Johannes Wilden

An innovative joining strategy in order to join zinc coated steels with minimized damaging of the coating

Metallic components undergo during complete lifetime complex loading conditions, which are a combination of mechanical, environmental as well as corrosive loading. The developments within the ULSAB-project allowed the enhancing of mechanical properties of steels, as for example for DP-and CP steel sorts. The corrosion resistance of high strength steels and of deep drawing steel can be only improved through coating technology, as for example through hot dipping in zinc or electrolytic zinc coating. Thermal joining of zinc coated steel sheets presents some difficulties, as the boiling temperature of zinc (907°C) is lower than the steel melting point. As a result of the eruption like evaporation of zinc, pores in the weld seam are present after solidification and the zinc coating near the weldment is spoilt and has to be reestablished. Furthermore the degassing of zinc influences negatively process conditions, as for example arc turbulences when MSG welding and wear of the electrons when resistance spot welding is performed. Even if solutions were developed, brazing of zinc coated steels with CuSi-alloys is nowadays mostly set when joining zinc coated steels. Nevertheless the high melting point of these filler materials (over 900°C) requires very restricted process strategies and damaging of the zinc coating near the brazing seam can’t be avoided.ZnAl-alloy filler wires, on the contrary, allow to achieve low joining temperatures similar to the melting point of zinc in the region between 400-500°C and offer new possibilities in order to join zinc coated steels without damaging of the zinc layer. In this paper investigations regarding the set of different ZnAl-alloys (Zn till ZnAl15) for soldering zinc coated steels are reported. Investigations were performed with an Nd:YAG and with a diode laser and confirm the suitability of these alloys for joining these steel types. Mechanical behaviour is reported as well, as a further confirmation of the successfully set of Zn-Alloys for soldering applications.Metallic components undergo during complete lifetime complex loading conditions, which are a combination of mechanical, environmental as well as corrosive loading. The developments within the ULSAB-project allowed the enhancing of mechanical properties of steels, as for example for DP-and CP steel sorts. The corrosion resistance of high strength steels and of deep drawing steel can be only improved through coating technology, as for example through hot dipping in zinc or electrolytic zinc coating. Thermal joining of zinc coated steel sheets presents some difficulties, as the boiling temperature of zinc (907°C) is lower than the steel melting point. As a result of the eruption like evaporation of zinc, pores in the weld seam are present after solidification and the zinc coating near the weldment is spoilt and has to be reestablished. Furthermore the degassing of zinc influences negatively process conditions, as for example arc turbulences when MSG welding and wear of the electrons when resistance spot weld...

In-situ synthesis of Y2W3O12 within a Co-based superalloy powder mixture

Abstract In this work, the synthesis of the negative thermal expansion ceramic Y2W3O12 within a Co-based superalloy was analyzed in detail. On the basis of thermal analysis of the synthesis from the feedstock powders Y2O3 and WO3 further experiments were conducted. Thermal behavior and stability of different mixtures of the powders Y2O3, WO3 and the Co-based superalloy were investigated. The qualitative and quantitative results of the synthesis separately and within the superalloy are presented as achieved by energy dispersive X-ray analysis, X-ray powder diffraction, differential thermal analysis and thermal gravimetric analysis. It was found that the synthesis of yttrium tungstate inside the Co-based superalloy is possible with a defined temperature program and a simple basic production process of powder mixtures.

Modification of solidification conditions through the application of pulsed Nd:YAG lasers

Welding and joining of different metallic materials such as steel, aluminum and titanium represents an increasing challenge for welding specialists, as intermetallic brittle phases form in the molten bath and grow to a connected chain during solidification. The brittleness of these phases leads to cracking and reduces the quality of the weld. One solution to avoid the negative effects of extended intermetallic phases is to directly and actively modify the motion behavior of the melt pool, in order to have intermetallics be broken up by the strong convection and attain a homogeneous and fine distribution in the solidified pool. Recent developments in laser technology allow for time dependent modulation of the laser power, so that Marangoni convection can be influenced as well. The basic effects of pulse shaping are presented in this paper for different material combinations.

