and presented in Fig. 8) is not very elevated one. This fact will have negative consequences for a laser treatment
Fig. 6. WE43-T2 surface layer SEM images.
Fig. 7. ZE41-T2 surface layer SEM images.
388
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
Fig. 8. Absorption for B-type samples.
at 1.06 mm, because high laser energy will be needed to obtain the wanted effects. The particularity of WE43-B samples is the presence of two plateaus. The first one, as seen in Fig. 8, is around 25% (absorbed energy), when the substrate material is still in solid state. This first plateau is quickly reached, for energies of 1 J. When the substrate material change its state from solid to melted one (‘‘M’’ points in Fig. 8), the absorbed energy rises again, but slowly this time, to reach the second plateau at 40% of absorbed energy. As for the ZE41-B samples, only one plateau can be seen, reached quite rapidly (from 5 J). The experiments were conducted until 40–45 J without obtaining melted points on the samples, the absorbed energy being almost constant at around 30%.
3.4.2. Absorption for T1-type samples In this case, a similar behaviour was observed for both studied materials (WE43 and ZE41), as shown in Fig. 9. Firstly, starting with energies slightly superior to 0 J, the absorption rises very fast to 40%, the maximum absorbed energy for B-type samples. Increasing further the incident energy will slowly increase the absorption level until a plateau, observed for the two types of materials around 65% of absorbed energy. Melted state is reached at 13 J, without increasing more than a few percent the absorbed energy. The identical behaviour of the two materials can be explained by the similarities in the layers that cover the surfaces, issued from the anodising process, as it was presented earlier in this paper. The absorption
Fig. 9. Absorption for T1-type samples (HAE).
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
level and profile open the opportunities for a more uniform laser surface treatment than in the case of Btype samples. The presence of oxides at the substrate surface plays an important role in the laser beam–material coupling effect. Even if the anodised layer has only a limited thickness (tens of microns), two major objectives can be reached with it: anti-corrosion protection and good laser beam–substrate interaction. Due to the reduced thickness, the elements from the anodised layer will not influence the results of the laser treatment (such as cladding, alloying, etc.). It must be recalled that all above considerations are valid only in the case of a laser beam at 1.06 mm wavelength. 3.4.3. Absorption for T2-type samples It was shown earlier that the two T1-type surface treatments were almost identical, for the two studied materials (WE43 and ZE41). As a result, the laser beam absorption occurs in a very similar manner. The same thing cannot be said about the T2-type samples. First of all, there are some differences in the deposited layer composition and in the colour, especially. These differences, as expected, have an important influence for laser beam absorption by the substrate, as seen in Fig. 10. Concerning the laser beam absorption for WE43T2, it can be observed that a first plateau is reached very quickly, at about 20% of the absorbed energy. The same behaviour was observed for WE43-B samples, as shown earlier. This plateau is maintained until incident energies of about 8 J, threshold at which melted state
389
appears. As a consequence, the absorbed energy increases rapidly to reach the second plateau, at about 50%. At this point, it can be said that the surface nature for WE43-T2 has a negative influence on the laser beam–material coupling effect, since it increases the quantity of reflected laser energy. A completely different behaviour occurs in the case of ZE41-T2 samples. The absorbed energy increases quickly until 70% to reach the first plateau, without passing to the melted state. All this happens for incident energies lower than 1 J. Increasing more the incident energy, above 3 J, melted state occurs. As a result, the absorbed energy increases to 80%, which constitutes the second and final plateau. Comparing the two variations in absorbed energy, it can be said that the T2 surface treatment applied to the ZE41 magnesium alloy allows a very good laser– material interaction because of the absorption regularity. The length of the first plateau in the variation of absorption, along with the difference between the first and the second plateau, cannot have a good effect for the laser–material interaction, especially in the coupling stage of the last one. 3.4.4. Absorption—WE43 Until now we have seen the differences between the energy absorption comparing the same surface treatments applied to different magnesium alloys (WE43 and ZE41). Let us take a look at the energy absorption for the same alloy (WE43) covered by two different layers (T1- and T2-types) or not covered at all (B-type). The
Fig. 10. Absorption for T2-type samples.
390
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
Fig. 11. Absorption for WE43 magnesium alloys.
