Local residual stress build-up during solidification of technical alloys during welding

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Uwe Reisgen

Head of Institute

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+49 241 80 93870

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1. Introduction:

Residual stress buildup and distortion are central problems within the production chain of welded components with a direct influence on the cost and precision of the final product. Despite the numerous research activities on this complex of topics, fundamental questions still exist regarding the development of residual stresses during welding and the resulting distortion. The essential complexes of topics here are:

  • Material behavior in the liquidus-solidus transition,

  • Heat dissipation and temperature fields, and

  • Transformation processes and microstructure development

To improve the understanding of residual stress buildup, the in situ measurement of strains during welding is an important tool, which, however, has only been used occasionally so far.has so far only been used sporadically due to the high experimental effort involved. In addition, the determination of reliable temperature-dependent material parameters is a basic prerequisite for model building.

The aim of the work of SP A2 is the development and testing of measurement methods for the in-situ detection of thermally induced strain processes, as well as the development of models to improve the understanding of the resulting residual stress structure in the technical component.

This work will be supported by the simulation of the thermal field and the melt pool geometry in SP A4. The validation of a thermomechanical model will be based on the work of subproject SP B7 and in cooperation with SP B3. By combining this with optical surface measurement methods (in accordance with SP A1, A7 and A8), a significantly higher resolution can be achieved so that the zones of high temperature and strain gradients immediately adjacent to the melt pool can be better recorded. The measurement of residual residual stresses is performed together with TP A7 and A12. By applying the optical strain measurement technique, a detailed investigation of crack initiation during the mechanical characterization of solder samples can be carried out together with A5. In addition, there is cooperation with SP A1 and A7, as well as with the IFSW of the University of Stuttgart, regarding the performance of in situ experiments with synchrotron radiation for process monitoring.

 

2. Phase 2:

2. 1 Multi wire arc welding for in-situ alloying in additive manufacturing

For material characterization, especially of tailored alloys as used in SP A7, material samples with minimum dimensions are sometimes required, which cannot be taken from the generated welds. Within the framework of SP A2, a methodology was therefore developed to produce tailored material samples by means of additive manufacturing and in-situ alloying, which can be generated in almost any dimensions and thus enable extensive characterization, for example by dilatometry.

At the same time, the effect of the tailored materials in the material composite can be investigated by additively generating corresponding multi-material test specimens.

 
  Additively manufactured multi-material composite specimen with LTT microstructure inserted in rows. Copyright: © SFB 1120 Figure 1 Additively manufactured multi-material composite specimen with LTT microstructure inserted in rows.
 
 

2.2 Test rig for in-situ acquisition of temperature-deformation curves for additive manufacturing

For the time-coupled in-situ recording of the temperature-deformation behavior of substrate plates in additive manufacturing, a corresponding test rig was set up. The spatially resolved deformation measurement is implemented by means of digital image correlation, the temperature measurement is carried out via thermocouples on the one hand and via thermocamera on the other hand. The test setup was successfully tested at LMD and WAAM and provides an important data basis for model testing and validation.

 
  DIC test rig for coupled spatially resolved temperature-deformation measurement. Copyright: © SFB 1120 Figure 2 DIC test rig for coupled spatially resolved temperature-deformation measurement.
 
 

2.3 Extension of the forming dilatometer with digital image correlation for spatially resolved deformation measurement

By using a quench dilatometer procured in the first application phase, the cooling rate-dependent shift of the transformation temperature can also be taken into account. Thermal expansion plays a decisive role in addition to the determination of flow curves. In particular for the transition from the solid-liquid to the solid phase, only a few approaches are known to date as to how a determination of flow curves can be successfully implemented experimentally. In the second phase, preparatory conversion and development measures were carried out for the use of the dilatometer procured for the determination of flow curves, which are to be finalized in the third phase.

Links to subprojects of project area A, which use such methods both diagnostically and in the later course for the determination of data, which enable compensation of precision-controlled influencing factors in the manufacturing processes used.

2.4 In-Situ-Beugungsexperimente

For an understanding of the residual stress buildup and an estimation of the residual stresses remaining in the material, a spatially and temporally precise recording of the deformations during welding is necessary. The determination of elastic deformations inside welded materials has been done only to a very limited extent. The previous experiments with neutron diffraction have shown that a higher local and temporal resolution is necessary due to lower measuring volumes and recording rates. Using energy dispersive X-ray diffraction at the synchrotron, deformations could be recorded in a measurement volume of 0.1x0.1x1.5 mm3 with an exposure time of one second. The measurement setup for this can be seen in Figure 4 The evaluation of the result spectra still takes some time of phase 2 but first results can be seen in Figure 3.

