- Email: [email protected]
Application of atomic absorption spectrometry to laser plume analysis
Vacuum 59 (2000) 338}348
Application of atomic absorption spectrometry to laser plume analysis X. Han , M. Futamata * Graduate School, Kitami Institute of Technology, 165 Koencho, Kitami-shi, Hokkaido 090-8507, Japan Department of Mechanical Engineering, Kitami Institute of Technology, 165 Koencho, Kitami-shi, Hokkaido 090-8507, Japan
Abstract In order to investigate the formation and properties of the plume induced by Nd}YAG laser, the absorption spectra of the elements contained in pure aluminum, aluminum alloy such as aluminum, magnesium, zinc, silicon, and titanium were analyzed by using atomic absorption spectrometry. The results have indicated that the elements having low melting and atomizing points were detected at "rst followed by the atomization of other elements with increasing laser power. Further, a basic discussion on the in#uence of compression "eld caused by the reaction of plume vaporizing was also done. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Nd}YAG laser; Laser plume; Atomic absorption spectrometry; Aluminum, Aluminum alloy and titanium
1. Introduction Laser beams are applied widely in the manufacturing "eld as a heat source with high energy density. The laser processing phenomenon becomes clearer. With regard to measurement di$culty, however, there are still many unknown factors about laser plume formation and its properties. Especially, it is recognized that the laser plume can provide information about the processing phenomenon. Therefore, it is necessary to study the formation mechanism and structure. The early works [1] have indicated that the behavior of laser plume relates closely to the wavelength of the laser. Besides, it has been shown that the laser plume induced by the CO laser consists of a weakly ionized plasma with high temperature and high density, but the one induced by the pulsed Nd}YAG laser consists of atomic steam [1] with high density and low temperature that is close to
* Corresponding author. Fax: #81-157-26-9218. E-mail address: [email protected] (M. Futamata). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 8 7 - 6
X. Han, M. Futamata / Vacuum 59 (2000) 338}348
339
Fig. 1. Schematic con"guration of Nd}YAG laser oscillator and Zeeman absorption spectrometer.
Fig. 2. Graphite furnace (a) and miniature cup (b).
the evaporating temperature. In this study, the absorption spectrum of each element contained in the target material irradiated with Nd}YAG laser is measured by using an atomic absorption spectrometer and the plume structure is discussed.
2. Experimental equipment and experimental method The schematic con"guration of the experimental system is shown in Fig. 1. It consists of the Nd}YAG laser equipment (NEC, SL117C) and the Zeeman atomic absorption spectrometer (HITACHI, 170-70). The laser equipment can cause the oscillation of the laser of 1.06 lm wavelength and maximum nominal output of 300 W in continuous wave (CW) or a Q switch pulse wave of maximum 99 kHz. The irradiating time can be controlled accurately by using a computer. The Zeeman atomic absorption spectrometer is excellent for the compensation of background absorption. A graphite atomizer with a cup-shaped cuvette was used as graphite furnace and a graphite miniature cup which has a sensitizing e!ect and makes it easy to put target material into graphite furnace was chosen. The cup-shaped cuvette and the miniature cup are shown in Figs. 2(a) and (b). The miniature cup is used to avoid direct contact between the cuvette and the target material. Therefore, the cuvette
340
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Table 1 Chemical compositions and physical properties of target materials
Pure aluminum (wt%) (JIS A1100) Aluminum alloy (wt%) (JIS A5083-H112) Vapor pressure (K) (100 Mpa) Atomization temp. (K) Boiling point (K)
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
0.12 0.13 2560 2303 2915
0.54 0.20 3008 1753 3027
0.13 0.01 2868 1733 2853
0.01 0.63 2373 1603 2425
0.01 4.58 1380 1553 1370
0.00 0.11 2753 1803 2485
0.01 0.00 1180 1103 1176
0.01 0.01 3533 2423 3535
Bal. Bal. 3485 2303 2759