Fig. 12. Absorption for ZE41 magnesium alloys.
absorption variation with incident impact energy is presented in Fig. 11. Almost identical behaviour is shown by B-type and T2-type substrates for low levels of incident energy. Both types of samples quickly reach their first plateau, at 20% of absorbed energy. Increasing the incident energy above 7 J, T2-type samples will absorb more laser energy than B-type, reaching the melted state at 50% of the absorbed energy. On their side, B-type samples stay in solid state until 13 J. Above this threshold melted state occurs and the material absorbs 40% of laser beam energy. Thus, the energy needed to melt the B-type samples surface is greater than in the case of T2-type samples. As a result, the coupling stage will occur later in the case of B-type samples, a fact with negative influence in the first part of a laser treatment. A better behaviour presents the T1-type sample, being more uniform than the two others. Moreover, the absorbed energy is greater than in the two other cases, reaching quite rapidly 60–65%. The melted state does not change the absorption level a lot, which increases only by 5%. However, the energy needed to melt T1type samples is greater than in the case of T2-type samples and this fact must be carefully considered in laser treatments. At this point, it can be concluded that the surface composition and brilliance of T2-type coating layers on WE43 decreases the substrate properties in laser treatments.
The coupling effect between the laser beam (wavelength: 1.06 mm) and the substrate occurs with difficulties, a fact with negative consequences for the entire process. Much better behaviour shows T1-type samples. The absorption increases very fast with the incident energy and becomes stable around 65–70% when the melted state occurs. However, the best behaviour of the three cases studied is shown by the T2-type samples. The absorption in this case rises quickly at 70%, the material being still in its solid state. Increasing the incident energy, the second plateau is reached, at 85%, when the melted state appears. As a consequence, we are dealing with a very interesting comportment for laser surface treatments, because the melted state occurs quickly and at low energy. An important part of laser beam is absorbed, which can only increase the efficiency of the entire process.
3.4.5. Absorption—ZE41 The absorption variation with incident energy in the case of ZE41 magnesium alloy is presented in Fig. 12. The first conclusion is that the B-type substrate does not have an adequate behaviour for laser treatments.
4. Conclusions The experiments, made for measuring the laser energy absorption on magnesium alloys (WE43 and ZE41), represent a step forward in the understanding of laser process. Since magnesium alloys are often used covered by protective coatings, it is important to know not only the interaction mechanisms between ascast magnesium alloys and a laser beam but also when micrometric layers cover their surfaces. Absorption is the first parameter that affects laser treatment processes and, often, the most important. The results of our research proved that the layers obtained with the both techniques (anodising and
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
mordancage processes) have a positive influence on the laser beam–material interaction and for the laser treatments in general. In the case of magnesium alloys (WE43 or ZE41) laser treatments, better results can be expected for ZE41-T2. Anodising process equally offers good properties for laser beam–material interaction. It must be noted that all these conclusions are valid only in the case of Nd:YAG lasers (wavelength 1.06 mm) and the energy provided by one laser pulse was measured. Future experiments, with a de-focalised laser beam will be carried out, in order to study the influence of this important parameter, which is the laser beam focalisation, on the absorption coefficient.
Acknowledgements This work was performed in the frame of the PRESTIGE research project, financed by the French Research Ministry. The authors equally want to thank the industrial partners involved in this project (Honsel
391
Fonderie Messier, PMA, CLAIRE) and the other partners (E´ cole Nationale Supe´ rieure d’Arts et Me´ tiers, E´ cole Centrale de Lyon and Universite´ Aix-Marseille II), for their support and interest. References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. [2] J.E. Gray, B. Luan, J. Alloy Compd. 336 (2002) 88–113. [3] F. Czerwinski, Acta Mater. 50 (2002) 2639–2654. [4] K. Funatani, Surf. Coat. Tech. 133–1334 (2000) 264–272. [5] A.L. Rudd, C.B. Breslin, F. Mansfeld, Corrosion Sci. 42 (2000) 275–288. [6] A.J. Eifert, J.P. Thomas, Scripta Mater. 40 (8) (1999) 929–935. [7] C. Sanchez, G. Nussbaum, P. Azavant, H. Octor, Mater. Sci. Eng. A 221 (1996) 48–57. [8] J.G. Wang, L.M. Hsiung, T.G. Nieh, M. Mabuchi, Mater. Sci. Eng. A 315 (2001) 81–88. [9] O. Perret, Ph.D. thesis, University of Burgundy, France, 2000. [10] M. Von Allmen, Laser Beam Interactions with Materials, second ed., Springer, Berlin, p. 31, ISBN 3-54059401-9.