Enhanced cladding quality through application of high frequency magnetic fields

The laser cladding process allows the generation of high quality coatings. Otherwise the circular cross section of the single tracks, which is mainly defined by surface tension and gravity, shows potential for optimization. For cladding of large areas a flat and broad geometry is favoured, slim walls allow enhancing the generation of 3-dimensional structures. The paper reports about a novel technology, which uses the skin effect due to the application of high frequency magnetic fields and the resulting surface pressure to adapt the track geometry towards the requirement of the application.

Improving laser cladding process conditions by inducing skin effect through high frequency magnetic field

Laser beam cladding represents a technology, which allows producing high quality coatings, compared with other thermal coating processes. For example, a good metallic bonding between coating and base material, a fine microstructure and good mechanical properties due to the rapid cooling are only some of the numerous advantages. Moreover, the very low dilution allows obtaining the desired metallurgic properties in a one-layer cladding, for example. Nevertheless the spreading of laser cladding in industrial production is very limited, as on the one side the process efficiency is very low and on the other hand the investment costs are very high compared to the actual benefits.The flexibility and the effectiveness of laser cladding can be largely improved through free forming (shaping) of the coating. The geometric shape of the coating seam is mainly defined by gravity and the surface tension of the melt. An additional force, as for example Lorentź force, can optimize the geometry in order to improve the proc...

Set of a high power diode laser in order to enhance wettability conditions and process speed in a controlled short arc brazing process of zinc coated steels

In the last years an increasing amount in strategies to join zinc coated steels can be observed. Brazing represents a profitable way in order to join zinc coated steels at low temperatures, reducing or avoiding zinc evaporation. Low temperature melting brazes materials as CuSi3 or CuAl8 are nowadays wide spread in automotive industry as well as in low cost sheet metal application. Nevertheless the working temperature of these materials is higher than the boiling temperature of zinc, so that a damaging occurs in the near of the joint region as well as pore formation in the braze bead. The advantages of these braze sorts are high wettability as well as high mechanical properties.In order to avoid completely zinc coating damaging the set of ZnAl-fillers was already investigated for laser brazing of zinc coated steels as well as for joining steel with aluminum. Further more thanks to new developments in arc wire technology ZnAl-electrodes can be used for MIG-brazing as well. In this case the conventional short arc, which normally doesn’t allow brazing with zinc wires, as eruption like evaporation occurs, is modified and controlled. The drop formation on the wire tip, the metal transfer and the heat input can be controlled and modified very precisely, so that processing of filler material can be performed easily for thin sheets (till 0,8-1,5 mm) and at low processing speed, below 0,3-0,4 m/min. The control of the heat input occurs due to reducing abruptly the current after short arc.The disadvantages connected with a lower heat input in the base material are a restricted wetting behavior of the zinc pool on the base material and the low brazing speed due to a very high cooling rate. The focus of the presented investigation is the development of a laser based strategy in order to enhance MIG-brazing conditions. A high power diode laser is positioned with a certain offset in front of the GMA-torch, in order to preheat the brazing site, so that wetting can occur easily and velocity increased. A “coldArc” device of EWM (Germany), which allows a controlled short arc and the control of the heat input, was used. As laser source, a fiber coupled high power diode laser with a maximum output of 3 kW was part of the setup. The strategy results in a better wettability and in an increase to 0,7 m/min (approx. 900 W laser power) due to the preheating effect of the sheets by a very low laser power. The influence of the laser power on the wetting is described for a model joint. Further investigations report the advantaged and the process conditions for joining zinc coated steels. The effect of the strategy on the joining of steel and aluminum represents a further topic of the investigation, which will be demonstrated with basic investigation.In the last years an increasing amount in strategies to join zinc coated steels can be observed. Brazing represents a profitable way in order to join zinc coated steels at low temperatures, reducing or avoiding zinc evaporation. Low temperature melting brazes materials as CuSi3 or CuAl8 are nowadays wide spread in automotive industry as well as in low cost sheet metal application. Nevertheless the working temperature of these materials is higher than the boiling temperature of zinc, so that a damaging occurs in the near of the joint region as well as pore formation in the braze bead. The advantages of these braze sorts are high wettability as well as high mechanical properties.In order to avoid completely zinc coating damaging the set of ZnAl-fillers was already investigated for laser brazing of zinc coated steels as well as for joining steel with aluminum. Further more thanks to new developments in arc wire technology ZnAl-electrodes can be used for MIG-brazing as well. In this case the conventional shor...

