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Laser Micro Welding of Mechatronics Components LASER MICRO WELDING OF MECHATRONICS COMPONENTS S.l. Daniel Besnea, PhD E...

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Laser Micro Welding of Mechatronics Components

LASER MICRO WELDING OF MECHATRONICS COMPONENTS S.l. Daniel Besnea, PhD Eng. Affiliation: Politehnica University of Bucharest Post address: Splaiul Independenţei nr. 313, sector 6, Bucharest E-mail:[email protected] Prof. Octavian Dontu, PhD Eng. Affiliation: Politehnica University of Bucharest Post address: str. Splaiul Independenţei nr. 313, sector 6, Bucharest E-mail:[email protected] Prof.Gheorghe I.Gheorghe, PhD. Eng. Affiliation: Director General INCDMTM Bucharest Post address: Sos.Pantelimon, nr. 6-8, sector 2, Bucharest E-mail: [email protected] Dr. ing. Ciobanu Robert Affiliation: Politehnica University of Bucharest Post address: str. Splaiul Independenţei nr. 313, sector 6, Bucharest E-mail: [email protected] Ing. Andrei Cuta Affiliation: Politehnica University of Bucharest Post address: str. Splaiul Independenţei nr. 313, sector 6, Bucharest E-mail: [email protected] Abstract - The paper presents a constructive solution regarding laser micro-welding of a new mechatronic component belonging to the aluminum alloy sensors category. The laser welding process of aluminum alloys is particularly complex because the aluminum alloy has a very high reflectivity and in most situations it requires a specific surface pretreatment. Due to the physical characteristics of the material and the precise positioning requirements of the parts that are to be assembled through laser micro welding, the system designed and realized has specific operating conditions. After laser micro welding, the sensor was tested and all initial conditions imposed were satisfied: the sensor was pressure tested with very good results and we didn't have difficulties to integrate the sensor in the mechatronic structure for which was designed. Keywords: micro-welding, Nd-YAG laser, sensors, positioning system.

1. Introduction The impressive progress registered in the science of materials, regarding the development of the advanced materials, would not have been accomplished without the appropriate development of the science of processing and the technology for assembling these new materials. The assembly is a technology implying an often neglected risk, dictating the success of the devices and components developed in all the industrial sectors. The constant success of the industrial production is conditioned by the continuous development of advanced and special materials assembling technology. CO2 and Nd:YAG laser welding has a significant share in the industrial production [1].

The Nd:YAG solid state lasers are used in welding small sized components, such as medical instruments especially for laparoscopic surgery or electronic parts and also in automotive industry, chemical industry with applications involving the usage of titanium, due its particular corrosion resistance and also special applications requiring superconductivity properties (niobium alloys) or the shape memory effect (shape memory nickel alloys). Laser welding is particularly used in assembling parts built of materials with different fusion points and hard to weld (for instance: high alloyed steel, precious metals, tungsten and molybdenum, tantalum, titanium, nickel and beryllium) with small thickness and a reduced heat affected zone in the assembled materials.

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Laser Micro Welding of Mechatronics Components Two laser assembling methods can be used: surface welding and in-depth material welding (fig.1).

Figure 2 – The parts that will be welded using the laser Figure 1 – Surface and in-depth welding The laser processing technologies increasingly take over the industrial process, due to the benefits they offer. Their advantages are clear and proven in current practice, in relation to classical industrial installations, such as flexibility, the ability to manufacture miniaturized components, repetitiveness and consistency of results, the ability to process very hard materials, facile automation. The mechanical and thermal constraints imposed by the materials in using these technologies are minimal, thus obtaining a high quality weld seam in high productivity conditions [2]. The finite elements analysis using the ANSYS 12 and CATIA V5 software, allows the quantification of the tensions and strains in the laser welded probes, the identification of the minimal resistance areas, with high risk of cracks, modelling and simulation of the elements that are to be assembled. Considering these analyzes, the optimal parameters of welding may be chosen, so that the areas of minimal resistance can be avoided. Due to the advantages the laser presents (strictly controlled field temperature and heating time, minimum thermal affecting of the area around the weld seam, high welding speed without requiring metal addition, the possibility of complete automation etc.), its use in achieving welded assemblies is increasing, particularly for materials that are difficult to weld through conventional methods such as stainless steels and some aluminum alloys [3]. The parts made of stainless steels can be welded in different ways, but it is important to choose the most appropriate method to obtain the welds with the best characteristics. It is important to note that regardless the chosen method, welding generally involves heating up the materials to their melting stage, which may lead to structural changes that might influence the characteristics of the thermally affected materials. Figure 2 presents the parts that are to be welded; the thickness of the sensor membrane is 0.09 mm and the housing is made of aluminum. In order to position and stiffen the membrane on the housing of the sensor, a 1.3 mm thick aluminum ring has been manufactured, the welding being performed on the outer contour.

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2. Presentation of the origianl solution In order to accomplish a proper laser weld, a laser generator which is appropriate to the joint type and materials and whose emission power and wavelength are leading to the best absorption of the laser radiation by the material, must be chosen. The parameters of the laser radiation, adjusted upon the characteristics of the welding materials and the disposal of the welded parts must ensure the raise of the Tms temperature from the joint area to a value which is superior to the melting temperature – Tmelting, the vaporization temperature Tvaporization, as following: Tmelting < Tms < Tvaporization

(1)

The laser action timeframe, or the displacement speed of the parts/beam is chosen so that the penetration of the melting frontline in the materials takes place before the evaporation of their superficial layer, basically the maximum melting depth being reached when the temperature of the material from the surface reaches the boiling point. The rapid heat conduction in the two materials from the joint area depends on the thermic diffusion – d of the materials and the type of contact between the parts that are being welded [4]. Taking into account the conditions, the expression of the temperature field in the welded materials[3], according to the flux intensity absorbed per unit area – I0, the radial distance - r from the laser beam center, the penetration depth of the material – z and the time – t, can be determined using the following expression: τ

T( r , z ,t )



z2

I a 2 d i e 4 dt e −r ( 4 dt +a ) = 0 ⋅ ⋅ k π ∫0 t (4dt + a 2 ) 2

2

(2)

where: τi= time length of the laser pulse By imposing certain conditions, the thermal time constant CtT can be defined as the timeframe in which the temperature of the profound mass of the melt has the same order of magnitude as the temperature of the laser irradiated surface.