Magnesium alloys (WE43 and ZE41) characterisation for laser applications Sorin Ignata,b,*, Pierre Sallamanda, Dominique Greveya, Michel Lambertinb a
Laboratoire LTm, Universite´ de Bourgogne, 12 Rue de la Fonderie, 71200 Le Creusot, France Ecole Nationale Supe´rieure d’Arts et Me´tiers, LaBoMaP, Rue Porte de Paris, 71250 Cluny, France
b´
Received in revised form 1 April 2004; accepted 1 April 2004
Abstract One of the most important parameters in laser treatment is the quantity of beam energy absorbed by the substrate. Despite its important role played in laser processes, this factor is rarely available for the laser sources wavelengths and at high temperatures reached during such treatments. A series of experiments were carried out in order to characterise, from this point of view, two types of magnesium alloys, WE43 and ZE41, often used in laser applications (cladding, alloying, welding, etc.). The results represent an important step in order to understand Mg-alloys behaviour under laser beam action. # 2004 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Laser treatment; Microstructure; Coating; Absorption
1. Introduction The industry, as well as the Scientist, focused their attention on the development of the promising magnesium alloys, due to the increasing demand for lightweight materials in aerospace and automobile applications. Magnesium and magnesium alloys present a number of interesting advantages, such as [1,2]: high specific strength, good castability, can be turned or milled at high speed, good weldability under controlled atmosphere, etc. Despite these properties, magnesium alloys are used with many precautions, due to some poor properties, such as [1,2]: high chemical
*
Corresponding author. Tel.: þ33-3-85-59-53-15; fax: þ33-3-85-73-11-20. E-mail address: [email protected] (S. Ignat).
reactivity and, in consequence, limited corrosion resistance (in some applications), low elastic modulus, limited high strength and creep resistance (compared to some other materials) at elevated temperatures, etc. The fact that magnesium alloys are used in various conditions, from indoor environment (in electronic applications, for example) to intermittent salt splashes (in automotive applications, for example) results in an important interest manifested by the scientific community for the oxidation phenomenon [3]. Since the corrosion resistance of magnesium alloys is mainly influenced by its impurity content, two means are generally used to improve this resistance, namely alloy design and surface modification treatment [4,5]. The most part of protective coatings applied to magnesium alloys (such as electrochemical plating, conversion coatings, anodising or organic coatings [2,4,5,6]), in order to improve its behaviour
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.04.002
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
in an industrial environment, are realised at ambient temperature. However, in the last decade, laser applications have created and consolidated their place between the regular processes used to treat the magnesium alloys (cladding, alloying, welding, etc.). Such processes expose these alloys at high temperatures and detrimental contact with oxidising media, despite the gas protection that can be applied. The material behaviour during such treatments is very hard to be evaluated, followed and/or anticipated with precision. The first important parameter in any laser process is the coupling effect between the laser beam and the treated material. This effect depends on numerous and incontrollable parameters and, thus, it cannot be predicted with accuracy. The experiments that we carried out were conducted to explain, firstly, the coupling effect between magnesium alloys (with or without protective layers) and Nd:YAG laser beam and, secondly, to give an indication about the effect of laser energy for such materials. Since the absorbed energy is the only one that has an effect for the material transformation, it is very important to measure this energy. As for the magnesium alloys choice, ZE41 and WE43 were characterised, two types commonly employed in laser applications. ZE41 shows a good creep resistance, better than AZ91, for example [7]. On the other hand, the Mg alloys that contains yttrium and/or rare earths (such as WE43) appear to have the best combination of mechanical properties and corrosion resistance [8].