consumption can be improved and the contamination problem can also be resolved. To again measure at the same position of the laser plume, we devised methods for manufacturing cuvette, miniature cup and target material according to the dimensions shown in Fig. 2. The industrial pure Al (JIS A1100) and Al}Mg alloy (JIS A5083-H112) were used as the main target materials and pure Ti for research was also chosen as a material for a part of the experiment. The chemical composition and thermal physical properties of pure Al and Al}Mg alloy that are related to this study are listed in Table 1. The absorption spectra of laser plume were measured in two cases of irradiation with CW laser and Q switch pulse laser for di!erent laser powers and irradiating times. A focusing lens of 100 mm focal length was used in the external optical system and the focal position was set at 0 mm. In order to compare the experiment with the results detected by irradiating with YAG laser, the absorption spectrum of each element has been measured by heating with a built-in heat source in the atomic absorption spectrometer. 3. Experimental results and discussion 3.1. Measurement of absorption spectrum by heating with a built-in heat source The absorption spectrum of each constituent element of the target material has been measured by heating with a built-in heat source in the spectrometer and the results are shown in Fig. 3. The experimental conditions were decided according to the heating condition speci"ed in the manual of the spectrometer. The drying temperature and the time were separately "xed to 400 K and 10 s, respectively, for all of the elements. The ashing time remained constant at 30 s and the ashing temperature was 800 K for Mg, Mn, Zn, 900 K for Fe, Cu, 1300 K for Al, Cr and 1800 K for Ti. The atomizing temperature was set to 2300 K for Mg, Zn, 2800 K for Mn, 3000 K for Fe, Cu, 3100 K for Al, Cr, Si, Ti and the atomizing time remained constant at 10 s. In this case, the absorption spectrum of each constituent element could be detected clearly in 100% for both pure Al and Al alloy. 3.2. Measurement of absorption spectrum and discussion of laser plume 3.2.1. Measurement of absorption spectrum From the principle of atomic absorption spectrometry, the absorption spectrum cannot be detected if the target is not heated to or above the atomizing temperature of the target element. But
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341
Fig. 3. Absorption spectra of constituent elements by heating with a built-in heat source.
sometimes even on heating to or above the atomizing temperature, the spectrum cannot be clearly detected. Within a very low concentration or when in a cluster state or becomes a compound state by mixing with other elements, the atomic absorption spectrum cannot be detected. The detection becomes di$cult when the atomization is delayed or the sensitivity becomes lower because of the chemical interference caused by coexisting elements. The peak height of the absorption spectrum means that the absorbance has been measured for each element contained in the pure Al and Al alloy. First, the discussion considers Al, Mg and Ti because Al is the main constituent of target material, Mg is an element having a lower atomizing temperature and Ti is less in content and has the highest atomizing temperature among all the constituent elements. Furthermore, in our experiment the light wavelength from the hollow cathode lamp was in good agreement with the wavelength shown in Fig. 3 for each element. Even when the experimental condition was the same, the absorption spectrum of the plume could not be always detected for any consituent element. Therefore, it was measured 5}10 times and the detection percentage F (F"the number of times detected/ the number of times measured) was calculated. A useful absorption spectrum is de"ned as a signal where the ratio (S/N) of the peak height of the absorption spectrum to the amplitude of the background absorption is equal to or more than 3. The relationship between the detection percentage and the laser power is shown in Fig. 4. The detection percentage depends on the laser power, the oscillating method and the constituent elements. On the whole, regardless of the element having the same content, the detection percentage of each element of pure Al is lower than that of Al alloy using the same oscillating method. Though the laser power at which the occurrence of plume can be con"rmed by eye estimation ranges from about 2 W for both the oscillating methods, Al, Mg and Ti cannot be detected at 2 W and further no other elements could be detected except Zn as shown in Table 2. In the case of Q switch pulsed laser, all of the elements can be detected in 100% with certain laser power, but some of the elements cannot be detected in 100% when irradiating with CW laser in the range of this experimental condition. This result indicates that the
342
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Fig. 4. Relationship between laser power and detection percentage in the case of CW laser and Q switch laser.
