- Email: [email protected]
Defect detection in wire welded joints using thermography investigations
Materials Science and Engineering B 177 (2012) 1239–1242
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Defect detection in wire welded joints using thermography investigations a,∗ a ´ atczak ˛ ˛ T. Swi , M. Tomczyk b , B. Wiecek , R. Pawlak b , R. Olbrycht a a b
Institute of Electronics, Technical University of Łód´z, Łód´z, Poland Institute of Electrical Engineering Systems, Technical University of Łód´z, Łód´z, Poland
a r t i c l e
i n f o
Article history: Received 12 October 2011 Received in revised form 22 December 2011 Accepted 3 March 2012 Available online 18 March 2012 Keywords: Laser microtechnology Laser wire bonding Active thermography Thermal impedance
a b s t r a c t The formation of gas voids inside the wire joints during laser welding may cause internal defects (cracks and porosity), that are invisible from outside. Authors propose the application of active thermography for detection of such defects. Thermal camera was used to acquire sequences of thermograms showing the joints during transient heating. Fourier analysis enabled phase value calculation, which is different for defective and non-defective samples. Laboratory results were confirmed by simulations on prepared two-dimensional model. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Laser technology can be applied to produce wire welded joints. The laser spot welding process is characterized by extremely fast heating and melting of a small volume of metal and next the high cooling rate (107 –109 K/s). The shape of the laser-welded wire joint results from the forces acting on the molten area of the material. The hydrodynamics of the molten pool connected among other with thermocapillary convection causes creation of weld joints with the specific shape buried with possible narrowings. The narrowings greater than 20% of cross section, which can degrade electrical and mechanical properties of joints, occur as a result of laser welding under non-optimal conditions. Absorbed energy of the laser beam causes melting of a wire tip. After the laser pulse, the molten metal starts cooling and rapid crystallization occurs. The molten region is formed into a ball as a result of interaction of surface tension and gravitational forces [1,2]. Similar processes take place during the ball formation in thermosonic wire-bonding and stranded wire welding [3,4]. Even in batch of similar welded joints there are joints which properties are unacceptable. The main reason of this situation could be inner defects in joints, like cracks and voids. The cracks appear due to thermal tensions, which are caused by very high rate of cooling and forming of multiphase structure of welded joint.
∗ Corresponding author. Tel.: +48 42 631 26 56. ´ atczak). ˛ E-mail address: [email protected] (T. Swi 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2012.03.009
The formation of gas voids inside the weld occurs at high power densities of the laser beam. During welding it comes to release of gases and the evaporation of metal. When the welding process is carried out under optimal conditions the gases and vapors leak into the atmosphere. Rapid capillary thermal vortices, formed at high energies, put them inside the joint. Cracks and gas voids (porosity) are unwanted defects, which could not be recognized by an ordinary visual inspection (Fig. 1a). In order to detect those defects without damaging samples, it is necessary to apply non-destructive methods such as ultrasonic defectoscopy, magnetic defectoscopy, thermographic method. Due to the small size of objects and other discontinuities and pores, it is a difficult task to recognize defects. Among the known defectoscopy methods proposed a special kind of active thermographic technique – transient thermography.
2. Thermovision measurements Authors prepared the set-up consisting of the thermal camera Cedip Titanium with macro extention rings, power generator, necessary meters and PC computer, to perform experiments using non-destructive testing method (NDT). Camera resolution was 640 × 512 pixels and NETD equal to 15 mK. Active thermography methods enable examination of the internal defects and thin layers in materials without mechanical intrusion and samples damage. In this techniques, object of investigations is stimulated by a heat flux and thermal response in time is aquisited. Next, using Fourier transform, the thermal response in frequency domain is calculated. Obtained amplitude and phase values of the complex thermal
1240
´wiatczak ˛ T. S et al. / Materials Science and Engineering B 177 (2012) 1239–1242
Fig. 1. Laser-welded wire joints: (a) view of joint of Ni/Ni wire (˚ = 0.5 mm) made by Nd:YAG laser pulse ( = 4 m s, power density 1×108 W/m2 ); (b) cross section of laser welded joint with big void inside, non visible by visual inspection.