383
2. Experimental details 2.1. Equipment The laser–matter interaction energetic efficiency is defined by: Z¼
Ea Ei
where Ei is the incident energy of the laser beam and Ea the energy absorbed by the irradiated material. The efficiency defined this way can also be called absorption coefficient. If we suppose that the absorbed photon energy is entirely transformed into calorific energy within the material, Ea energies can be obtained using a calorimeter, in our case an isothermal differential Calvet micro-calorimeter (SETARAM C80D) [9]. The first step in this kind of experiments is the laser ‘‘real energy’’ determination Ei. By real energy, we understand the energy that physically arrives at the impact zone, after passing by the optical transmission and focalisation system. Ei energy, which corresponds to one laser pulse, can be measured by trapping the beam at a hollow sphere made in an absorbing material. The laser beam comes from a pulsed Nd:YAG laser (l ¼ 1:06 mm) and the sphere is machined in a graphite cylinder (Fig. 1). At the end of the laser pulse, the electric jack controlled by a microprocessor rapidly plunges into the calorimeter cells carrying silica rods, having inside its bottom part the graphite
Fig. 1. CALVET micro-calorimeter.
384
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
cylinder. Since this movement is very fast, the lost amount of energy (during the cell transportation to the bottom) can be neglected. After laser energy calibration, absorption measurements for all the substrate types were conducted. The absorbed energy Ea, provided each time by one laser pulse, is established using a similar principle as for the calibration: the thermal flows difference is measured between the two ‘‘cells’’ of the micro-calorimeter, the laboratory cell giving the thermal energy to measure (Ea), while the other cell is the reference one. The laser employed being the same as above, at the end of the laser pulse, two electric jacks controlled by a microprocessor rapidly plunge into the calorimeter cells, carrying two silica rods having two absolutely identical (in dimensions and mass) steel cells. As described previously, the fast movement of the two cells makes that the lost energy (during the cell transportation to the bottom) can be neglected. The steel cells carry the studied sample and, respectively, the witness one (reference cell), as shown in Fig. 1. The absorbed energy measured this way does not include that of the plume formed occasionally, when laser beam arrives at samples surface, which can be considered insignificant compared to Ea. This method can be applied in a lot of experimental set-up, which is an important consequence of our experimental configuration. However, it is a very sensitive method because the laser source used during the experiments must be stable in time and all the pulses delivered by the laser source identical. The main advantage of this technique is the fact that the measurements can be conducted even when the sample material passes to the melted state. Each measure takes between one and two hours, as the impact energy is low or high. To obtain coherent and reproducible values for absorption coefficient, each measure was replicated at least three times. The laser beam was focused on a 2 mm circular spot, at the sample surface. The spot dimension is, thus, in concordance with the real treatment conditions, applied to this kind of alloys in our laboratory, for the welding process. The sample dimensions were imposed by the measuring device, having 10 mm 10 mm 2 mm. Since the witness and the lab cell must have the same mass, each sample has 300 mg. For a better reproducibility, the surface of B-type (see Section 2.2) samples was polished and cleaned. The polishing process was
Table 1 Substrate additional elements content [7] WE43 (wt.%)
ZE41 (wt.%)
Y (4.11) Nd (2.28) Dy (0.27) Gd (0.19) Yb (0.09) Zr (0.45)
Zn (4.37) La (0.32) Ce (0.78)
conducted to have a plane surface but not a mirror one, because such a surface has a negative influence for laser beam–material initial interaction. 2.2. Materials Two magnesium alloys are investigated, WE43 and ZE41, whose standard composition are reported in Table 1 [7]. Besides the as-cast materials from the two types of magnesium alloys, two types of coated substrates were used, the coatings being realised by a HAE treatment (anodising process) and a conversion treatment (mordancage process), respectively. These standard coating processes are generally applied in order to improve the corrosion resistance. Chemical analysis of these coatings is provided in the third paragraph of this paper. Further in the paper, the first treatment will be noted T1 and the second T2. The substrate without treatment will be referred as the B letter. Thus, practically six types of materials/surfaces are studied: WE43-B, WE43-T1, WE43-T2, ZE41-B, ZE41-T1 and ZE41-T2.