Flux free joining of zinc coated steel as well as steel/aluminum through laser and controlled short arc technology

The developments in the nineties regarding light weight construction in the automotive industry lead to an increasing set of so called Multi-Material-Mix. The light weight properties of aluminum, magnesium and titanium advantaged the use of these material classes together with new developed high strength steels. Due to the very different metallurgical properties of these materials new joining strategies are necessary. In the case of steel components the corrosion resistance is enhanced through zinc coating, which should remain undamaged during processing. The low evaporation point of zinc as well as the low temperature difference between melting and boiling point requires new rules when joining, as zinc evaporation can occur. In the case of welding of dissimilar materials as steel/aluminum joints intermetallic phases in the melt pool are formed, so that stress cracking can be detected already during processing.In the last years several methods in order to join steels and dissimilar metallic materials were...

Use of electromagnetic induced forces for active forming of coating geometry

Laser beam cladding represents a technology, which allows high quality coating properties towards other thermal coating processes as for example a good metallic bonding between coating and base material, a fine microstructure and good mechanical properties due to the rapid cooling are reached. Moreover the very small dilution allows to ensure the desired metallurgic properties in for example a one layer cladding. Nevertheless the spreading of laser cladding in industrial production is very limited, as on the one side the process efficiency is very low and on the other hand the investment costs are very high compared to the actual benefits.During cladding a coating material, which can be feed in form of powder or filler metal, is molten together with a restricted part of base material and the melt bath spreads on the surface of the component. If its possible during the short interval before solidification to actively form the melt bath, for example widening, flexibility as well as acceptance of laser cladd...

Direct laser pyrolysis of nanostructured micro components

The following paper discusses the production of free form nanostructure components for micro system engineering or rapid prototyping applications. In contrast to previous procedures, e.g. laser sintering of ceramics powders, the ceramic components will be directly produced by laser generation from a liquid precursor. The component will be built up layer by layer, thus shrinkage and crack formation due to entrapped gas or internal stress can be avoided. During the conversion from polymer precursor to ceramic the two processes polymerisation and pyrolysis take place. Furthermore, the energy for this step by step conversion will supplied by a pulsed Nd:YAG laser. To achieve a reaction within a defined small area around the focus point, a decrease of energy density on the path of the laser beam through the liquid precursor is necessary. One way to obtain this is to split the laser beam in two or more beams. On the other hand, material and processing development is important to control the absorption properties of the precursor and the assembling of the produced ceramic.The following paper discusses the production of free form nanostructure components for micro system engineering or rapid prototyping applications. In contrast to previous procedures, e.g. laser sintering of ceramics powders, the ceramic components will be directly produced by laser generation from a liquid precursor. The component will be built up layer by layer, thus shrinkage and crack formation due to entrapped gas or internal stress can be avoided. During the conversion from polymer precursor to ceramic the two processes polymerisation and pyrolysis take place. Furthermore, the energy for this step by step conversion will supplied by a pulsed Nd:YAG laser. To achieve a reaction within a defined small area around the focus point, a decrease of energy density on the path of the laser beam through the liquid precursor is necessary. One way to obtain this is to split the laser beam in two or more beams. On the other hand, material and processing development is important to control the absorption propertie...