Fig. 2. Non destructive testing set-up.
response enable determination of thermal and geometrical properties of investigated objects [5–7]. Described NDT set-up is shown in Fig. 2. Electrical connections between two nickel wires with 0.5 mm diameter made by laser welding have been used as a subject of the research. The samples with and without internal defects, welded according to described method, were chosen for the research. Figures below (Fig. 3) present exemplary connections made by laser welding with one of the biggest investigated defects in form of an air bulb (a) and proper connection without any defects (b). In order to estimate thermal impedance of the investigated samples, authors applied one of the most popular active thermography technique – transient thermography [8,9]. Wire welded joints were heated by 3 A DC current. Heating for each sample lasted 100 s to obtain the characteristic frequency of 10 mHz after Fourier transform. Applied excitation signal had the transient character (step function). Heating process of investigated connections was recorded with the thermal camera. Considering the fact that temperature
Fig. 3. Investigated samples, with defect (a) and without (b).
measured by the thermal camera strongly depends on the emissivity coefficient, investigated surfaces of the laser welded joints were coated by soot, therefore the emissivity coefficient was assumed to be 0.95. For each sample, the sequence of the thermograms of the thermal response was recorded in the time domain. Next, the authors measured the mean value of temperature in the region of interest (circle located on the top surface of welded joints) for every time instant. This process was carried out both for joints with and without defects inside. Exemplary frame of the recorded sequence is presented in Fig. 4. The distribution of temperature values in time domain for connections with and without air bulb inside are shown in Fig. 5. Plots in the picture above present thermal response in time with removed DC constant (equal to ambient temperature). This approach was applied in order to perform fast Fourier transform. Using Matlab software, authors calculated thermal impedance, according to the Eq. (1) for all samples for all frames of the thermogram sequences, and obtained complex values of the thermal response in frequency domain [9]. Next, phase values of the thermal impedance were compared for samples with and without defect. Based on those values, it was possible to differentiate samples into two groups – defective and non-defective. In case of exemplary sample with defect the average phase value calculated for the frequency of 40 mHz (fourth component) was equal to −45◦ and −51◦
Fig. 4. Exemplary frame of the recorded sequence. Resolution of the thermal camera: 640 × 512, magnification with 120 mm extension rings: 2 times, resolution: 7 m per pixel.
´wiatczak ˛ T. S et al. / Materials Science and Engineering B 177 (2012) 1239–1242
1241
Fig. 7. Schematic diagram of physical model. Fig. 5. Exemplary thermal responses in time domain.
for sample without defect. Furthermore, Nyquist plots were drawn (Fig. 6) to illustrate differences in phase values between described two groups of samples. The phase shift between sample with and without defect is marked as ϕ (in average 6◦ ). The value of thermal impedance calculated for all samples is proportional to the power loss in conductive element according to Eq. (1). Z(t) =
T (t) , P
Z(jω) =
T (jω) P(jω)
(1)
where Z is the thermal impedance, T is the temperature gradient, P is the power and P = P0 × 1(t). 3. Thermal modelling To confirm the thermovision measurements, the thermal models of the described connection with and without defects were created. With reference to laboratory investigations, during simulations the modelled structure was heated by the current flow and the thermal response in time domain was obtained. The series of the thermal simulation in Comsol software was performed. This package solves non-linear systems of partial differential equations (PDE) by the finite element method (FEM). In the model the multiphysics approach, offering a possibility of relating thermal and electrical phenomena to each other, was used. The mathematical model for conductive heat transfer is described by the heat equation (2): Cp
∂T − k ∇ 2 T = qv ∂t
(2)
where is the material density, T is the temperature, Cp is the specific heat, t is the time, k is the thermal conductivity, and qv is the power density. The process of heat exchange with the environment results from radiation and convection and is described by the expression: h(Tinf − T), where h is a coefficient of heat exchange and Tinf is the temperature of the ambient. Coefficient h is strongly dependent on temperature and sample geometry (especially for thin wires) [10,11]. The heat generated is the result of power losses in conductive element. The amount of heat is proportional to the square of the absolute value of electric current density J in the material. Current density, in turn, is proportional to the electric field potential V. Resistivity is a coefficient of proportionality. The heat dissipated in the material is described by Eq. (3): qv = 0 (1 + ˛(T − T0 )) · |J|2
(3)
where 0 is the resistivity in 293 K, ˛ is the thermal coefficient of resistivity, and J is the current density. The schematic diagram of physical model is shown in Fig. 7. The parameters used in the simulation are presented in the Table 1. The phase value of thermal response was calculated for two cases (with and without defect) in the same manner as for laboratory research. For the defective structure this value equals to −47◦ , and −50◦ for non-defective. Results obtained from simulations with prepared model are in agreement with observations that non-defective sample has greater modulus of phase value. Authors prepared Nyquist plots for the model (shown in Fig. 8). As shown in the plot, the character of thermal response obtained from simulations with finite element method on the prepared model is coherent with the results that authors obtained during their research with active thermography. 4. Results and conclusion Thermal response in time domain was the results of both the thermovision measurements and simulations. In order to convert the domain from time into frequency and to obtain the thermal Table 1 Parameters used in the simulation.