3. Results and discussions 3.1. WE43 and ZE41 chemical composition analysis Before any study carried out on a material, its chemical composition and structure must be known. Previous experiments made for the two types of magnesium alloys (WE43 and ZE41) permitted to have an indication concerning the chemical composition, described in Table 1 [7]. SEM images of the two materials are presented in Fig. 2 (for ZE41) and Fig. 3 (for WE43), along with an SEM–EDS profile, in order to obtain the distribution of the main addition
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
385
Fig. 2. ZE41 microstructure and composition profile (on 245.1 mm).
elements. In the both cases, the addition elements are concentrated at grain boundaries. 3.2. T1 layer (anodising technique) The anodising treatments applied on the two types of magnesium alloys are quite similar. Both coatings look alike and can hardly be distinguished visually by their aspect or colour. Some compositional differences can be observed using SEM images (Fig. 4 for WE43 and Fig. 5 for ZE41) and especially SEM quantitative analysis (Table 2). However, the main layer’s constitutive elements are the same and in similar proportions, as we can see in Table 2, looking at the O, Al and
Mn percentages. It must be noted that Nd was also detected (present in the magnesium alloys), but it was not quantified since our data base does not contain references on it. 3.3. T2 layer (mordancage process) Despite the presence of the same main constitutive elements (Table 3), the protective layers that cover the two types of magnesium alloys present some differences, as it can be seen when comparing Figs. 6 and 7. It must be said that even visually, the two coatings are very different, WE43-T2 covering layers being more brilliant than those of ZE41-T2.
Fig. 3. WE43 microstructure and composition profile (on 281.6 mm).
386
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
Fig. 4. WE43-T1 surface layer SEM images.
Fig. 5. ZE41-T1 surface layer SEM images.
3.4. Laser beam absorption (at 1.06 mm)
decreases, in general, with the temperature because the apparent electron-lattice collision time shortens. In addition, hot metals are reactive and irreversible changes in reflectance due to chemical reactions at the surface (oxidation, roughness, etc.) tend to occur, except under high vacuum conditions. The total absorptance of a metal can be thought of as composed of three contributions, due to free electrons, interband transitions and surface effects. The reduced reflection
As explained in literature [10], metals are not immune to beam-induced changes in their optical properties: their reflectance usually decreases under irradiation. The effect is thermal in nature and makes metals susceptible to thermal runaway, particularly in the infrared domain where reflectances tend to be high. Moreover, the bulk reflectance of metals Table 2 SEM quantitative analysis—T1 coat layers Analysed zone
WE43: Fig. 4-X200 ZE41: Fig. 5-X200
Element Mg (wt.%)
Al (wt.%)
Mn (wt.%)
K (wt.%)
O (wt.%)
Y (wt.%)
Zn (wt.%)
Si (wt.%)
Ca (wt.%)
P (wt.%)
Ce (wt.%)
46.35 46.52
10.71 9.59
5.06 4.65
0.50 0.66
36.48 35.23
0.89 –
– 1.01
– 0.14
– 0.14
– 1.48
– 0.58
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
387
Table 3 SEM quantitative analysis—T2 coat layers Analysed zone
WE43: Fig. 6-X200 ZE41: Fig. 7-X200
Element Mg (wt.%)
Cr (wt.%)
Zr (wt.%)
K (wt.%)
O (wt.%)
Y (wt.%)
Zn (wt.%)
Si (wt.%)
Ca (wt.%)
S (wt.%)
Ce (wt.%)
69.93 46.33
8.08 16.31
– 1.06
– 2.10
17.84 24.71
4.04 –
– 7.24
– 0.29
0.12 0.16
– 0.71
– 1.10
under laser irradiation (which may or may not indicate enhanced absorption) can be the result of the two following reasons: mechanical deformation of the surface, possible at moderate irradiance, or plasma effects, which require irradiances sufficient for strong evaporation. Another encountered effect is that above room temperature the conductivities of typical metals decrease roughly linearly with temperature, and the same holds for liquid metals at too high temperatures.
Upon melting, most metals show an additional drop in conductivity [10]. The above considerations can also be employed in order to explain the absorption coefficient variation in the cases studied below. 3.4.1. Absorption for B-type samples The energy absorption measured in this case (and presented in Fig. 8) is not very elevated one. This fact will have negative consequences for a laser treatment
Fig. 6. WE43-T2 surface layer SEM images.
Fig. 7. ZE41-T2 surface layer SEM images.