Table 2 Laser power of detection beginning (start power A) and laser power of detection beginning (start power B) in 100% (W)
Pure alloy CW Start Start Q Start Start Al alloy CW Start Start Q Start Start
power power power power power power power power
A B A B A B A B
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
10 150 20 21 10 50 21 23
25 * 16 17 50 * 17 19
10 * 20 23 10 100 23 25
10 * 20 21 50 60 15 16
5 50 15 20 5 40 10 11
* * * * 5 * 20 24
1 25 5 9 * * * *
50 * 19 25 11 150 8 25
5 * 15 25 5 200 10 20
Not measured. Not con"rmed.
high peak value of Q switch pulse laser promotes the formation of the plume more than CW laser and this is in good agreement with the research result about plume sound by the authors [2]. Figs. 5 and 6 show the relationship between the peak heights of the absorption spectra of the target elements mentioned above and the laser power. We tried to perform the experiment under the same experimental condition, but the experimental results show some extent of variation. Even
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Fig. 5. E!ect of laser power on peak height with CW laser.
so, with increasing laser power, an increasing tendency of the peak height is indicated on the whole. In atomic absorption analysis the peak height, which means absorption intensity (absorbance), is proportional to the atomic concentration up to a certain range. Consequently, the increase of the peak height means that in the laser plume, the atomic concentration of the corresponding element increases simultaneously while the laser power increases. On the other hand, as described in Ref. [3] about the relationship between the removal mass and the laser power, the removal mass increases as the laser power increases. Though the removal mass is not necessarily equal to the plume quantity, it may be said that the proportion of the plume quantity and the atomized particles increase as the removal mass increases. As for the dispersion, there is no single cause, but it may be considered that the composition of the plume is di!erent because of chemical interference and
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Fig. 6. E!ect of laser power on peak height with Q switch laser.
non-homogeneity of the element distribution in the irradiating "eld of the target material. Even though Al is the main constituent element, the detection is di$cult as shown in Figs. 5 and 6 in case of both CW laser and Q switch pulse laser. Especially, for pure Al target material it cannot be detected in 100% when irradiating with CW laser. The atomizing temperature of Al is higher, but this cannot explain why the detection is so di$cult. As will be explained later the temperature of irradiating "eld is high enough to atomize any constituent element of both the target materials. In atomic absorption analysis, the Al atom lifetime is very short [4] and is easily in#uenced by chemical interference from other elements, for example, it compounds with Mg to form the stable MgAl O , to make the atomization di$cult. In short, the chemical interference can be considered as the cause and this result shows that the in#uence of the chemical interference on the atomization is larger than the atomizing temperature of an element. Fig. 7 shows the detection percentage and the peak height for Zn in pure Al in the case of the CW laser. Even if the laser power is 1 W, the detection percentage is only about 70% of the high levels. The dispersion of the peak height is also
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345
Fig. 7. Relationship between laser power, detection percentage and peak height of Zn with CW laser.
smaller than that of Al and Mg. This is because Zn can exist stably in an isolated atom state since it is an element which is not in#uenced easily by chemical interference and lower atomizing temperature. The surface temperature of the irradiating "eld (¹ , ¹ ) can be calculated approx!5 / imately with the following formula [5] for CW laser and Q switch pulsed laser: ¹ "2bE/pjd, ¹ "2bF /(pjco) , !5 / where b is the absorption coe$cient, d the diameter of laser beam (cm), E the laser power (W), j the thermal conductivity (cal/cm 3C s), F the power density (W/cm), c the speci"c heat (cal/3C g), and o the density (g/cm). For aluminum, substituting b"1, j"0.49, c"0.27, o"2.7, d"5;10\ in the above formulas the surface temperature is found to be above 3000 K when the laser power is 10 W for CW laser, 0.6 W for Q switch pulse laser. Even considering the loss by re#ection and evaporation this temperature is beyond the Ti atomization temperature (2430 K) which is the highest among all the constituent elements. So it is high enough for all the elements to atomize. However, there are elements such as Zn which can be detected at lower power in higher detection percentage and there are also elements such as Cu shown in Fig. 8 which has a lower atomization temperature and is weakly in#uenced by chemical interference but its detection percentage is lower. This proves that there are a number of factors that in#uence atomization. 3.2.2. Discussion about the laser plume As described earlier, it is a prerequisite of detection that the target element should be atomized. However, even when atomized it cannot be detected if it is in a cluster state or compounds with other elements to form new compounds. The laser power of the detection beginning depends on the elements as shown in Table 2. In other words, by the principle of the atomic absorption analysis, if an element can be detected the temperature of the plume must at least equal or exceed the minimum atomizing temperature of that element. As mentioned in Section 3.2.1 when using the built-in heat source, if Ti can be detected at a temperature of 3100 K in 100%, then the plume
346
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Fig. 8. Relationship between laser power, detection percentage and spectrum peak height of Cu with CW laser.
