UNIVERSITY OF CRAIOVA FACULTY OF MECHANICAL ENGINEERING HABILITATION THESIS ASSOCIATE PROFESSOR,

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1 UNIVERSITY OF CRAIOVA FACULTY OF MECHANICAL ENGINEERING HABILITATION THESIS ASSOCIATE PROFESSOR, PhD SORIN VASILE SAVU

2 HABILITATION THESIS Developments of new joining and processing technologies for advanced materials Associate Professor Sorin Vasile SAVU, PhD University of Craiova Faculty of Mechanical Engineering

3 (A-I) ABSTRACT My entire professional activity after becoming PhD in 2009 (public presentation of the thesis was held in 2008 and the governmental confirmation with the issue of the PhD Diploma arrived in March 2009) was strictly related to scientific research, to teaching and training and to project and institutional management. The habilitation thesis has two major parts: the first part is presents the scientific work performed after PhD thesis and the second part is dedicated to academic work. Section B-I. Achievements and Developments Plans The scientific research work was performed within the, within the University of Craiova (UCV) ( ) as Lecturer and later Associate Professor and within the Romanian Welding Society (ASR) ( ) as welding expert. The topics targeted in my research activity can be grouped in five chapters as follows: Sensors and Automation Technology, Joining and Microjoining Technology, Microwave Technology, Laser Technology and Developments in EQF 4-6 Education and Training Systems. These topics were selected taking into account my specialization as electrical engineer and expert in hybrid welding technologies. Chapter 1 Sensors and Automation Technology This chapter represents a continuation of the research work done during the elaboration of my PhD which was related to the development of new nanostructured sensors for laser-arc hybrid welding. The research was focused on three parts: elaboration of the new magnetic nanostructured sensors, the optimization of the microwave heating process for the elaboration of solid magnetic materials and the improvement of performance of the contacts of automated power factor corrections installations The first part was as a continuation of the studies performed in my PhD. thesis. I was focused on two different magnetic materials: magnetic nanofluids and magnetic solid materials. Therefore the research performed was focused on the influence of the gravitational flow in a torus of the magnetic nanofluids and how can be modelled and improved its energy in order to increase the sensing properties as well as to the elaboration of new solid materials with higher magnetic properties. The researches ended with elaboration and testing of two types of sensors used in sensing of the relative position of the laser beam and TIG head, as well as the improving the performance of a tennis player by sensing the movement and speed of the ball. In the second part the research was focused on the elaboration of analytical solution to match the impedance of the load to the impedance of the transmission line for microwave heating process. Also a computer code has been written in order to be applied as scenarios for the three stubs movements. The results of the research were the improvement of microwave power transfer from the generator to the load. In the third part the research was performed taking into account the big number of defects occurred in switching process of the capacitive loads from the power factor improvement installations. The research was requested by the private sector and funded by the national programme for research, development and innovation PN III Innovation cheques. The project entitled Optimization of constructive elements of the protection and electrical switchgear to improve the lifetime of the product REACTIV has dealt with automation switching of the capacitor banks from an industrial process. The outcomes of the project 3

4 were new composites materials based on W-Cu alloys which have been tested to over switches using a devices especially designed for that application. The second part of the research has been done to improve the electrical contact of the knife-fork system of the fuses. Chapter 2 - Joining and Microjoining Technology This chapter deals with joining and microjoining of the materials (polymers, metals and composites materials) as part of my research in the field of welding technologies. Covering three types of materials, presented above, the research was focused mainly on the designing and setting up the welding equipments as well as the characterization of the welds. First part of the chapter is dedicated to polymer welding. Relevant researches developed after PhD thesis in the field of polymers joining/microjoining have been done in order to study the behaviour of polymers, in terms of flowing process, when they are heated. Two different processes were tested on polymers: heated butt tool/electrofusion welding and ultrasonic micro vibrations in polymers welding. Regarding the first welding process of the polymers, the results of the research have revealed that lower heating during welding is bringing higher elongation viscosity. That means that the plasticity of the polymer is increasing in the analysed area. The next in my research related to polymer welding was dedicated to combining of polymers joining with ultrasound activation in order to increase the flow rate of the polymers. The results of the experimental research has revealed an increase with more than 70 % of the flow rate when ultrasound micro vibration was applied to the process. The second part of the chapter deals with technologies and processes for welding of the metals. Taking into account that the developments in doctoral research were oriented towards to elaboration materials for sensors and actuators, the joining of metals have been performed in order to be used in electronic motherboards of the sensing systems. Two different microjoining processes have been researched: eutectic bonding with ultrasonic activation of Sn-Ag-Cu solder in order to reduce the formation of intermetallic compounds and the thermosonic bonding of the metal wires in order to obtain joints with low electrical resistance. Ultrasonic irradiation was revealed to have influence on the dimension of the IMCs formed at the interface between the solder and the base metal and in the solver. It has been observed that the increasing of the ultrasounds frequency decreases the thickness of the Cu3Sn layer which is usually forming at the interface between the solder and the base metal. Applying ultrasonic field up to 55kHz frequency the quantity of formed Cu3Sn is decreasing and the developed layer is about 60-65% (from maximum value of 18μm thickness at 320 o C soldering temperature and 50s time of the process to 14μm for f=45khz and 10μm for f=55khz) from the thickness which is recorded for classic eutectic bonding. The development of thermosonic bonding process was the result of the need to improve the productivity of the bonding process with ultrasound. The micro-joints of MEMS, which are related to electrical connections, are often realised by using thermosonic microbonding process. Because of that the main elements subjected to thermosonic microbonding are wire or thin plates (foils). As base materials for joining have been selected copper-copper (Cu-Cu), aluminium-copper (Al-Cu) and (Al-Al). During the experimental programme have been obtained 6 pairs of joints, the research being performed for microbonding temperature T = (100 0, ) C and an ultrasonic activation time tus = 0.4 s. For each joint have been performed electrical resistance measurements using a Signametric DMM PCI 7 ½ digits digital multimeter. The method of measurements was 4Ω (4 points measurement). The 4

