DVS_Bericht_368_LP

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Process monitoring in electron beam welding by optimizing the in-process sensor technology by means of backscattered electrons and optical emission S. Gach, Aachen, S. Olschok, U. Reisgen, Aachen The demands of the market on manufacturing companies and their products require a time and cost efficient production. There is an ever-increasing need to make welding production more transparent and to ensure and document processes and quality in order to be able to meet the requirements of customers as well as laws and regulations. The extension of sensor technology in electron beam welding can contribute to the achievement of these objectives - by integrating the detection of discrete wavelength ranges of optical process emissions, optimizing the detection of backscattered electrons and correlating the measurement signals to process changes and events, irregularities can be detected in-situ. The presented investigations include the further development of the detection and analysis of backscattered electrons and the utilization of optical sensors for the electron beam process. Weldings are performed with the simulation of typical failure scenarios. The data are evaluated with the aim to find temporal and causal correlations to the induced defects. It is investigated whether the correlation to welding defects of the combination of optical and conventional sensor technology is also possible with the optimized backscatter electron sensor technology alone. In the field of optical sensors the identification of characteristic optical process emissions by spectroscopy and a selection of suitable photodiodes and band-pass filters based on these results is investigated. 1 Introduction Process monitoring in electron beam welding is usually limited to recording machine data such as acceleration voltage, beam current or lens currents and, if necessary, the correction values of the beam device. These enable process disturbances resulting from beam generation or beam guidance to be detected, but do not provide any information about a process disturbance from the actual impact zone of the beam on the component. Sporadically, the detection of the sample or component current and the measurement of the backside passing electron current at full penetration welds are carried out with the capillary open downwards. While the sample current is primarily relevant for the determination of the efficiency, the penetrating current allows at least a detection of a continuously opened capillary. A reliable assignment of signals to welding defects does not exist at present. 2 State of the art When an electron beam interacts with a material, the kinetic energy of the electrons is converted into thermal energy. The majority of the energy is transferred to the conduction electrons in the lattice structure, which results in a higher lattice vibration, which in turn is equivalent to a higher temperature. Although efficiencies >90% are achieved, especially in deep welding, part of the energy is converted into secondary effects: As a result of the energy transfer to the conduction electrons, optically visible radiation and thermal radiation are emitted from the material. In addition, X-ray radiation is released, which is emitted directly by the beam electrons during their deceleration, called X-ray braking radiation, as well as the emission of the characteristic X-ray radiation, which is emitted by the hit material atoms. [1] [2] Not all beam electrons participate equally in the energy conversion, some are diffracted in the Coulomb fields and leave the material with only a small loss of energy. These are called backscattered electrons. During energy transfer, individual electrons of the lattice compound of the material may receive sufficient energy that they exceed the exit energy to leave the material compound and leave the material as free electrons. These are called secondary electrons and can be differentiated from the backscattered electrons by their significantly lower velocity, respectively energy. In addition to the effects already described, vapourisation of the material occurs, especially in deep welding. The metal vapour, similar to a PVD process, spreads out in a hemispherical shape over the vapour capillary in a vacuum and coats everything that is exposed to it. The vapour contains positively charged metal vapour ions. [3][4] The described side effects all originate from the effective range of the electron beam and can thus be regarded as information carriers of the welding process. Detected by means of suitable detectors and correlated with associated process disturbances, they could form a basis for process monitoring systems in the future. 6 DVS 368
3 Experimental procedure and analysis Backscattered electrons are detected by means of a detector unit consisting of eight individual detector plates, which are mounted in a ring around the beam path on the chamber ceiling, Figure 1 left. The individual detector plates are made of graphite and can be aligned orthogonally to the beam spot by means of a cam disk. Electrons hitting the etector late are ie to ron via a earin reitor nt e voltae ro at te nt t allows the detection of the incident electrons, Figure 1 (middle). The analogue converter from National Instruments (Ni9205) provides a cumulative sampling rate of 250kHz. Figure 1. Backscatter detector in installed condition (left), circuit of the measurement setup shunt and detector gap (middle), Row data signal record (inverted) of a backscattered electron measurement (right) The optical process emission is recorded using the Ocean HDX-XR spectrometer with an integration time of 6ms in a etectale avelent rane eteen n an n otical re i ie tro a vac feetro into te orin caer an aline to te roce colliatin len enre cient aralleli e len i rotecte fro vaoriation y te roce ae y a rotective la an a coaial rotective a o eli 40mnl/min). Comparative measurements using the DIABEAM beam measurement system [5] show no adverse cane in ea iaeter e to te vole o te caer rere ettle at a tale eiliri -3 mbar. 4 Results 4.1 Backscattered Electrons Several challenges exist during the acquisition of backscattered and secondary electrons. Firstly, the raw data of the accattere electron inal o noiy eareent recor ere are neative eection recore a ol be expected from the detection of backscattered and secondary electrons, but also positive signals are occur, Figure 1 (right). The reason for latter is that positively charged ions hit the detector plates, being ejected by the plasma jet from the vapour capillary. If positively charged ions meet the detector plate at the same time as negative electron tey erae eac oter inal n tatitical averae ti lea to a noie of te inal eenin of te amount of negative and positive particles. Secondly, metal vapour that emits from the capillary and spreads throughout the vacuum chamber accumulates on te etector late it increain layer ticne a in a yicalvaoreoition roce i falie the measurement due to a changed degree of absorption of the detector material. The measurement results stabilise if a homogeneous vapour deposition layer occurred on the detector surfaces. Ventilation of the vacuum chamber leads to a change in the absorption conditions as well due to oxidation. Successful, reliable measurements can only be achieved by deliberately coating the detectors directly before welding, using material of the same kind as being welded. eenin on te ele alloy no tainale rface layer i fore n oe alloy layer eel o fro te etector surfaces. Depending on the alloy, this makes reliable measurement impossible. Two possibilities are currently being evaluated to improve the noise ratio - on the one hand, increasing the sampling rate of the analogue converter, which probably reduce the noise through temporal equalisation. For this purpose, the electron beam welding system will be equipped with a measuring computer with fast analogue inputs, which will allo te etector late to e canne at to er cannel a clearer teoral ierentiation eteen positive vapour ions and electrons will be possible. On the other hand, protecting the detector plates via a coaxial inert a vole o i a econ aroac i tecnie i alreay cceflly in e in te el of laer ea welding under vacuum (LaVa) to protect the laser optics [6]. It will possibly minimise vaporisation of the detector late ic avoi te neative eect of layerin an aitionally it i reventin te eavy an lo vaor ion fro reacin te etector late ile te lit an fat electron ol e only litly aecte ll te analye DVS 368 7
Page 1 and 2: 2021 DVS-BERICHTE International Ele
Page 3 and 4: IEBW - International Electron Beam
Page 5 and 6: Welcome Are you interested in effic
Page 7: Applications U. Reisgen, S. Olschok
Page 10 and 11: Figure 1. General basic content for
Page 12 and 13: lto te leon ncle concern elin in v
Page 16 and 17: presented below were carried out on
Page 18: Figure 4. Optical emission of an el

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