2nd Materials Technology Colloquium Chemnitz (14./15. 10. 1999)
After the great success of the 1st Chemnitz Colloquium on Materials Engineering in September 1998, the organisers at the Chair of Composite Materials at Chemnitz University of Technology decided to continue it in the following year. While the 1st Chemnitz Colloquium on Materials Technology gave a general overview of new research activities and –trends in materials technology, the 2nd Colloquium focused on soldering.
The aim was to present the current state of soldering technology and to highlight new trends in research and development. Representatives from renowned research institutes and companies were recruited to give the 81 participants a comprehensive insight into brazing technology. Participating organisations were the German Association for Welding and Allied Processes (DVS), the Specialist Society for Brazing of the DVS, the German Society for Materials Science (DGM), the Association for Thermal Spraying (GTS), the German Society for Electroplating and Surface Technology (DGO) and the German Copper Institute (DKI).
A poster show and a tour of the department demonstrated the capabilities of the Department of Composite Materials at Chemnitz University of Technology. During the festive evening there were numerous opportunities to meet old friends and acquaintances again or to make new contacts.
The colloquium was opened by Prof. v. Borczyskowski, Rector of the TU Chemnitz. Dr Boecking then brought the greetings of the DVS. In his introductory lecture, Prof. Wielage, TU Chemnitz, gave an overview of the status and development trends in soldering technology.
The aim was to present the current state of soldering technology and to highlight new trends in research and development. Representatives from renowned research institutes and companies were recruited to give the 81 participants a comprehensive insight into brazing technology. Participating organisations were the German Association for Welding and Allied Processes (DVS), the Specialist Society for Brazing of the DVS, the German Society for Materials Science (DGM), the Association for Thermal Spraying (GTS), the German Society for Electroplating and Surface Technology (DGO) and the German Copper Institute (DKI).
A poster show and a tour of the department demonstrated the capabilities of the Department of Composite Materials at Chemnitz University of Technology. During the festive evening there were numerous opportunities to meet old friends and acquaintances again or to make new contacts.
The colloquium was opened by Prof. v. Borczyskowski, Rector of the TU Chemnitz. Dr Boecking then brought the greetings of the DVS. In his introductory lecture, Prof. Wielage, TU Chemnitz, gave an overview of the status and development trends in soldering technology.
Soldering of non-metallic materials
The application of simulation methods to soldering technology tasks was explained by Mr Schüler, TU Chemnitz, using the example of a ceramic-steel solder joint. The targeted optimisation of the soldering process requires an understanding of the physical and chemical processes that take place during soldering. Numerical models allow the investigation and optimisation of the influence of the essential parameters on the formation of the brazed joint.
Two main topics are the focus of interest. These are, on the one hand, the diffusion-controlled reaction mechanisms that lead to the transformation of the ceramic surface into a reaction layer that can be wetted by the solder and, on the other hand, the strength problem, in particular the thermally induced residual stresses that arise in metal-ceramic joints due to the different thermal expansion of the joining partners involved.
Dr Klose, TU Chemnitz described the brazing of diamond. This material can be joined by active brazing using titanium-containing active brazing alloys based on CuSn or AgCu. Due to the high oxygen affinity of the active element, brazing must be carried out in a high vacuum or under inert gas. In addition, in the presence of air or other chemically active substances, graphitisation of the diamond can occur from about 900 K onwards. In an inert atmosphere, graphitisation begins at about 1800 K.
The main areas of application for brazed diamond materials are machining tools in mechanical engineering, such as dressing tools for grinding wheels or milling cutters.
Using practical examples, Prof. Blank-Bewersdorff, FH Hof (formerly Sulzer Winterthur) showed the problems that can occur when brazing metallic with non-metallic materials. The material combinations to be joined are carbide-steel, SiC-graphite, carbide-TiAl6V4 and asbestos substitute steel. According to the requirement profile, a solution can usually be developed through a suitable solder selection and process optimisation.