The Romanian Review Precision Mechanics, Optics & Mechatronics, 2014, No. 46

Laser Micro Welding of Mechatronics Components When the laser action timeframe is much lower (the displacement speed is too high) than the thermal time constant CtT of the material, an homogenous melt

of the materials around the weld seam will not be obtained, having unfavorable consequences over the quality of the welded assemblies[4].

Figure 3 – The 3D model of the welding device Because the laser SWP 6002, from the specialty laboratory only allows the CNC controlled displacement on three axes XYZ, in order to accomplish a circular weld seam, a small, compact device must be designed [5,6], allowing the performance of a rotation movement (A), around the X axis, accomplished with an electronically controlled stepper motor (fig. 3). Because the technological working parameters, the impulse energy (J), the impulse timeframe (ms), the

nominal power (W), the maximum power of an impulse (kW) and also the size of the focal spot must be correlated with the type of welded alloy[7,8], an electronic driver for the stepper motor has been designed (fig. 3), which allows the correlation of the rotation movement of the parts that will be laser welded, with the optimal working parameters, while the laser action timeframe is precisely controlled, allowing to obtain a flawless weld seam[8].

Figure 4 – The stepper motor electronic driver The electronic block presented in figure 4, utilized for the stepper motor command is based on a PIC16F1937 microcontroller. It was programed with the ICSP method using a PICKit2 programmer, programmed in C++, compiled using CCSPIC compiler. The signal amplification of the digital signals that drive the stepper motor are amplified using a Darlington driver, which has built in diodes for

inductive loads. The command pins are also connected to LEDs for visualizing the commands. The speed of the motor is determined by the drop of voltage using a potentiometer connected to one of the digital inputs of the microcontroller. The direction of the motor is determined by a switch (fig.5).

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Laser Micro Welding of Mechatronics Components

Figure 5 – The developed device and electronic driver for the stepper motor control The whole system is monitored by a PC using a FTDI interface, so that the user can modify the speed and direction so that it can get synchronized with the laser beam so that it can be obtained a quality laser weld (the surface of the new point needs to be at least 80% of the surface of the new point). The proposed system was tested with successful results (fig. 6).

industry (power and chemical equipment, aircrafts), recent applications have also been reported in the process of medical systems and components manufacturing, including laparoscopic instruments, electronics or the nuclear and consumer goods industry. Other applications refer to titanium alloys welding, which are used in several domains, due to their particular corrosion resistance, for medical components (implants or exploratory probes, which must ensure compatibility with the human body), for welding dental alloys, where substantial upgrades of the traditional assembly methods have been obtained. This paper presents a complex study regarding the laser welding of a new mechatronic component belonging to the pressure sensors category made by aluminum alloy. Due to very high reflectivity of aluminum alloys, fast oxidation, the absorption of gases from the surrounding medium, special difficulties arise at laser beam welding. Besides these aspects, the paper presents a constructive solution of a device that optimizes the sensor laser welding process. Acknowledgement: The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132395 4. References

Figure 6 – The SWP 6002 welding system – (a); image of the parts that will be welded (acquired from the stereomicroscope), 10X zoom – (b); the welding device, in the workspace – (c); the welded parts – (d) 3. Conclusions One of the main reasons why there is a limited propagation of aluminum alloys in mechanical structures is that they are very difficult to weld. In these particular situations, laser welding is an efficient solution due to its ability to strictly control the inlet temperature. Moreover, between the input and the output variables, we can formulate a mathematical model to obtain a quality weld. Several applications covering various domains exist, due to the special advantages that laser welding offers (the possibility of welding different materials, with complex shapes and dimensions, accomplishing a high welding speed and a high quality weld mesh, in a fully automated process). For instance, besides the industrial applications known from the automotive

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[1] William M. Steen, Laser Material Processing, Springer Verlag London, 1999; [2] Costa, A., Quintino, L., Soldatura laser de metais duros, Rev. ISQ –Technologia Qualidade, nr. 41/42/2001, p. 43-47; [3] M. Piersica, S. Nedelcu, D. Luculescu, Metode de modelare a procesului de prelucrare cu laser, ed. Albastra, Cluj Napoca, 2006; [4] I. Avarvarei, O. Dontu, D. Besnea, I. Voiculescu, R. Ciobanu, Laser welding of stainless steel capsules, Optoelectronics and advanced materials– rapid communications, Vol. 4, No. 11, November 2010, p. 1894 – 1897; [5] Mircea Mihail Popovici, Modelarea virtuala 3 D în construcţia de maşini, Editura Printech, Bucureşti, 2005; [6] D. Besnea, O. Dontu, N. Alexandrescu, G. Gheorghe, P. Beca, A. Abalaru-Tehnologii de fabricatie asistate de calculator pentru executia unor component mecatronice, Ed. Printech, Bucuresti, 2008; [7] Narikiyo T., Fujinaga S., ş.a., Optimisation of welding using two Nd-YAG laser beams combined at work piece surface, Science and Technology of Weling and Joining, 2000; [8] Sekhar N, Reed R., Power beam welding of thick section nickel base super alloys, Science and Technology of Welding and Joining, vol. 7, 2002.

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