5 evaluation of electrical resistance of the bonded materials has been done in order to determine the best technical solution for MEMS circuits. The results shown that coppercopper joints presented the lowest electrical resistance and the aluminium-aluminium joints presented the highest electrical resistance. An important aspect must be highlighted related to the values measured for thermosonic bonding of dissimilar materials copper-aluminium. The values of electrical resistance are close to the average of the values recorded for Cu-Cu or Al-Al which give the opportunity to use dissimilar joints in electronic circuits in order to decrease the price of products. The third part of the chapter is dedicated to the welding of composites materials. The idea of the research came as a continuation of the researches in the field of sensors and actuators. Both applications require materials some advantages like almost zero thermal expansion coefficient, low density, low electrical resistance, low friction coefficient, low or high stiffness and satisfactory ductility. The first step in my research was to elaborate composites materials and then to try to weld them without major modification of the structure and properties of the base materials. Therefore some composites materials with cylinder shapes (CuAl-SiC, CuSn10-C and W-Cu-Ni) were elaborated using powder metallurgy applications. The sintered samples were subjected to laser welding process. The main conclusions released were: the welding of W-Cu-Ni was almost impossible. It was necessary to have long pulses (12 18 ms), but even using these precautions, the stability was low. By increasing the pulse power, a constant and stabile melting started at about 1,000 W, but very fast, at about 1,600 W the process became a cutting process; CuSn10-C presented the lowest weldability, for all the parameters and without preheating the stability was low: high spattering, violent explosions and excessive burning of the materials; In the case of the CuAl-SiC, by modifying the parameters of the heat source, different types of melting of the base metal were obtained. Even if all the regimes were specific to welding, some melting processes developed as in the cutting processes modes. Every situation was characterised by high instability and that came from the porous structure of the base metal, structure that is specific to the sintered materials. Chapter 3 Microwave Technology Microwave technology and applications represents one of the main research directions after my PhD. thesis. The researches have been conducted through a postdoctoral scholarship ( ) Research on microwaves interaction with composites magnetic materials for inductive sensors in order to optimize the fabrication processes of the sensing elements. The main goal of the researches was to study the interaction between microwave one-direction fields and nanostructured magnetic materials (ferrites type M and W) in order to develop new materials with high sensing properties for inductive sensors as well as the development of new fabrication processes using microwaves as primary thermal source. Theoretical models for thermal field evolution inside the materials processed in microwave field have developed. Taking into account the large spectre addressed the research activities can be grouped in: elaboration of nanostructured magnetic materials using microwave heating, materials processing in microwave field and structural degeneration using microwave technology. First part of the research was dedicated to the elaboration of nanostructured magnetic materials using microwave heating. The topics covered by the research were related to the improving of the heating mechanism in order to reach the sintering temperature, the controlling the microwave heating process in order to avoid the thermal runaway 5