Fig. 9. Relationship of vertical distance from the surface of workpiece and peak height, detection percentage for pure Ti.
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347
temperature at which Ti can be detected may be estimated to be 3100 K or above. Fig. 9 shows the peak height of the target element Ti- and Fe-containing pure Ti target material along the vertical direction of plume. According to the structure of this spectrometer, when obstructing the light from the hollow cathode lamp by the dross which was formed around the irradiating point of the target material after irradiating, the measurement of the absorption spectrum begins from a position 0.1 mm above the surface of the target material. Though a dispersion exists in the measured value, the peak height becomes low with increase of the distance from the surface of the target material and the detection percentage indicates a similar tendency except at one point (Fig. 9(a)). The result obtained shows that a temperature di!erence exists along the vertical direction of the laser plume. It is well known that, in general, as the temperature becomes lower the in#uence of chemical interference becomes stronger. Because of this temperature di!erence, the isolated atoms compound again with other particles. Therefore, the detection of the absorption spectrum becomes di$cult. To sum up, the absorption spectrum of each element in the plume cannot always be detected and the peak height is also di!erent depending on the irradiating condition and the target material. By this, it may be inferred that the plume contains the isolated atoms which can be detected and the substances of cluster state which cannot be detected. Regarding the reasons for the cluster formation, besides the chemical interference, the compression "eld [6] produced by the target material evaporating explosively can also play an important role.
4. Conclusion In this study, the peak height of the absorption spectrum for each constituent element in the laser plume has been measured and the detection percentage has also been calculated. The peak height and the detection percentage relate to the laser irradiating conditions and the target material. Elements like Zn, which have lower melting point, lower atomizing temperature and weak chemical interference were detected at "rst. It was found that a temperature di!erence exists along the vertical direction of the laser plume. The plume is composed of isolated atoms and the substances of cluster state produced by the chemical interference and the compression "eld.
References [1] Matsunawa A. Memoirs of the grant-in-aid for developmental scienti"c research(B) of the education ministry of Japan, 1988. p. 8. [2] Futamata M, Han X et al. Preprint of the National Meeting of JWS, vol. 61, 1997. p. 318}9 (in Japanese). [3] Hirata E. Master's thesis, Kitami Institute of Technology, Japan, 1996. p. 42, 65 (in Japanese). [4] Fuwa K, Shimomula S et al. Atomic absorption analysis, vol. II. Hirokawa Publishing Co., Japan, 1990. p. 516 (in Japanese). [5] Kobayashi A. Laser processing. Kaihatsu Publishing Co., Japan, p. 62}5 (in Japanese). [6] Gatzweiler W, Maischner D, et al. SPIE 1988;1012:141}7.
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Xizhuang Han received the BE degree from Nanjing University of Science and Technology of China, the ME Degree from the Kitami Institute of Technology (KIT) of Japan and is presently studying at the Graduate school of KIT of Japan. He is a member of Japan Welding Society and his research interests are the application of laser processing and the analysis of its phenomenon. Masami Futamata received the doctor of engineering degree from the Osaka University and a professorship at the department of mechanical engineering of Kitami Institute of Technology (KIT) in Japan. His area of research is the analysis of laser processing phenomenon and control of its process, the development of a new scienti"c material via the thermal spraying. He is a member of the Japan Welding Society, the High Temperature Society of Japan and Japan Society of Mechanical Engineers, etc. He is also the head of Hokkaido branch o$ce of the High Temperature Society of Japan, director of cooperative research center of KIT. He was awarded the OKDA memorial award of welding science and technology.