Fig. 6. Exemplary Nyquist plots obtained from thermographic measurements.
Parameter
Value
Thermal conductivity Resistivity at reference temp. Temperature coefficient Density Heat capacity Heat transfer coefficient
90.9 W/(m K) 69.3e−9 m 0.005866 K−1 8908 kg/m3 540 J/(kg K) 150 W/(m2 K)
1242
´wiatczak ˛ T. S et al. / Materials Science and Engineering B 177 (2012) 1239–1242
its sizes, so it is possible to estimate quality of this connections, in this way. In this paper authors described a new application of the non-destructive method with the use of active thermography. Proposed method could be useful tool to detect internal structure defects in laser welded joints. Acknowledgement Authors thank to Ministry of Science and Higher Education of Poland for the financial support (ref. no. 3601/B/T02/2009/36). References
Fig. 8. Nyquist plots – result of FEM simulation.
impedance values, Fourier transform was performed. In the next step, the Nyquist plots were drawn and the thermal impedance values and phase of thermal response obtained both from simulations and measurements were compared. The differences between thermal impedance values calculated from the model and obtained during the research could arose due to non-ideal laboratory conditions and very small scale of samples. It was nearly impossible to eliminate the problem of camera shake as well as the influence of extension macro rings on the recorded thermogram sequences. Furthermore, the real sample was described by the two-dimensional model which was precise, but not exact. In case of very thin wires it was difficult to determine the correct value of the heat transfer coefficient. The thermal impedance value of the laser welded connection directly depends on internal defects and
[1] L.J. Huang, P.S. Ayyaswamy, I.M. Cohen, International Journal of Heat and Mass Transfer 38 (9) (1995) 1637–1645. [2] L.J. Huang, P.S. Ayyaswamy, I.M. Cohen, International Journal of Heat and Mass Transfer 38 (9) (1995) 1647–1659. [3] A. Pequegnat, H.J. Kim, M. Mayer, Y. Zhou, J. Persic, J.T. Moon, Microelectronics Reliability 51 (January (1)) (2011) 43–52. [4] R. Pawlak, M. Tomczyk, Metoda laserowego spawania ultra-cienkich przewodów wielodrutowych 9. Sympozjum Techniki Laserowej – conference materials, Szczecin, 2009, pp. 179–180. [5] X. Maldague, F. Galmiche, A. Ziadi, Infrared Physics and Technology 43 (2002). [6] C. Meola, G. Carlomagno, A. Squillace, G. Giorleo, Measurement Science and Technology 13 (2002). ˛ [7] B. Wiecek, S. Zwolenik, R. Danych, T. Wajman, M. Lis, Pomiary Automatyka Kontrola 11 (2002). ´ atczak, ˛ ˛ [8] G. De Mey, B. Vermeersch, J. Banaszczyk, T. Swi B. Wiecek, M. Janicki, A. Napieralski, International Journal of Heat and Mass Transfer 50 (2007). ´ atczak, ˛ ˛ [9] R. Olbrycht, B. Wiecek, G. Gralewicz, T. Swi G. Owczarek, QIRT Journal 4 (2) (2007). [10] R. Pawlak, A. Rosowski, M. Tomczyk, M. Walczak, Materials Science and Engineering B 176 (March (4)) (2011) 344–351. [11] J. Rymaszewski, M. Lebioda, E. Korzeniewska, Materials Science and Engineering B 176 (March (4)) (2011) 334–339.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Defect detection in wire welded joints using thermography investigations a,∗ a ´ atczak ˛ ˛ T. Swi , M. Tomczyk b , B. Wiecek , R. Pawlak b , R. Olbrycht a a b
Institute of Electronics, Technical University of Łód´z, Łód´z, Poland Institute of Electrical Engineering Systems, Technical University of Łód´z, Łód´z, Poland
a r t i c l e
i n f o
Article history: Received 12 October 2011 Received in revised form 22 December 2011 Accepted 3 March 2012 Available online 18 March 2012 Keywords: Laser microtechnology Laser wire bonding Active thermography Thermal impedance
