Helmut Wohlfahrt

Residual Stresses in Welded Joints – Sources and Consequences

Residual stresses in welded joints are a consequence of hindered shrinkage of the heated zones and in materials like steels where phase transformations occur of the combined effect of hindered volume expansion. With these two elementary processes any residual stress distribution can be described conclusively if the cooling conditions and the material properties like the transformation behaviour are known. The grade of restraint of the weld seam is the most important boundary condition which is responsible for the opportunity that high residual stresses may occur. The grade of restraint is given by the ratio between the heated zones and the apparent colder material and is not primarily depending on the size of a component. Every way to reduce the grade of restraint offers principally the possibility of an increasing distortion. Many experiments show, that a certain control of the welding parameters which should be carefully adjusted on the properties of each material enables a combination of favourable strength and toughness properties with acceptable residual stresses.

Electron beam activated brazing of cubic boron nitride to tungsten carbide cutting tools

Abstract New tool materials are needed for efficient and high-grade finishing of difficult-to-work materials such as cladded and sprayed layers, cast iron and hardened steels. Advanced superhard materials based on diamond are in common use. However, cubic boron nitride (CBN), as a material for efficient tools, seems to be better than natural diamond. Obtaining reliable cubic boron nitride–tungsten carbide joints is an essential problem in the production of cutting tools with this superhard material. There are different methods of fixing CBN into a tool body. One of them, electron beam activated brazing, seems to be very promising. An electron beam, used as a heat source in this process, enables the precise control of heat delivered to the tool parts during brazing as well as high cleanliness of the joint due to lack of gas impurities in the vacuum environment. Using an experimental design method based on Taguchi techniques for quality engineering, it was found that the heating time, the electron beam current and the roughness of the CBN surface influence mostly the joint shear strength.

Residual Stress Calculations And Measurements — Review And Assessment Of The Iiw Round Robin Results

A Round Robin Programme was established by a working group of Commission X in 1997 in order to evaluate the possibilities of residual stress and distortion prediction (RSDP) in welded structures and to validate and benchmark prediction codes based on finite element simulation of the welding process. Calculations on residual tresses have been carried out on an austenitic steel plate with well-defined material and geometrical data, welding conditions and with the proposal to use the kinematic hardening model. In addition, a second programme for measurements of residual stresses in this plate has been started in 2003. Measurements have been taken on three austenitic steel plates using the X-ray measuring method as well as neutron diffraction methods and different kinds and variations of the hole drilling method. Although a rough agreement between the measured results and the calculated ones was found in principle, distinct differences have been observed in detail — especially for the longitudinal residual stresses. The calculations indicated nearly constant tensile stresses of the order of the initial yield strength in the weld seam and in the adjacent heat affected zone (HAZ). The measurements however revealed stress maxima in the HAZ with values clearly higher than the initial yield strength. Therefore additional calculations have been carried out with different hardening models in order to investigate their influence on the residual stress results. In fact, calculations performed with the isotropic hardening model could approve these stress maxima quite exactly. In further detailed investigations, it could be shown that the reason for the stress maxima is the strain hardening in the HAZ due to thermal stresses during the welding process. Finally basic considerations could explain that the kinematic hardening model, including the Bauschinger effect, cannot account for this hardening effect. Literature indicates that the Bauschinger effect is at least not fully effective at high working temperatures of austenitic steels. Altogether, the calculation programme has shown that a rough estimation of the residual stress state after welding is possible without any special distinction of the hardening model to be used. For a more precise and detailed information, the chosen hardening model is of significant relevance.

Application of the Local Fatigue Strength Concept for the Evaluation of Post Weld Treatments

Many investigations have revealed the beneficial effect of mechanical surface hardening in combination with induced compressive residual stresses, which lead to an improved fatigue strength. The benefit will be the higher, the higher the quality of the weld is. On the other hand methods like shot peening are useless, if the weld toe profile is poor and the connected notch effect strong. Then an increased fatigue strength is only possible after a post weld treatment which produces a smoother shape of the weld toe. Experiments reveal that methods, which are working in order to increase the intensity of the penetration depth, must not necessarily produce the best fatigue performance. High intensities of the mechanical surface treatments, especially in ductile aluminium alloys, may produce new geometrical defects, which may compensate the benefit of the deeper penetration depth of cold hardening and compressive residual stresses. A systematic evaluation of the results of improvement methods is possible with the help of the local fatigue strength concept which compares the local strength properties and the local load stresses.

Application of room-temperature infrared photodetectors in high-speed laser beam diagnostics of industrial CO 2 lasers

The paper presents applications of newly developed room-temperature mercury based photo- detectors (single-element and array type) in the measurement of infrared CO2 laser radiation during material processing. A specially designed measuring device to determine the intensity profile of high-power CO2 lasers in two perpendicular cross-sections with a time- resolution up to 10 kHz for a whole line-scan is presented. The data-acquisition and analyzing equipment based on a digital-signal-processor (DSP) enables us to calculate parameters which characterize the temporal stability of the laser mode on-line. Experimental results illustrate the possibilities of the device. Moreover, the time-resolved measurement of laser beam parameters during material processing serves as a tool to investigate the influence of the laser beam on the dynamic process behavior. Optical signals obtained from the laser beam welding process are correlated with laser beam power variations to determine the grade of interaction.