Two main topics are the focus of interest. These are, on the one hand, the diffusion-controlled reaction mechanisms that lead to the transformation of the ceramic surface into a reaction layer that can be wetted by the solder and, on the other hand, the strength problem, in particular the thermally induced residual stresses that arise in metal-ceramic joints due to the different thermal expansion of the joining partners involved.
Dr Klose, TU Chemnitz described the brazing of diamond. This material can be joined by active brazing using titanium-containing active brazing alloys based on CuSn or AgCu. Due to the high oxygen affinity of the active element, brazing must be carried out in a high vacuum or under inert gas. In addition, in the presence of air or other chemically active substances, graphitisation of the diamond can occur from about 900 K onwards. In an inert atmosphere, graphitisation begins at about 1800 K.
The main areas of application for brazed diamond materials are machining tools in mechanical engineering, such as dressing tools for grinding wheels or milling cutters.
Using practical examples, Prof. Blank-Bewersdorff, FH Hof (formerly Sulzer Winterthur) showed the problems that can occur when brazing metallic with non-metallic materials. The material combinations to be joined are carbide-steel, SiC-graphite, carbide-TiAl6V4 and asbestos substitute steel. According to the requirement profile, a solution can usually be developed through a suitable solder selection and process optimisation.
Soft soldering
In a lecture on alternative soldering concepts for soft soldering, Dr. Martinez, TU Chemnitz, showed that soft soldering technology is on the verge of a radical change, since in Europe, the USA and Japan legislative initiatives are under discussion to ban the use of lead and lead alloys with a few exceptions. It seems to be irrelevant for the public discussion in Europe that only 0.6% of lead consumption is accounted for by electronic solders. However, with a few exceptions, these contain lead.
Despite intensive research, no equivalent alternatives have yet been found. The basis of alternative solders is formed by tin. The alloying additions are silver, copper, zinc, bismuth, indium or antimony. The newly developed lead-free soft solders have melting temperatures that are significantly higher than the melting point of the currently frequently used tin-lead eutectic. However, many electronic components are not designed for the higher soldering temperatures required with them, so that their reliability is not guaranteed.
Therefore, two research trends are particularly interesting for soft solder development. One is the targeted lowering of the melting temperatures of lead-free solders through the addition of further alloying elements and the other is the development of reaction solders. These are made of two or more components that react with each other only at soldering temperature. During soldering, the re-melting temperature increases so that the soldered joints can be used at higher operating temperatures.
Dr. Ahrens, Fraunhofer Institute for Silicon Technology Itzehoe described the metallurgy and some processes of soft soldering. The metallurgical bonding of the solder to the base material takes place via a boundary layer of intermetallic phases. When wetting tin-lead solder on copper materials, for example, copper dissolves in the solder. Stable nuclei of the intermetallic phases Cu3Sn and Cu6Sn5 are formed at the interface. Lead does not participate in this reaction.
The widespread use of soft soldering in electrical engineering is based on the low process temperatures and the good automatability. With automated mass soldering processes such as wave and reflow soldering, large quantities can be produced rationally.
Dr. Vogel, Siemens Amberg, described the selective soldering of electronic components using practical examples. Mini-wave soldering is a very good alternative to manual soldering. The problems presented showed that the fluxes released for wave soldering cannot be used for mini wave soldering without further ado. In addition, mini-wave soldering generally involves higher temperatures for which the electronic components are not designed. Most components can nevertheless withstand the increased soldering temperatures, but they must be subjected to careful quality control or the increased requirements must be specified with the component supplier and the components adapted accordingly.
Despite intensive research, no equivalent alternatives have yet been found. The basis of alternative solders is formed by tin. The alloying additions are silver, copper, zinc, bismuth, indium or antimony. The newly developed lead-free soft solders have melting temperatures that are significantly higher than the melting point of the currently frequently used tin-lead eutectic. However, many electronic components are not designed for the higher soldering temperatures required with them, so that their reliability is not guaranteed.