Application of atomic absorption spectrometry to laser plume analysis X. Han , M. Futamata * Graduate School, Kitami Institute of Technology, 165 Koencho, Kitami-shi, Hokkaido 090-8507, Japan Department of Mechanical Engineering, Kitami Institute of Technology, 165 Koencho, Kitami-shi, Hokkaido 090-8507, Japan
Abstract In order to investigate the formation and properties of the plume induced by Nd}YAG laser, the absorption spectra of the elements contained in pure aluminum, aluminum alloy such as aluminum, magnesium, zinc, silicon, and titanium were analyzed by using atomic absorption spectrometry. The results have indicated that the elements having low melting and atomizing points were detected at "rst followed by the atomization of other elements with increasing laser power. Further, a basic discussion on the in#uence of compression "eld caused by the reaction of plume vaporizing was also done. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Nd}YAG laser; Laser plume; Atomic absorption spectrometry; Aluminum, Aluminum alloy and titanium
1. Introduction Laser beams are applied widely in the manufacturing "eld as a heat source with high energy density. The laser processing phenomenon becomes clearer. With regard to measurement di$culty, however, there are still many unknown factors about laser plume formation and its properties. Especially, it is recognized that the laser plume can provide information about the processing phenomenon. Therefore, it is necessary to study the formation mechanism and structure. The early works [1] have indicated that the behavior of laser plume relates closely to the wavelength of the laser. Besides, it has been shown that the laser plume induced by the CO laser consists of a weakly ionized plasma with high temperature and high density, but the one induced by the pulsed Nd}YAG laser consists of atomic steam [1] with high density and low temperature that is close to
* Corresponding author. Fax: #81-157-26-9218. E-mail address: [email protected] (M. Futamata). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 8 7 - 6
X. Han, M. Futamata / Vacuum 59 (2000) 338}348
339
Fig. 1. Schematic con"guration of Nd}YAG laser oscillator and Zeeman absorption spectrometer.
Fig. 2. Graphite furnace (a) and miniature cup (b).
the evaporating temperature. In this study, the absorption spectrum of each element contained in the target material irradiated with Nd}YAG laser is measured by using an atomic absorption spectrometer and the plume structure is discussed.
2. Experimental equipment and experimental method The schematic con"guration of the experimental system is shown in Fig. 1. It consists of the Nd}YAG laser equipment (NEC, SL117C) and the Zeeman atomic absorption spectrometer (HITACHI, 170-70). The laser equipment can cause the oscillation of the laser of 1.06 lm wavelength and maximum nominal output of 300 W in continuous wave (CW) or a Q switch pulse wave of maximum 99 kHz. The irradiating time can be controlled accurately by using a computer. The Zeeman atomic absorption spectrometer is excellent for the compensation of background absorption. A graphite atomizer with a cup-shaped cuvette was used as graphite furnace and a graphite miniature cup which has a sensitizing e!ect and makes it easy to put target material into graphite furnace was chosen. The cup-shaped cuvette and the miniature cup are shown in Figs. 2(a) and (b). The miniature cup is used to avoid direct contact between the cuvette and the target material. Therefore, the cuvette
340
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Table 1 Chemical compositions and physical properties of target materials
Pure aluminum (wt%) (JIS A1100) Aluminum alloy (wt%) (JIS A5083-H112) Vapor pressure (K) (100 Mpa) Atomization temp. (K) Boiling point (K)
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
0.12 0.13 2560 2303 2915
0.54 0.20 3008 1753 3027
0.13 0.01 2868 1733 2853
0.01 0.63 2373 1603 2425
0.01 4.58 1380 1553 1370
0.00 0.11 2753 1803 2485
0.01 0.00 1180 1103 1176
0.01 0.01 3533 2423 3535
Bal. Bal. 3485 2303 2759