6 phenomena during heating process, the identification the phase transition in materials using new technique for differential thermal analysis in microwave field and the characterization of the materials properties obtained in microwave field. The second part of the research has been focused on microwave welding. Different types of materials (polymers, metals, amorphous solids and composites) were subject of experimental welding applications. Regarding microwave welding of polymers, the welding experiments have been realized using two precursors such ceramic plates and alumina powder. The goal of the research was to evaluate the behaviour of the HDPE 100 during heating and how DSC method is useful to evaluate the thermal behaviour of the named polymer. The result of the thermal test revealed high rate for the crystallization process. The second important parameter studied in my research was the elongation viscosity which gives the dimensions of the mechanical characteristics of the weld. The results have shown low values for the elongation viscosity, which means not very fluid material and a necessity to apply high temperatures during the processing by welding or plastic deformation. Regarding the relaxation modulus which indicates the plasticity of the material it can be observed that the microwave heating produced a decreasing of the plasticity with the increasing of the amount of heat introduced into the material. The decreasing of the relaxation modulus recorded for the passing from 65 W to 130 W power is in the range of 12-20%. Such decrease of the plasticity could affect the weld, if the weld is subjected to high loading during the exploitation (for example in piping systems). The decreasing shows a behaviour specific to the ageing of the material due to the microwave heating, but the risk of cracking remains low. The relation between the relaxation modulus and the stress proves linearity for any value of the specific deformation. When increase the specific deformation, the relaxation modulus is decreasing. The decreasing is important and it reveals the influence of the reticular structure on the plasticity of the polymer. Regarding the microwave welding of metals I tried to develop some applications related to eutectic microbonding. During the microwave welding process two issues have been studied: to succeed the heating process using microwaves as thermal source and to prevent the plasma arc initiation when a high frequency electromagnetic field interacts with metal materials. The macroscopic analysis of the microjoints has shown that the microjoints presents some material defects explained by the existence of the imperfect contact between base materials, by the poor wetting and the insufficient amount of solder alloy, or a combination of too low temperatures. Other researches in the field of microwave welding of metals were focused on obtaining of microjoints of electrical wires using plasma microwave. Taking into account that all my researches were somehow connected with the development of sensors and actuators, the microjoints obtained in microwave plasma were subject of electrical evaluation The values of the electrical resistance have shown that the changes of the electrical resistance after the joining are minimal for laser welding case, but, the cost of this equipment are much more higher than a microwave generator. It was observed that the electrical resistance for laser welding is similar to the electrical resistance obtained using plasma microwave procedure. The next step in my research after the experiments realized for polymers and metal welding was focused on glass welding in microwave field. The researches performed indicated that the process is possible but during the cooling period the risks of cracking were very high and the joints had low quality. Therefore I tried to control the cooling process in order to avoid 6

7 the cracking process. I designed and executed an experimental microwave-resistance hybrid welding device. The results obtained in terms of the process stability were better than single microwave welding but the quality of the welds was not very good due to the massive flow of the glasses in the viscous state. This point is close to thermal runaway point and therefore the process can be unstable. By decreasing the temperature, the welding process cannot be obtained and by increasing the temperature, the danger of appearance of the thermal runaway phenomena is very high. Having the experimental microwave-resistance hybrid device I tried to use it in the welding process of the metal-ceramic composites. The reason of the experimental program was to create a map of the temperatures developed in the both base and solder materials, in order to have a good image of the heating phenomenon and to be able to improve the heating procedure to obtain optimum soldering process. A ceramic powder has been introduced in order to force the microwave heating mechanism. The results obtained that the difference between forced and unforced heating mechanism was that using conditioned microwave-resistance heating the temperature reached after 6 seconds were over comparative with unforced heating mechanism where there temperature where around C, which is lower than necessary for the melting of the solder. The third part of my research were dedicated to the structural degeneration of the materials in microwave field. The research was approached as follow of the burning process developed by microwaves actions on some materials. Some textiles and expired drugs were used in my research. On the other hand the emissions of fumes during burning process were recorded. The results obtained have shown that the smoke emission during burning has significantly lower values for each of the four monitored gases: NO, NO2, CO, CO2, and in terms of environmental impact they remain in the danger zone only in the case of conventional burning. Chapter 4 Laser Technology My involvement in the development of laser technologies for material processing in general but focused on welding technologies raised as an extension of the researches performed during PhD studies. The doctoral studies have been oriented to the improvement of the laser-arc welding systems through high precision nanostructured sensors. The next step in the development of hybrid laser welding technologies was to obtain preliminary information related to laser technology which were used later in hybrid applications. This chapter deals with four parts of laser technology: laser processing - several researches developed after the PhD thesis were focused on applications using laser technology; surface improvement - the researches have been oriented to the improvement of the materials surfaces in order to obtain better mechanical properties; Laser marking - the studies have been focused on the research of the behaviours of the polymers during their interaction with laser beam; hybrid technologies - new hybrid welding processes composed from 2 thermal sources have been developed as a step forward in obtaining synergies in welding processes First part of the chapter is dedicated to laser processing. Two different materials (polymers and composites materials) have been subject of the research work. In the case of polymers welding a new laser device based on SLD (solid laser diode) was designed. The results obtained have shown that the materials with high transparency even the current was the maximum allowed by the electrical performance of the SLD, the materials had not been joined. Better results were obtained for polymers foils with low transparency. In the case of composites materials the research related to laser processing of the composites materials 7