Therefore, two research trends are particularly interesting for soft solder development. One is the targeted lowering of the melting temperatures of lead-free solders through the addition of further alloying elements and the other is the development of reaction solders. These are made of two or more components that react with each other only at soldering temperature. During soldering, the re-melting temperature increases so that the soldered joints can be used at higher operating temperatures.
Dr. Ahrens, Fraunhofer Institute for Silicon Technology Itzehoe described the metallurgy and some processes of soft soldering. The metallurgical bonding of the solder to the base material takes place via a boundary layer of intermetallic phases. When wetting tin-lead solder on copper materials, for example, copper dissolves in the solder. Stable nuclei of the intermetallic phases Cu3Sn and Cu6Sn5 are formed at the interface. Lead does not participate in this reaction.
The widespread use of soft soldering in electrical engineering is based on the low process temperatures and the good automatability. With automated mass soldering processes such as wave and reflow soldering, large quantities can be produced rationally.
Dr. Vogel, Siemens Amberg, described the selective soldering of electronic components using practical examples. Mini-wave soldering is a very good alternative to manual soldering. The problems presented showed that the fluxes released for wave soldering cannot be used for mini wave soldering without further ado. In addition, mini-wave soldering generally involves higher temperatures for which the electronic components are not designed. Most components can nevertheless withstand the increased soldering temperatures, but they must be subjected to careful quality control or the increased requirements must be specified with the component supplier and the components adapted accordingly.
Aluminium brazing
The brazing of aluminium has numerous applications, especially in the manufacture of heat exchangers. For these components with their many inaccessible joints, brazing is an excellent joining technology. The available brazing processes were presented by Mr Basler, Behr Industrietechnik Stuttgart, using various heat exchanger designs.
Salt bath brazing is increasingly taking a back seat to vacuum and Nocolok inert gas brazing. Another process for brazing aluminium alloys is ultrasonic brazing, which can be used especially for simple geometries. Industrially, it is used to braze the reverse bends of heat exchangers.
The currently most widely used process for brazing heat exchangers was described in more detail by Dr. Belt, Solvay Hannover. In this process, the oxide layer on the aluminium surface is broken up by a fluoride non-corrosive flux. Reoxidation must be prevented by an N2 protective gas atmosphere. The process allows continuous production in continuous furnaces.
Aluminium brazing with conventional brazing alloys based on AlSi requires brazing temperatures in the range of the melting temperatures of most aluminium alloys. This means that most aluminium alloys cannot be brazed, and for others there is a risk of base material melting.
Prof. Schöndorf, FH Köln, therefore presented alternatives to the AlSi system that lie in the borderline area between soft and hard soldering. With solders based on ZnAl, aluminium alloys from 381°C can be brazed. Non-corrosive fluxes based on CsAlF complexes are available for these solders, the effective temperature range of which starts at about 430°C. This means that a wide range of aluminium alloys can be brazed. A wide range of aluminium alloys can be brazed with these fluxes. A special brazing atmosphere is not required. The brazed joints have good strength and corrosion properties.
The ZnAl soldering system cannot be used in a vacuum due to the low vapour pressure of zinc. However, a flux-free process is available with ultrasonic soldering, which was presented by Mr Trommer, TU Chemnitz. In this process, the joint is immersed in a molten solder bath. The effect of ultrasound creates cavitation on the joining surfaces, which breaks up the oxide layer and allows the joining surface to be wetted by the solder.
The development trends in the brazing of copper heat exchangers were shown by Dr. Türpe, Deutsches Kupfer-Institut. In automotive engineering, copper radiators, which were used exclusively in the past, are increasingly being displaced by aluminium radiators. However, copper radiators are still used, especially when there are high demands on corrosion resistance or reparability. In contrast to aluminium brazing with AlSi solders, there is a clear difference between the brazing temperature and the melting range of the base materials of about 300 K, so that there is no danger of base material melting when brazing copper heat exchangers. In addition, copper materials can be soldered flux-free without much effort. Copper materials can also be used in dry ammonia refrigeration systems.