consumption can be improved and the contamination problem can also be resolved. To again measure at the same position of the laser plume, we devised methods for manufacturing cuvette, miniature cup and target material according to the dimensions shown in Fig. 2. The industrial pure Al (JIS A1100) and Al}Mg alloy (JIS A5083-H112) were used as the main target materials and pure Ti for research was also chosen as a material for a part of the experiment. The chemical composition and thermal physical properties of pure Al and Al}Mg alloy that are related to this study are listed in Table 1. The absorption spectra of laser plume were measured in two cases of irradiation with CW laser and Q switch pulse laser for di!erent laser powers and irradiating times. A focusing lens of 100 mm focal length was used in the external optical system and the focal position was set at 0 mm. In order to compare the experiment with the results detected by irradiating with YAG laser, the absorption spectrum of each element has been measured by heating with a built-in heat source in the atomic absorption spectrometer. 3. Experimental results and discussion 3.1. Measurement of absorption spectrum by heating with a built-in heat source The absorption spectrum of each constituent element of the target material has been measured by heating with a built-in heat source in the spectrometer and the results are shown in Fig. 3. The experimental conditions were decided according to the heating condition speci"ed in the manual of the spectrometer. The drying temperature and the time were separately "xed to 400 K and 10 s, respectively, for all of the elements. The ashing time remained constant at 30 s and the ashing temperature was 800 K for Mg, Mn, Zn, 900 K for Fe, Cu, 1300 K for Al, Cr and 1800 K for Ti. The atomizing temperature was set to 2300 K for Mg, Zn, 2800 K for Mn, 3000 K for Fe, Cu, 3100 K for Al, Cr, Si, Ti and the atomizing time remained constant at 10 s. In this case, the absorption spectrum of each constituent element could be detected clearly in 100% for both pure Al and Al alloy. 3.2. Measurement of absorption spectrum and discussion of laser plume 3.2.1. Measurement of absorption spectrum From the principle of atomic absorption spectrometry, the absorption spectrum cannot be detected if the target is not heated to or above the atomizing temperature of the target element. But
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341
Fig. 3. Absorption spectra of constituent elements by heating with a built-in heat source.
sometimes even on heating to or above the atomizing temperature, the spectrum cannot be clearly detected. Within a very low concentration or when in a cluster state or becomes a compound state by mixing with other elements, the atomic absorption spectrum cannot be detected. The detection becomes di$cult when the atomization is delayed or the sensitivity becomes lower because of the chemical interference caused by coexisting elements. The peak height of the absorption spectrum means that the absorbance has been measured for each element contained in the pure Al and Al alloy. First, the discussion considers Al, Mg and Ti because Al is the main constituent of target material, Mg is an element having a lower atomizing temperature and Ti is less in content and has the highest atomizing temperature among all the constituent elements. Furthermore, in our experiment the light wavelength from the hollow cathode lamp was in good agreement with the wavelength shown in Fig. 3 for each element. Even when the experimental condition was the same, the absorption spectrum of the plume could not be always detected for any consituent element. Therefore, it was measured 5}10 times and the detection percentage F (F"the number of times detected/ the number of times measured) was calculated. A useful absorption spectrum is de"ned as a signal where the ratio (S/N) of the peak height of the absorption spectrum to the amplitude of the background absorption is equal to or more than 3. The relationship between the detection percentage and the laser power is shown in Fig. 4. The detection percentage depends on the laser power, the oscillating method and the constituent elements. On the whole, regardless of the element having the same content, the detection percentage of each element of pure Al is lower than that of Al alloy using the same oscillating method. Though the laser power at which the occurrence of plume can be con"rmed by eye estimation ranges from about 2 W for both the oscillating methods, Al, Mg and Ti cannot be detected at 2 W and further no other elements could be detected except Zn as shown in Table 2. In the case of Q switch pulsed laser, all of the elements can be detected in 100% with certain laser power, but some of the elements cannot be detected in 100% when irradiating with CW laser in the range of this experimental condition. This result indicates that the
342
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Fig. 4. Relationship between laser power and detection percentage in the case of CW laser and Q switch laser.