388
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
Fig. 8. Absorption for B-type samples.
at 1.06 mm, because high laser energy will be needed to obtain the wanted effects. The particularity of WE43-B samples is the presence of two plateaus. The first one, as seen in Fig. 8, is around 25% (absorbed energy), when the substrate material is still in solid state. This first plateau is quickly reached, for energies of 1 J. When the substrate material change its state from solid to melted one (‘‘M’’ points in Fig. 8), the absorbed energy rises again, but slowly this time, to reach the second plateau at 40% of absorbed energy. As for the ZE41-B samples, only one plateau can be seen, reached quite rapidly (from 5 J). The experiments were conducted until 40–45 J without obtaining melted points on the samples, the absorbed energy being almost constant at around 30%.
3.4.2. Absorption for T1-type samples In this case, a similar behaviour was observed for both studied materials (WE43 and ZE41), as shown in Fig. 9. Firstly, starting with energies slightly superior to 0 J, the absorption rises very fast to 40%, the maximum absorbed energy for B-type samples. Increasing further the incident energy will slowly increase the absorption level until a plateau, observed for the two types of materials around 65% of absorbed energy. Melted state is reached at 13 J, without increasing more than a few percent the absorbed energy. The identical behaviour of the two materials can be explained by the similarities in the layers that cover the surfaces, issued from the anodising process, as it was presented earlier in this paper. The absorption
Fig. 9. Absorption for T1-type samples (HAE).
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
level and profile open the opportunities for a more uniform laser surface treatment than in the case of Btype samples. The presence of oxides at the substrate surface plays an important role in the laser beam–material coupling effect. Even if the anodised layer has only a limited thickness (tens of microns), two major objectives can be reached with it: anti-corrosion protection and good laser beam–substrate interaction. Due to the reduced thickness, the elements from the anodised layer will not influence the results of the laser treatment (such as cladding, alloying, etc.). It must be recalled that all above considerations are valid only in the case of a laser beam at 1.06 mm wavelength. 3.4.3. Absorption for T2-type samples It was shown earlier that the two T1-type surface treatments were almost identical, for the two studied materials (WE43 and ZE41). As a result, the laser beam absorption occurs in a very similar manner. The same thing cannot be said about the T2-type samples. First of all, there are some differences in the deposited layer composition and in the colour, especially. These differences, as expected, have an important influence for laser beam absorption by the substrate, as seen in Fig. 10. Concerning the laser beam absorption for WE43T2, it can be observed that a first plateau is reached very quickly, at about 20% of the absorbed energy. The same behaviour was observed for WE43-B samples, as shown earlier. This plateau is maintained until incident energies of about 8 J, threshold at which melted state
389
appears. As a consequence, the absorbed energy increases rapidly to reach the second plateau, at about 50%. At this point, it can be said that the surface nature for WE43-T2 has a negative influence on the laser beam–material coupling effect, since it increases the quantity of reflected laser energy. A completely different behaviour occurs in the case of ZE41-T2 samples. The absorbed energy increases quickly until 70% to reach the first plateau, without passing to the melted state. All this happens for incident energies lower than 1 J. Increasing more the incident energy, above 3 J, melted state occurs. As a result, the absorbed energy increases to 80%, which constitutes the second and final plateau. Comparing the two variations in absorbed energy, it can be said that the T2 surface treatment applied to the ZE41 magnesium alloy allows a very good laser– material interaction because of the absorption regularity. The length of the first plateau in the variation of absorption, along with the difference between the first and the second plateau, cannot have a good effect for the laser–material interaction, especially in the coupling stage of the last one. 3.4.4. Absorption—WE43 Until now we have seen the differences between the energy absorption comparing the same surface treatments applied to different magnesium alloys (WE43 and ZE41). Let us take a look at the energy absorption for the same alloy (WE43) covered by two different layers (T1- and T2-types) or not covered at all (B-type). The
Fig. 10. Absorption for T2-type samples.
390
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
Fig. 11. Absorption for WE43 magnesium alloys.