Dynamic behavior of the laser beam cutting and welding process due to instabilities of industrial high-power CO 2 lasers

The temporal beam power fluctuations of a high-power CO2 laser are analyzed according to their frequency spectrum and its time dependent variation. Simultaneously to the laser beam diagnostic, signals obtained from the light emission during the cutting and welding process have been detected. By comparison of corresponding spectra the interaction between beam oscillations and process dynamics is studied. The influence on the surface structure of the processed material is discussed especially in the case of laser beam cutting. For this the spatial surface morphology is also analyzed by means of local fast Fourier transformation. The overall result is that the spectra are not constant, but show time-dependent behavior. Thus a more detailed correlation analysis is necessary for full description of dynamical beam--process-interactions.

Fast spatial-resolved diagnostics of high-power CO2 laser beams

A laser beam diagnostic device is presented, which allows the determination of the intensity profiles of high-power carbon-dioxide lasers with high time-resolution. The detector of this device consists of two linear arrays of room-temperature HgCdTe-detectors, arranged perpendicularly to each other across the center of the beam. The data of the 70 detector elements is acquired simultaneously at rates up to 15 kS/sec for single shot events and several 100 kS/sec for repetitive laser pulses. Due to a use of a digital signal processor (DSP) and an especially adapted software, the device is capable of analyzing the temporal behavior and the stability of the intensity distribution on-line. The calculation of the local variance and mean values enables the dependence of the lasers short- and long-term stability to be investigated due to changes in the resonator alignment, the stability of the power supply, the gas composition, etc. For the pulse-mode of a laser, its transient behavior, for example changes in the intensity distribution, can be determined with high time-resolution. This is achieved by the method of sequential equivalent time sampling. The calculation of further statistical functions by the DSP makes it possible to estimate the uniformity of the laser pulses on-line as well. Using a partially transmitting mirror in the beam delivery system, measurements can be performed during material processing.

Space- and time-resolved laser beam diagnostics of high-power CO2 lasers during material processing

A measuring device for the intensity profile of high-power CO2 laser beams based on a linear pyroelectric detector array is presented. It enables the user to measure variations of the mode structure up to several kHz with a one dimensional spatial resolution. Periodic as well as stochastic fluctuations of the local intensity are analyzed, leading to fundamental considerations to characterize the temporal stability of laser beams. Simultaneously to the laser beam diagnostic, signals obtained from the cutting and welding process have been detected. By the comparison of these signals with the time-resolved measurement of the intensity profile, the influence of laser mode instabilities on the material processing is documented.

Anwendung des lokalen Dauerfestigkeitskonzepts zur Bewertung der Wirksamkeit von Schweißnahtnachbehandlungsmaßnahmen

Abstract Many investigations have revealed the beneficial effect of mechanical surface hardening in combination with induced compressive residual stresses leading to improved fatigue strength. The benefit will be higher, the higher the quality of the weld is. On the other hand, methods like shot peening are useless if the weld toe profile is poor and the connected notch effect strong. Increased fatigue strength is then only possible after a post weld treatment which produces a smoother shape of the weld toe. Experiments reveal that methods which aim to increase the intensity of the penetration depth do not necessarily produce the best fatigue performance. High intensities of the mechanical surface treatments, especially in ductile aluminium alloys, may produce new geometrical defects which may work against the benefit of the deeper penetration depth of cold hardening and compressive residual stresses. A systematic evaluation of the results of improvement methods is possible with the help of the local fatigue strength concept, which compares the local strength properties and the local load stresses.

Fast spatial-resolved beam diagnostics for material processing by industrial CO2 lasers

Due to the increasing range of high-speed and high-accuracy applications in material processing, especially in laser beam welding and cutting, the temporal stability of the laser beam parameters becomes more and more important. In this paper a laser beam diagnostic device is presented, that allows the determination of the intensity-profiles of high- power CO2 lasers with high time-resolution. The detector of this device consists of two linear arrays of room- temperature HgCdTe-detectors, arranged perpendicularly to each other across the center of the beam. The data of the 70 detector elements is acquired simultaneously at rates up to 15 kS/sec for single shot events and several 100 kS/sec for repetitive laser pulses. Due to the use of a digital signal processor (DSP) and an especially adapted software, it is possible to analyze the fluctuations of the intensity distribution on-line. By help of a partially transmitting mirror in the beam delivery system, measurements can be performed during material processing. Therefore, the interaction of the laser beam source itself with the material processing due to beam reflection as well as influences of the industrial environment to the laser can be detected. The calculation of the local variance and mean values enables the dependence of the lasers short- and long-term stability to be investigated due to changes in the resonator alignment, the stability of the power supply, the gas composition etc., as well as to the influence of the processing. For the pulse-mode of a laser, its transient behavior like changes of the intensity distribution can be determined with high time-resolution. For the improvement of drilling processes, the calculation of further statistical functions by the DSP makes it possible to estimate the uniformity of the laser pulses on-line as well.