Marcell Gáspár

Efficient Increase of the Productivity of GMA Welding of AHSS Using Flux Cored Wire

In order to guarantee the outstanding mechanical properties of high strength steel joints during the welding, preheating, strictly limited linear heat input and interpass temperature should be applied. These parameters together define a narrow optimal t8.5/5 cooling range, and result low productivity. Since significant rise cannot be achieved in the productivity by increasing the heat input due to the metallographycal limitations, steel producers cooperating with filler metal producers have developed flux cored wires for the improving of productivity. These tubular wires can be suitable for raising the melting rate; however, their application may have risks regarding the weld quality (slag inclusions). In this article, welding experiments for AHSS (Advanced High Strength Steel) will be demonstrated for the comparison of two flux cored wires (basic and metal powder) to a traditional solid wire.

Resistance Spot Welding of 7075 Aluminium Alloy

New generations of high strength aluminium alloys are widely used in automotive industry. 7075 (AlZn5.5MgCu) is an aluminium alloy with zinc as the primary alloying element (according to EN 485-2 standard). The 7075 has outstanding strength properties, but the elongation is limited between 5 and 11%. This means an obstacle when this alloy is aimed to use in car body elements. The goal of the present research work is to analyse the weldability of this alloy through resistance spot welding (RSW) tests. Experiments are performed on 1 mm thick 7075 sheets in T6 condition with different surface pre-treatment methods and the weld lobe was determined for constant electrode force. A Tecna 8007 RSW equipment with a TE 550 control unit was applied, which has been recently developed with a tool for the application of post pressing force. The spot welded joints were examined by macro testing, tensile shear and hardness tests. The properties of RSW joint from 7075 were compared with the 6082 alloy.

The Effect of Solution Annealing and Ageing During the RSW of 6082 Aluminium Alloy

In the automotive industry there is a growing tendency for the application of high strength aluminium alloys. In spite of their significant role in weight reduction there are still obstacles for their wider use due to their limited formability and weldability. Hot forming and in-die quenching (HFQ) process was recently developed for the forming of car body sheets. During the HFQ technology the sheet metal forming should be performed in a solution annealed condition. In the solution annealed condition the aluminium alloys have lower strength and better formability properties. The forming process is followed by a precipitation hardening which is generally connected with the painting of body parts (bake hardening). Besides the formability the implementation of HFQ has an effect on the weldability properties, too. HFQ must have an effect on the resistance spot welding (RSW) of aluminium sheets since the weld nuggets are produced after the HFQ, in the assembly part of the production chain, when the aluminium alloy is in a solution annealed and formed condition. The final properties of the welded joints are determined by the precipitation hardening which is the final step of the whole production process. The present research work aims to investigate the effect of the HFQ process on the weldability of AA6082-T6 aluminium alloy. The properties of the RSW joints are examined in different conditions (T6 delivery condition, solution annealed, precipitation aged). The materials tests include conventional macro testing, hardness tests and tensile-shear tests extended with EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscatter Diffraction) tests in order to characterize the distribution of alloying elements and to analyze the grain structure.

High Cycle Fatigue Investigations on High Strength Steels and Their GMA Welded Joints

High cycle fatigue tests were performed on two strength categories of high strength steels, on quenched and tempered (S690 and S960) and thermomechanical (S960) types, on base materials and their gas metal arc welded joints, and on different matching conditions. The planning and optimization of welding technologies based on investigations under cyclic loading conditions were built upon a large number of investigations and statistical evaluation of the test results. Statistical approach was already applied during the preparation of the investigations, which have been allowed the expansion of the scope of the results and the increasing of their reliability. The article demonstrates and evaluates the results comparing with each other and with literary data.

Toughness Examination of Physically Simulated S960QL HAZ by a Special Drilled Specimen

Based on the welding heat cycle models physical simulators are capable for the creation of critical heat-affected zones (HAZ). The simulated HAZ areas can be examined by various material testing methods (e. g. Charpy V-notch impact test) due to their increased homogeneous volume compared to their extension in real welding experiments. In our research work relevant technological variants (t 8.5/5 = 2.5…30 s) for gas metal arc welding technology were applied during the HAZ simulation of S960QL steel (EN 10025-6), and the effect of cooling time on the coarse-grained HAZ was analysed. In thermo-mechanical simulators the achievable cooling rate is always the function of specimen geometry and the presence of external cooling. Therefore a special drilled specimen with external cooling was developed for performing a shorter (t 8.5/5 = 2.5 s) cooling than 5 s, which cannot be realized on the conventional Gleeble specimen. Heat cycles were determined according to the Rykalin 3D model. The properties of the selected coarse-grained (CGHAZ) zone were investigated by scanning electron microscope, hardness test and Charpy V-notch impact test.

Role of the Physical Simulation for the Estimation of the Weldability of High Strength Steels and Aluminum Alloys

The physical simulation is an ultimate innovative way to develop the welding processes. The paper introduces the connection between weldability and physical simulation, hot-cracking sensibility, the Gleeble 3500 thermo-mechanical physical simulator, respectively. Four kinds of materials were investigated and different kinds of physical simulation test methods were made such as, identification of the Nil-Strength Temperature (NST), hot tensile tests (on heating and on cooling parts of the welding simulation curve are also investigated). Furthermore, Heat Affected Zone (HAZ) tests are being introduced. The future approaches of the research are also exposed.

Fatigue Crack Propagation Limit Curves for High Strength Steels and their Application for Engineering Critical Assessment Calculations

There are different prescriptions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth. The research work aimed (i) to determine fatigue crack propagation limit curves for high strength steels and their welded joints, based on the Paris-Erdogan law; (ii) to use the determined limit curves for engineering critical assessment (ECA) calculations. Experiments were performed on different high strength steels and their welded joints; and the propagating cracks in the specimens represent the different possible locations of the real cracks in the structural elements. Fatigue crack growth tests were executed by ΔK-decreasing and constant load amplitude methods. The evaluation process consists of six steps, and by means of the selected values a statistical method can be proposed for determination of the limit curves. Engineering critical assessment calculations were performed on a welded structural element having crack like defects.