Fig. 12. Absorption for ZE41 magnesium alloys.
absorption variation with incident impact energy is presented in Fig. 11. Almost identical behaviour is shown by B-type and T2-type substrates for low levels of incident energy. Both types of samples quickly reach their first plateau, at 20% of absorbed energy. Increasing the incident energy above 7 J, T2-type samples will absorb more laser energy than B-type, reaching the melted state at 50% of the absorbed energy. On their side, B-type samples stay in solid state until 13 J. Above this threshold melted state occurs and the material absorbs 40% of laser beam energy. Thus, the energy needed to melt the B-type samples surface is greater than in the case of T2-type samples. As a result, the coupling stage will occur later in the case of B-type samples, a fact with negative influence in the first part of a laser treatment. A better behaviour presents the T1-type sample, being more uniform than the two others. Moreover, the absorbed energy is greater than in the two other cases, reaching quite rapidly 60–65%. The melted state does not change the absorption level a lot, which increases only by 5%. However, the energy needed to melt T1type samples is greater than in the case of T2-type samples and this fact must be carefully considered in laser treatments. At this point, it can be concluded that the surface composition and brilliance of T2-type coating layers on WE43 decreases the substrate properties in laser treatments.
The coupling effect between the laser beam (wavelength: 1.06 mm) and the substrate occurs with difficulties, a fact with negative consequences for the entire process. Much better behaviour shows T1-type samples. The absorption increases very fast with the incident energy and becomes stable around 65–70% when the melted state occurs. However, the best behaviour of the three cases studied is shown by the T2-type samples. The absorption in this case rises quickly at 70%, the material being still in its solid state. Increasing the incident energy, the second plateau is reached, at 85%, when the melted state appears. As a consequence, we are dealing with a very interesting comportment for laser surface treatments, because the melted state occurs quickly and at low energy. An important part of laser beam is absorbed, which can only increase the efficiency of the entire process.
3.4.5. Absorption—ZE41 The absorption variation with incident energy in the case of ZE41 magnesium alloy is presented in Fig. 12. The first conclusion is that the B-type substrate does not have an adequate behaviour for laser treatments.
4. Conclusions The experiments, made for measuring the laser energy absorption on magnesium alloys (WE43 and ZE41), represent a step forward in the understanding of laser process. Since magnesium alloys are often used covered by protective coatings, it is important to know not only the interaction mechanisms between ascast magnesium alloys and a laser beam but also when micrometric layers cover their surfaces. Absorption is the first parameter that affects laser treatment processes and, often, the most important. The results of our research proved that the layers obtained with the both techniques (anodising and
S. Ignat et al. / Applied Surface Science 233 (2004) 382–391
mordancage processes) have a positive influence on the laser beam–material interaction and for the laser treatments in general. In the case of magnesium alloys (WE43 or ZE41) laser treatments, better results can be expected for ZE41-T2. Anodising process equally offers good properties for laser beam–material interaction. It must be noted that all these conclusions are valid only in the case of Nd:YAG lasers (wavelength 1.06 mm) and the energy provided by one laser pulse was measured. Future experiments, with a de-focalised laser beam will be carried out, in order to study the influence of this important parameter, which is the laser beam focalisation, on the absorption coefficient.
Acknowledgements This work was performed in the frame of the PRESTIGE research project, financed by the French Research Ministry. The authors equally want to thank the industrial partners involved in this project (Honsel
391
Fonderie Messier, PMA, CLAIRE) and the other partners (E´ cole Nationale Supe´ rieure d’Arts et Me´ tiers, E´ cole Centrale de Lyon and Universite´ Aix-Marseille II), for their support and interest. References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. [2] J.E. Gray, B. Luan, J. Alloy Compd. 336 (2002) 88–113. [3] F. Czerwinski, Acta Mater. 50 (2002) 2639–2654. [4] K. Funatani, Surf. Coat. Tech. 133–1334 (2000) 264–272. [5] A.L. Rudd, C.B. Breslin, F. Mansfeld, Corrosion Sci. 42 (2000) 275–288. [6] A.J. Eifert, J.P. Thomas, Scripta Mater. 40 (8) (1999) 929–935. [7] C. Sanchez, G. Nussbaum, P. Azavant, H. Octor, Mater. Sci. Eng. A 221 (1996) 48–57. [8] J.G. Wang, L.M. Hsiung, T.G. Nieh, M. Mabuchi, Mater. Sci. Eng. A 315 (2001) 81–88. [9] O. Perret, Ph.D. thesis, University of Burgundy, France, 2000. [10] M. Von Allmen, Laser Beam Interactions with Materials, second ed., Springer, Berlin, p. 31, ISBN 3-54059401-9.