a b s t r a c t The formation of gas voids inside the wire joints during laser welding may cause internal defects (cracks and porosity), that are invisible from outside. Authors propose the application of active thermography for detection of such defects. Thermal camera was used to acquire sequences of thermograms showing the joints during transient heating. Fourier analysis enabled phase value calculation, which is different for defective and non-defective samples. Laboratory results were confirmed by simulations on prepared two-dimensional model. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Laser technology can be applied to produce wire welded joints. The laser spot welding process is characterized by extremely fast heating and melting of a small volume of metal and next the high cooling rate (107 –109 K/s). The shape of the laser-welded wire joint results from the forces acting on the molten area of the material. The hydrodynamics of the molten pool connected among other with thermocapillary convection causes creation of weld joints with the specific shape buried with possible narrowings. The narrowings greater than 20% of cross section, which can degrade electrical and mechanical properties of joints, occur as a result of laser welding under non-optimal conditions. Absorbed energy of the laser beam causes melting of a wire tip. After the laser pulse, the molten metal starts cooling and rapid crystallization occurs. The molten region is formed into a ball as a result of interaction of surface tension and gravitational forces [1,2]. Similar processes take place during the ball formation in thermosonic wire-bonding and stranded wire welding [3,4]. Even in batch of similar welded joints there are joints which properties are unacceptable. The main reason of this situation could be inner defects in joints, like cracks and voids. The cracks appear due to thermal tensions, which are caused by very high rate of cooling and forming of multiphase structure of welded joint.
∗ Corresponding author. Tel.: +48 42 631 26 56. ´ atczak). ˛ E-mail address: [email protected] (T. Swi 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2012.03.009
The formation of gas voids inside the weld occurs at high power densities of the laser beam. During welding it comes to release of gases and the evaporation of metal. When the welding process is carried out under optimal conditions the gases and vapors leak into the atmosphere. Rapid capillary thermal vortices, formed at high energies, put them inside the joint. Cracks and gas voids (porosity) are unwanted defects, which could not be recognized by an ordinary visual inspection (Fig. 1a). In order to detect those defects without damaging samples, it is necessary to apply non-destructive methods such as ultrasonic defectoscopy, magnetic defectoscopy, thermographic method. Due to the small size of objects and other discontinuities and pores, it is a difficult task to recognize defects. Among the known defectoscopy methods proposed a special kind of active thermographic technique – transient thermography.
2. Thermovision measurements Authors prepared the set-up consisting of the thermal camera Cedip Titanium with macro extention rings, power generator, necessary meters and PC computer, to perform experiments using non-destructive testing method (NDT). Camera resolution was 640 × 512 pixels and NETD equal to 15 mK. Active thermography methods enable examination of the internal defects and thin layers in materials without mechanical intrusion and samples damage. In this techniques, object of investigations is stimulated by a heat flux and thermal response in time is aquisited. Next, using Fourier transform, the thermal response in frequency domain is calculated. Obtained amplitude and phase values of the complex thermal
1240
´wiatczak ˛ T. S et al. / Materials Science and Engineering B 177 (2012) 1239–1242
Fig. 1. Laser-welded wire joints: (a) view of joint of Ni/Ni wire (˚ = 0.5 mm) made by Nd:YAG laser pulse ( = 4 m s, power density 1×108 W/m2 ); (b) cross section of laser welded joint with big void inside, non visible by visual inspection.