Prof. Sepold, BIAS Bremen presented laser soldering as a suitable joining process for aluminium mixed constructions. The process enables the joining of aluminium materials with various other metallic materials. The use of lasers leads to strong temperature gradients in the joining zone, which strongly limits the growth of strength-reducing intermetallic phases. This requires an optimisation of the temperature-time curve via the control of the laser.
In the concluding lecture by Prof. Wobst, ILK Dresden, an insight was given into the design of heat exchangers. The ban on CFCs and HCFCs places increased demands on the strength of heat exchangers, as higher operating pressures are required for the alternative refrigerants. In addition, the differences in material values must be taken into account in the design.
Salt bath brazing is increasingly taking a back seat to vacuum and Nocolok inert gas brazing. Another process for brazing aluminium alloys is ultrasonic brazing, which can be used especially for simple geometries. Industrially, it is used to braze the reverse bends of heat exchangers.
The currently most widely used process for brazing heat exchangers was described in more detail by Dr. Belt, Solvay Hannover. In this process, the oxide layer on the aluminium surface is broken up by a fluoride non-corrosive flux. Reoxidation must be prevented by an N2 protective gas atmosphere. The process allows continuous production in continuous furnaces.
Aluminium brazing with conventional brazing alloys based on AlSi requires brazing temperatures in the range of the melting temperatures of most aluminium alloys. This means that most aluminium alloys cannot be brazed, and for others there is a risk of base material melting.
Prof. Schöndorf, FH Köln, therefore presented alternatives to the AlSi system that lie in the borderline area between soft and hard soldering. With solders based on ZnAl, aluminium alloys from 381°C can be brazed. Non-corrosive fluxes based on CsAlF complexes are available for these solders, the effective temperature range of which starts at about 430°C. This means that a wide range of aluminium alloys can be brazed. A wide range of aluminium alloys can be brazed with these fluxes. A special brazing atmosphere is not required. The brazed joints have good strength and corrosion properties.
The ZnAl soldering system cannot be used in a vacuum due to the low vapour pressure of zinc. However, a flux-free process is available with ultrasonic soldering, which was presented by Mr Trommer, TU Chemnitz. In this process, the joint is immersed in a molten solder bath. The effect of ultrasound creates cavitation on the joining surfaces, which breaks up the oxide layer and allows the joining surface to be wetted by the solder.
The development trends in the brazing of copper heat exchangers were shown by Dr. Türpe, Deutsches Kupfer-Institut. In automotive engineering, copper radiators, which were used exclusively in the past, are increasingly being displaced by aluminium radiators. However, copper radiators are still used, especially when there are high demands on corrosion resistance or reparability. In contrast to aluminium brazing with AlSi solders, there is a clear difference between the brazing temperature and the melting range of the base materials of about 300 K, so that there is no danger of base material melting when brazing copper heat exchangers. In addition, copper materials can be soldered flux-free without much effort. Copper materials can also be used in dry ammonia refrigeration systems.
Prof. Sepold, BIAS Bremen presented laser soldering as a suitable joining process for aluminium mixed constructions. The process enables the joining of aluminium materials with various other metallic materials. The use of lasers leads to strong temperature gradients in the joining zone, which strongly limits the growth of strength-reducing intermetallic phases. This requires an optimisation of the temperature-time curve via the control of the laser.
In the concluding lecture by Prof. Wobst, ILK Dresden, an insight was given into the design of heat exchangers. The ban on CFCs and HCFCs places increased demands on the strength of heat exchangers, as higher operating pressures are required for the alternative refrigerants. In addition, the differences in material values must be taken into account in the design.