Table 2 Laser power of detection beginning (start power A) and laser power of detection beginning (start power B) in 100% (W)
Pure alloy CW Start Start Q Start Start Al alloy CW Start Start Q Start Start
power power power power power power power power
A B A B A B A B
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
10 150 20 21 10 50 21 23
25 * 16 17 50 * 17 19
10 * 20 23 10 100 23 25
10 * 20 21 50 60 15 16
5 50 15 20 5 40 10 11
* * * * 5 * 20 24
1 25 5 9 * * * *
50 * 19 25 11 150 8 25
5 * 15 25 5 200 10 20
Not measured. Not con"rmed.
high peak value of Q switch pulse laser promotes the formation of the plume more than CW laser and this is in good agreement with the research result about plume sound by the authors [2]. Figs. 5 and 6 show the relationship between the peak heights of the absorption spectra of the target elements mentioned above and the laser power. We tried to perform the experiment under the same experimental condition, but the experimental results show some extent of variation. Even
X. Han, M. Futamata / Vacuum 59 (2000) 338}348
343
Fig. 5. E!ect of laser power on peak height with CW laser.
so, with increasing laser power, an increasing tendency of the peak height is indicated on the whole. In atomic absorption analysis the peak height, which means absorption intensity (absorbance), is proportional to the atomic concentration up to a certain range. Consequently, the increase of the peak height means that in the laser plume, the atomic concentration of the corresponding element increases simultaneously while the laser power increases. On the other hand, as described in Ref. [3] about the relationship between the removal mass and the laser power, the removal mass increases as the laser power increases. Though the removal mass is not necessarily equal to the plume quantity, it may be said that the proportion of the plume quantity and the atomized particles increase as the removal mass increases. As for the dispersion, there is no single cause, but it may be considered that the composition of the plume is di!erent because of chemical interference and
344
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Fig. 6. E!ect of laser power on peak height with Q switch laser.
non-homogeneity of the element distribution in the irradiating "eld of the target material. Even though Al is the main constituent element, the detection is di$cult as shown in Figs. 5 and 6 in case of both CW laser and Q switch pulse laser. Especially, for pure Al target material it cannot be detected in 100% when irradiating with CW laser. The atomizing temperature of Al is higher, but this cannot explain why the detection is so di$cult. As will be explained later the temperature of irradiating "eld is high enough to atomize any constituent element of both the target materials. In atomic absorption analysis, the Al atom lifetime is very short [4] and is easily in#uenced by chemical interference from other elements, for example, it compounds with Mg to form the stable MgAl O , to make the atomization di$cult. In short, the chemical interference can be considered as the cause and this result shows that the in#uence of the chemical interference on the atomization is larger than the atomizing temperature of an element. Fig. 7 shows the detection percentage and the peak height for Zn in pure Al in the case of the CW laser. Even if the laser power is 1 W, the detection percentage is only about 70% of the high levels. The dispersion of the peak height is also
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345
Fig. 7. Relationship between laser power, detection percentage and peak height of Zn with CW laser.
smaller than that of Al and Mg. This is because Zn can exist stably in an isolated atom state since it is an element which is not in#uenced easily by chemical interference and lower atomizing temperature. The surface temperature of the irradiating "eld (¹ , ¹ ) can be calculated approx!5 / imately with the following formula [5] for CW laser and Q switch pulsed laser: ¹ "2bE/pjd, ¹ "2bF /(pjco) , !5 / where b is the absorption coe$cient, d the diameter of laser beam (cm), E the laser power (W), j the thermal conductivity (cal/cm 3C s), F the power density (W/cm), c the speci"c heat (cal/3C g), and o the density (g/cm). For aluminum, substituting b"1, j"0.49, c"0.27, o"2.7, d"5;10\ in the above formulas the surface temperature is found to be above 3000 K when the laser power is 10 W for CW laser, 0.6 W for Q switch pulse laser. Even considering the loss by re#ection and evaporation this temperature is beyond the Ti atomization temperature (2430 K) which is the highest among all the constituent elements. So it is high enough for all the elements to atomize. However, there are elements such as Zn which can be detected at lower power in higher detection percentage and there are also elements such as Cu shown in Fig. 8 which has a lower atomization temperature and is weakly in#uenced by chemical interference but its detection percentage is lower. This proves that there are a number of factors that in#uence atomization. 3.2.2. Discussion about the laser plume As described earlier, it is a prerequisite of detection that the target element should be atomized. However, even when atomized it cannot be detected if it is in a cluster state or compounds with other elements to form new compounds. The laser power of the detection beginning depends on the elements as shown in Table 2. In other words, by the principle of the atomic absorption analysis, if an element can be detected the temperature of the plume must at least equal or exceed the minimum atomizing temperature of that element. As mentioned in Section 3.2.1 when using the built-in heat source, if Ti can be detected at a temperature of 3100 K in 100%, then the plume
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Fig. 8. Relationship between laser power, detection percentage and spectrum peak height of Cu with CW laser.