 
 
First results of diffraction tests using synchrotron radiation strain distribution in the centre of the material Copyright: © SFB 1120 Figure 3 First results of diffraction tests using synchrotron radiation strain distribution in the centre of the material
 
 

The distribution of the strain is largely continuous in some cases the analyses still have to be reconstructed but the experiments can already be seen as a success and a great contribution to the understanding of the structure of residual stresses.

 
 

3.1 Problem definition

The knowledge available to date on the development and buildup of residual stresses during welding of metallic materials is based on solid mechanics models in combination with assumptions on solidification and transformation processes. So far, transient physical processes have not been taken into account for macroscopic problems in current models. Powerful observation methods and measurement techniques can now be used to verify and supplement existing models and assumptions, enabling more precise prediction and compensation of resulting residual stress and distortion levels. A technical application of additive manufacturing by arc processes requires a control of the occurring distortions and residual stresses by The quantification of thermal strains continues to be essential for answering open questions on material- and stress-related cracking phenomena, such as hot cracking and hydrogen-induced cold cracking. In addition, any further development of measurement methods for obtaining temperature-dependent material data represents progress towards consistent numerical welding simulation. This is dependent on efficient algorithms and accurate material data in addition to suitable, physically based models.

3.2 Scope of the workplan

The overall objective of the subproject is to understand the buildup of residual stresses immediately after solidification from the melt by means of strain measurements using in situ measurement methods and to improve the understanding by comparison with solid mechanics models. Component stresses and distortions are later determined from the measured values by conversion, so that methods and procedures for compensation can be derived from them. The focus here is on arc welding as a model system. In the project phase applied for here, the focus is on the validation and improvement of models for residual stress buildup in welded components. Here, specialized experiments are to be developed that allow an increase in the spatial resolution of the strain measurement methods developed in the first phase and enable the mapping of the experimental observations in structural simulation software. In addition to general utility for improving the prediction of residual stress development in welded components, the goal is to increase the contour accuracy of additive deposition welded components. Additive manufacturing of metallic components is currently a particular focus of scientific research in the field of production engineering, as it can make a significant contribution towards resource-efficient manufacturing [9]. This is achieved in particular by saving semi-finished products in near-net-shape additive manufacturing compared to machining manufacturing from solid starting materials. The particular advantage of additive manufacturing by arc welding with wire-shaped filler metal is the very high deposition rates, which enable economical production of even large components. At the same time, the need for models to predict the heat field, distortion and residual stress profile is particularly great here due to the complex geometries and the high proportion of weld metal. The following sub-goals can be named for the project duration:

  • Development of improved experiments for strain measurement near the melt pool with high spatial resolution.

  • Validation and improvement of existing solid mechanics models for the calculation of weld residual stresses by transferring the experimental results into a numerical model.

  • Method development for the determination of high temperature material properties

  • Analysis of residual stress buildup in near-net-shape additive arc welded components.

 
 

3.3 Methods

The methodologies used are essentially based on the approaches developed in the second phase. Using the test setups developed for spatially resolved temperature and deformation measurement, the work in the third phase concentrates primarily on generating data on material and component behavior. Based on this, simulation models are optimized, which ultimately form the basis for controlling the manufacturing processes.

 
  Test rig for advanced measurement acquisition and model validation Copyright: © SFB 1120 Figure 4 Test rig for advanced measurement acquisition and model validation
 
 

The procedure can be subdivided into:

Metrological acquisition of material and process data

  • Determination of deformation behavior also in the high-temperature range for technical alloys

  • Cooling rate based determination of material microstructure

  • In situ strain measurement with synchrotron radiation or neutron radiation for the determination of the influence of melt pool geometry

Model-based process control

  • Development/adaptation of demonstrators

  • Residual stress and distortion simulation of demonstrator

  • Welding tests to map simulation results on demonstrator

Feeding back of precision-determining parameters

  • Processing of test results from A02 and other subprojects

  • Modeling by combining material characteristics, measurement results and simulation results

Validation and transfer of model using measurement methods and inverse approach