8 was performed in order to improve the behaviour of the sintered composites, when they were processed using laser technology, through an ultrasonic aided vibration. The conclusions released after experiments were: the laser welding process of the metallic matrix composite materials and the quality of the welds are influenced by the level of the powder compaction, powder that is the primer of the composites; the compaction determines the density of the sintered piece and the level of the weld s porosity, but in the same time an influence on the spattering during the welding process has been revealed; a possible method to improve the density (with about 0.5%) of the composite material consists in the vibration of the powder before and during the pressing process; the best results have been obtained by vibration of the powdered mixture before the pressing process The second part of the chapter deals with surface improvement of the materials with high strength, low weight and outstanding corrosion using laser technology. In my research I used as base material Ti-Al-V and the surface improvement consisted of applying two laser heating cycles: heating to melting and high rate ( c/s) cooling and heating to melting and high rate ( c/s) cooling followed by annealing treatment in order to reduce the brittleness by reducing the volume of the Widmanstätten structure volume. As general conclusion the improving of the Ti6Al4V surface wear resistance is possible by all applied methods. The cheapest method is the simply heat and cool down, according to cycle 1, but the improvement of the functionality of the surface is lower than necessary. The generation of special wear resistant layer on the surface of the Ti6Al4V was the solution with the optimal technical results, but not the most economic. The heating produced a decreasing of the thermal conductivity and that could partially explain the modification of the structure. In the same time, due to the heating process, oxidizing of the surface occurred and the kinetics of the oxidizing process can be controlled by controlling the heating time. The third part of the chapter is dedicated to the laser marking of polymers. Taking into account that many researches performed in welding technology were focused on polymers, I tried to establish how are affected the polymers when they are heated using a laser beam. As base material, HDPE 100 was used in the experimental programme. The selection of HDPE is based on the large number of applications that require marking and on the large volume of production; HDPE is generally known for its large strength to density ratio and is usually employed in the fabrication of plastic bottles, resistant pipes and other plastic materials. The experimental programme consisted in specific thermo-mechanical analysis of the polymer with and without burning during laser marking process, analysis that is dedicated to understand the evolution of the thermo-mechanical characteristic of the polymer after the marking process. The fourth part of the chapter represents a step forward in laser processing technology. The experience gained through different laser applications studied before and after PhD. thesis gave the opportunity to research the laser-hybrid processes. Starting with laser-tig welding the research was focused on the coupling phenomenon for the three specific situations depends both on the TIG arc characteristics (average current value, TIG current ratio, TIG frequency) and the hybrid process geometrical parameters (distance between processes and laser beam Laser Beam TIG torch angle) with impact on the weld seam geometrical characteristics and macroscopic aspect. Welding experiments were done in both TIG-LASER and LASER-TIG variant. The next hybrid process approached was related to the coupling of laser beam with ultrasounds in order to achieve the synergy from both processes. The hybrid 8

9 laser-ultrasonic (thermosonic) process is proposed to be applied by two bonding steps: first of all LASER preheating, to increase the plasticity of the gold wire and to reduce the cooling speed (necessary to avoid cracking of the bond) and a second step that is the bonding process. No brake time between the two steps were considered. Both the two bonding heads were in position. The LASER started the preheating and when the copper substrate reached specific temperature (values between 50 and C were considered in order to determine optimum per situation) ULTRASONIC impulses have been transmitted in order to perform the bonding. The experimental research was performed in conditions specific to the electronic industry: welding thin gold wires on copper substrate. The diameter of the wires was 450 µm and the thickness of the substrate was 400 µm. The second experiment consisted of the preheating using ultrasounds and welding using hybrid laser-ultrasonic process. Chapter 5 - Developments in EQF 4-6 Education & Training systems The research developed in the past 10 years gave scientific results which have been published in articles in journals or conference proceedings. Taking into account that I work in tertiary education system, I tried to use the expertise gained in the research activity to the elaboration and development of new courses both for secondary and tertiary level. Moreover, as member in the research teams from national and international projects, I contributed to the development of common education and training materials in order to harmonize the education and training system in Europe. The research activity was focused on two directions related to curricula/course development: welder s qualification and inland vessel crew qualification. Section B-II. Scientific, Professional and Academic Future Developments Plan This section is dedicated to my future plans both for academic and research activities. Taking into account my work performed until now my concerns related to scientific research will cover the following topics: research on sensing properties of the materials as well as the elaboration of nano and microstructured sensors, research on the development of new hybrid welding systems and processes, research on microwave technology related to materials processing and the research on selective catalyst systems activated with microwave for inland vessels. In terms of academic activity the topics which I consider to be relevant for my development as teacher and trainer will be: the improvement and continuous updating of the curricula and lessons materials (courses, seminars and practice laboratory), the improvement of the laboratories infrastructure and introduction of new practical works, the introduction of the students in scientific research through their participation in research projects, the continuous improvement of teaching and assessment methods and the harmonization of the courses at European level. 9