Fig. 2. Non destructive testing set-up.
response enable determination of thermal and geometrical properties of investigated objects [5–7]. Described NDT set-up is shown in Fig. 2. Electrical connections between two nickel wires with 0.5 mm diameter made by laser welding have been used as a subject of the research. The samples with and without internal defects, welded according to described method, were chosen for the research. Figures below (Fig. 3) present exemplary connections made by laser welding with one of the biggest investigated defects in form of an air bulb (a) and proper connection without any defects (b). In order to estimate thermal impedance of the investigated samples, authors applied one of the most popular active thermography technique – transient thermography [8,9]. Wire welded joints were heated by 3 A DC current. Heating for each sample lasted 100 s to obtain the characteristic frequency of 10 mHz after Fourier transform. Applied excitation signal had the transient character (step function). Heating process of investigated connections was recorded with the thermal camera. Considering the fact that temperature
Fig. 3. Investigated samples, with defect (a) and without (b).
measured by the thermal camera strongly depends on the emissivity coefficient, investigated surfaces of the laser welded joints were coated by soot, therefore the emissivity coefficient was assumed to be 0.95. For each sample, the sequence of the thermograms of the thermal response was recorded in the time domain. Next, the authors measured the mean value of temperature in the region of interest (circle located on the top surface of welded joints) for every time instant. This process was carried out both for joints with and without defects inside. Exemplary frame of the recorded sequence is presented in Fig. 4. The distribution of temperature values in time domain for connections with and without air bulb inside are shown in Fig. 5. Plots in the picture above present thermal response in time with removed DC constant (equal to ambient temperature). This approach was applied in order to perform fast Fourier transform. Using Matlab software, authors calculated thermal impedance, according to the Eq. (1) for all samples for all frames of the thermogram sequences, and obtained complex values of the thermal response in frequency domain [9]. Next, phase values of the thermal impedance were compared for samples with and without defect. Based on those values, it was possible to differentiate samples into two groups – defective and non-defective. In case of exemplary sample with defect the average phase value calculated for the frequency of 40 mHz (fourth component) was equal to −45◦ and −51◦
Fig. 4. Exemplary frame of the recorded sequence. Resolution of the thermal camera: 640 × 512, magnification with 120 mm extension rings: 2 times, resolution: 7 m per pixel.
´wiatczak ˛ T. S et al. / Materials Science and Engineering B 177 (2012) 1239–1242
1241
Fig. 7. Schematic diagram of physical model. Fig. 5. Exemplary thermal responses in time domain.
for sample without defect. Furthermore, Nyquist plots were drawn (Fig. 6) to illustrate differences in phase values between described two groups of samples. The phase shift between sample with and without defect is marked as ϕ (in average 6◦ ). The value of thermal impedance calculated for all samples is proportional to the power loss in conductive element according to Eq. (1). Z(t) =
T (t) , P
Z(jω) =
T (jω) P(jω)
(1)
where Z is the thermal impedance, T is the temperature gradient, P is the power and P = P0 × 1(t). 3. Thermal modelling To confirm the thermovision measurements, the thermal models of the described connection with and without defects were created. With reference to laboratory investigations, during simulations the modelled structure was heated by the current flow and the thermal response in time domain was obtained. The series of the thermal simulation in Comsol software was performed. This package solves non-linear systems of partial differential equations (PDE) by the finite element method (FEM). In the model the multiphysics approach, offering a possibility of relating thermal and electrical phenomena to each other, was used. The mathematical model for conductive heat transfer is described by the heat equation (2): Cp
∂T − k ∇ 2 T = qv ∂t
(2)
where is the material density, T is the temperature, Cp is the specific heat, t is the time, k is the thermal conductivity, and qv is the power density. The process of heat exchange with the environment results from radiation and convection and is described by the expression: h(Tinf − T), where h is a coefficient of heat exchange and Tinf is the temperature of the ambient. Coefficient h is strongly dependent on temperature and sample geometry (especially for thin wires) [10,11]. The heat generated is the result of power losses in conductive element. The amount of heat is proportional to the square of the absolute value of electric current density J in the material. Current density, in turn, is proportional to the electric field potential V. Resistivity is a coefficient of proportionality. The heat dissipated in the material is described by Eq. (3): qv = 0 (1 + ˛(T − T0 )) · |J|2
(3)
where 0 is the resistivity in 293 K, ˛ is the thermal coefficient of resistivity, and J is the current density. The schematic diagram of physical model is shown in Fig. 7. The parameters used in the simulation are presented in the Table 1. The phase value of thermal response was calculated for two cases (with and without defect) in the same manner as for laboratory research. For the defective structure this value equals to −47◦ , and −50◦ for non-defective. Results obtained from simulations with prepared model are in agreement with observations that non-defective sample has greater modulus of phase value. Authors prepared Nyquist plots for the model (shown in Fig. 8). As shown in the plot, the character of thermal response obtained from simulations with finite element method on the prepared model is coherent with the results that authors obtained during their research with active thermography. 4. Results and conclusion Thermal response in time domain was the results of both the thermovision measurements and simulations. In order to convert the domain from time into frequency and to obtain the thermal Table 1 Parameters used in the simulation.