Fig. 9. Relationship of vertical distance from the surface of workpiece and peak height, detection percentage for pure Ti.
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temperature at which Ti can be detected may be estimated to be 3100 K or above. Fig. 9 shows the peak height of the target element Ti- and Fe-containing pure Ti target material along the vertical direction of plume. According to the structure of this spectrometer, when obstructing the light from the hollow cathode lamp by the dross which was formed around the irradiating point of the target material after irradiating, the measurement of the absorption spectrum begins from a position 0.1 mm above the surface of the target material. Though a dispersion exists in the measured value, the peak height becomes low with increase of the distance from the surface of the target material and the detection percentage indicates a similar tendency except at one point (Fig. 9(a)). The result obtained shows that a temperature di!erence exists along the vertical direction of the laser plume. It is well known that, in general, as the temperature becomes lower the in#uence of chemical interference becomes stronger. Because of this temperature di!erence, the isolated atoms compound again with other particles. Therefore, the detection of the absorption spectrum becomes di$cult. To sum up, the absorption spectrum of each element in the plume cannot always be detected and the peak height is also di!erent depending on the irradiating condition and the target material. By this, it may be inferred that the plume contains the isolated atoms which can be detected and the substances of cluster state which cannot be detected. Regarding the reasons for the cluster formation, besides the chemical interference, the compression "eld [6] produced by the target material evaporating explosively can also play an important role.
4. Conclusion In this study, the peak height of the absorption spectrum for each constituent element in the laser plume has been measured and the detection percentage has also been calculated. The peak height and the detection percentage relate to the laser irradiating conditions and the target material. Elements like Zn, which have lower melting point, lower atomizing temperature and weak chemical interference were detected at "rst. It was found that a temperature di!erence exists along the vertical direction of the laser plume. The plume is composed of isolated atoms and the substances of cluster state produced by the chemical interference and the compression "eld.
References [1] Matsunawa A. Memoirs of the grant-in-aid for developmental scienti"c research(B) of the education ministry of Japan, 1988. p. 8. [2] Futamata M, Han X et al. Preprint of the National Meeting of JWS, vol. 61, 1997. p. 318}9 (in Japanese). [3] Hirata E. Master's thesis, Kitami Institute of Technology, Japan, 1996. p. 42, 65 (in Japanese). [4] Fuwa K, Shimomula S et al. Atomic absorption analysis, vol. II. Hirokawa Publishing Co., Japan, 1990. p. 516 (in Japanese). [5] Kobayashi A. Laser processing. Kaihatsu Publishing Co., Japan, p. 62}5 (in Japanese). [6] Gatzweiler W, Maischner D, et al. SPIE 1988;1012:141}7.
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Xizhuang Han received the BE degree from Nanjing University of Science and Technology of China, the ME Degree from the Kitami Institute of Technology (KIT) of Japan and is presently studying at the Graduate school of KIT of Japan. He is a member of Japan Welding Society and his research interests are the application of laser processing and the analysis of its phenomenon. Masami Futamata received the doctor of engineering degree from the Osaka University and a professorship at the department of mechanical engineering of Kitami Institute of Technology (KIT) in Japan. His area of research is the analysis of laser processing phenomenon and control of its process, the development of a new scienti"c material via the thermal spraying. He is a member of the Japan Welding Society, the High Temperature Society of Japan and Japan Society of Mechanical Engineers, etc. He is also the head of Hokkaido branch o$ce of the High Temperature Society of Japan, director of cooperative research center of KIT. He was awarded the OKDA memorial award of welding science and technology.