Olivera Popović

The effect of welding process and shielding atmosphere on the AlMg4.5Mn weld metal properties

Abstract In this paper, the AlMg4.5Mn alloy has been welded using tungsten inert gas (TIG) and metal inert gas (MIG) processes with different gas shielding atmospheres. Tensile strength, hardness, impact and fracture toughness, fatigue crack growth parameters (ΔKth, da/dN), as well as microstructure were determined. By comparing results for different gas mixtures, the main conclusion for TIG welding was that increased helium content has an important effect on toughness and fatigue crack growth parameters, whereas its effect on other mechanical properties is not significant. On the other hand, in the case of MIG welding, adding helium does not affect mechanical properties, except the fatigue crack growth rate. It was also established that adding nitrogen (TIG welding) and oxygen (MIG welding) reduces toughness and increases crack growth rate, so their use in shielding mixtures is not recommended.

Surface Welding as a Way of Railway Maintenance

Since its early days the development of railway systems has been an important driving force for technological progress. From the 1840s onward a dense railroad network was spread all over the world. Within a few decades railway became the predominant traffic system carrying a steadily increasing volume of goods and number of passengers. This rapid development was accompanied by substantial developments in many areas such as steel production, engine construction, civil engineering, communication, etc (Zerbst et al., 2005). The railway industry worldwide is introducing heavier axle loads, higher vehicle speeds, and larger traffic volumes for economic transportation of goods and passengers. Increasing demands for high-speed services and higher axle loads at the turn of the 21st century account for quite new challenges with respect of material and technology as well as safety issues. The main factors controlling rail degradation are wear and fatigue, which cause rails to become unfit for service due to unacceptable rail profiles, cracking, spalling and rail breaks. Degradation of rail is microstructure and macrostructure sensitive and there is a complicated interaction between wear mechanisms, wear rates, fatigue crack initiation and growth rates, which affect rail life (Eden et al.,2005; Kapoor et al.,2002). Defects such as squats and wheelburns occur even in the most modern and well maintained railway networks and, as a broad general rule, every network develops one such defect each year, every two kilometers. At least one European railway network suffers almost 4000 rail fractures every year. Although such fractures are rarely dangerous when actively managed, they entail a high replacement cost and can be disruptive to the network (Bhadeshia,2002). The replacement of such defects with a short rail section is expensive and not always desirable as it introduces two new discontinuities in the track in the form of two aluminothermic weld that destroy the advantages obtained with long hot-rolled rail.

Uticaj zaštitne atmosfere i brzine dovođenja žice na stabilnost mig/mag postupka zavarivanja

Rezime: Stabilnost procesa zavarivanja ima značajan uticaj na postizanje propisanog nivoa kvaliteta zavarenog spoja. Povećanje stabilnosti utiče na smanjenje mogućnosti pojave nesavršenosti, odnosno grešaka u samom spoju. Stabilnost procesa zavarivanja se ogleda u stabilnosti električnog luka. Promene dužine luka tokom zavarivanja utiču na varijaciju parametara zavarivanja, tačnije na jačinu struje i napon. Na osnovu praćenja parametara zavarivanja u realnom vremenu može se izvršiti statistička analiza dobijenih rezultata. U radu je prikazan uticaj brzine dovođenja žice i vrste zaštitne atmosfere na stabilnost MIG/MAG postupka zavarivanja pri upotrebi metalne punjene elektrodne žice.

Microstructure in the joining zone during the friction welding of the two dissimilar steels

Purpose The purpose of this paper is to point out the possibilities for friction welding of dissimilar steels which are used in various industries. In addition, friction welding is a welding method that is applied for executing the very responsible joints. This research is focused on friction and tribological processes in the friction plane of the two pieces during the welding. Design/methodology/approach The present study research has been conducted based on the experimental testing of cylindrical specimens and results are analyzed. Findings The austenite grain size is affected by several factors through the friction process phase and the compacting phase during the welding. The very fine grain is the consequence of the high degree of the plastic deformation of the near-the-contact layers even in the friction phase. The viscous layer, which is formed during the stable friction phase, is the area where the moving of matter occurs according to a very complex mechanism. Originality/value The paper contains useful results from the area of conventional friction welding of dissimilar steels and it can be very useful to researchers and engineers who deal with similar problems.

Application of the High Strength Steel HARDOX 450 for Manufacturing of Assemblies in the Military Industry

The problems related to welding of the high strength steel, aimed for manufacturing of welded structures operating in the prominent wear conditions, is considered in this paper. The paper presents an analysis of possibilities and prescribing the technology for welding the high strength steel HARDOX 450. The methodology for estimate the weldability of this steel was established in the theoretical part of the paper, as well as calculations of the welding parameters, while the hardness was investigated in details and the macro and micro structures of the individual welded joint zones were estimated in the experimental part. Obtained experimental results could be usefully applied for selecting and establishing the optimal welding technology for structures made of this steel. The technology thus established and the GMAW welding regime can enable successful execution of the welded joints and reduce the possibility for appearance of flaws and cracks to a minimum.

Methods for calculating the preheat temperature welding the high strength steels

With preheating of steel the cooling speed of a heat affected zone decreases, which prevents appearance of structure prone to cold cracks. On the other hand, too small cooling speed, in this area, results with growth the metal grains which, also, leads as to the higher prone to cracks. Common used method for calculation of preheating temperature for high strength steels ar Itto - Bessyo method, and method by standard SRPS EN 1011-2. This paper presents the methods for calculating the preheating temperature of micro aloyed steel P460 NL1. Analysis of the results obtained from both methods shows significant differences in the amounts calculated preheat temperature. It was concluded that both methods have limitations that must be taken into account in their application in practice.

Influence of gas shielding atmosphere on weldability of AIMg4.5Mn alloy

In this paper is shown the influence of gas shielding atmosphere on the weldability of AlMg4.5Mn alloy. Prepared plates were welded using tungsten inert gas (TIG) with four different shielding atmospheres. Metallographic testing, tensile testing and hardness testing of welded joints were carried out. It has been shown that composition of shielding atmosphere has effect on the weld appearance, depth of penetration, overflowing of filler material, porosity in the weld metal, as well as on values of tensile strength and hardness. Weldments obtained by welding in a shielding atmosphere Ar + 50% He + 0.015% N2 have the best appearance of the weld metal, good overflowing of filler material and satisfactory width to hight ratio. It was also established that these welds have the lowest weld metal porosity.

Massive Open Online Courses (MOOC) and Its Possibilities as Instrument of Formal, Nonformal, Informal and Lifelong Learning

In this chapter, we present the definition, development, and providers of massive open online courses (MOOC). We also explain the advantages and disadvantages of MOOC. We also present the structure of participants of MOOC, and also the motivation of participants is analyzed. Some basic statistics of realization and success of participants are also displayed. In one part of this chapter, the authors present case study of MOOC implementation in formal education in Belgrade Polytechnic (BP) College. For informal learning, a MOOC for welding is presented. This MOOC is planned to be organized on faculty of mechanical engineering at Belgrade University. During MOOC, participants also expanded their knowledge base and in that way they created possibilities for rerun of MOOC on a higher level. In that way, a participant can attend the same course after a couple of years and then will upgrade his knowledge. This attribute makes MOOC an instrument for lifelong learning.