Fig. 6. Exemplary Nyquist plots obtained from thermographic measurements.
Parameter
Value
Thermal conductivity Resistivity at reference temp. Temperature coefficient Density Heat capacity Heat transfer coefficient
90.9 W/(m K) 69.3e−9 m 0.005866 K−1 8908 kg/m3 540 J/(kg K) 150 W/(m2 K)
1242
´wiatczak ˛ T. S et al. / Materials Science and Engineering B 177 (2012) 1239–1242
its sizes, so it is possible to estimate quality of this connections, in this way. In this paper authors described a new application of the non-destructive method with the use of active thermography. Proposed method could be useful tool to detect internal structure defects in laser welded joints. Acknowledgement Authors thank to Ministry of Science and Higher Education of Poland for the financial support (ref. no. 3601/B/T02/2009/36). References
Fig. 8. Nyquist plots – result of FEM simulation.
impedance values, Fourier transform was performed. In the next step, the Nyquist plots were drawn and the thermal impedance values and phase of thermal response obtained both from simulations and measurements were compared. The differences between thermal impedance values calculated from the model and obtained during the research could arose due to non-ideal laboratory conditions and very small scale of samples. It was nearly impossible to eliminate the problem of camera shake as well as the influence of extension macro rings on the recorded thermogram sequences. Furthermore, the real sample was described by the two-dimensional model which was precise, but not exact. In case of very thin wires it was difficult to determine the correct value of the heat transfer coefficient. The thermal impedance value of the laser welded connection directly depends on internal defects and
[1] L.J. Huang, P.S. Ayyaswamy, I.M. Cohen, International Journal of Heat and Mass Transfer 38 (9) (1995) 1637–1645. [2] L.J. Huang, P.S. Ayyaswamy, I.M. Cohen, International Journal of Heat and Mass Transfer 38 (9) (1995) 1647–1659. [3] A. Pequegnat, H.J. Kim, M. Mayer, Y. Zhou, J. Persic, J.T. Moon, Microelectronics Reliability 51 (January (1)) (2011) 43–52. [4] R. Pawlak, M. Tomczyk, Metoda laserowego spawania ultra-cienkich przewodów wielodrutowych 9. Sympozjum Techniki Laserowej – conference materials, Szczecin, 2009, pp. 179–180. [5] X. Maldague, F. Galmiche, A. Ziadi, Infrared Physics and Technology 43 (2002). [6] C. Meola, G. Carlomagno, A. Squillace, G. Giorleo, Measurement Science and Technology 13 (2002). ˛ [7] B. Wiecek, S. Zwolenik, R. Danych, T. Wajman, M. Lis, Pomiary Automatyka Kontrola 11 (2002). ´ atczak, ˛ ˛ [8] G. De Mey, B. Vermeersch, J. Banaszczyk, T. Swi B. Wiecek, M. Janicki, A. Napieralski, International Journal of Heat and Mass Transfer 50 (2007). ´ atczak, ˛ ˛ [9] R. Olbrycht, B. Wiecek, G. Gralewicz, T. Swi G. Owczarek, QIRT Journal 4 (2) (2007). [10] R. Pawlak, A. Rosowski, M. Tomczyk, M. Walczak, Materials Science and Engineering B 176 (March (4)) (2011) 344–351. [11] J. Rymaszewski, M. Lebioda, E. Korzeniewska, Materials Science and Engineering B 176 (March (4)) (2011) 334–339.