Nenad Radović

Influence of FSW parameters on formation quality and mechanical properties of Al 2024-T351 butt welded joints

Abstract The influence of R/v ratio on joint quality in 2024-T351 aluminum alloy was studied. Specimens were subjected to friction stir welding with the rotation rates of 750, 950 and 1180 r/min and welding speed between 73 and 190 mm/min, providing R/v ratio between 5.00 and 10.27. The welded joints were tested by means of both non-destructive (visual, penetrant and X-ray inspection) and destructive (metallographic, tension and hardness) testing. In all specimens typical zones are revealed, with corresponding differences in grain size. Tensile efficiency of the joints obtained is in the range of 52.2% to 82.3%. The results show that the best quality is obtained at R/v ratio of 8.06, 10.17 and 10.27. This behavior is attributed to the assumption that the material flows around the pin with an optimal speed, i.e. sufficient amount of material is available to fill the gap and prevent tunnel formation. R/v ratio also showed influence on hardness distribution, onion features and crack initiation/propagation zones.

Influence of Cr, Mn and Mo Addition on Structure and Properties of V Microalloyed Medium Carbon Steels

The effect of austenitizing temperature and Cr, Mo and Mn addition on microstructure and mechanical properties of V microalloyed medium carbon steel has been studied by means of metallography and mechanical testing. The addition of Cr, Mn and Mo leads to a decrease in yield strength (YS) by approximate 100 MPa in comparison to the base steel. It is assumed that Mn and Mo increase hardenability by promoting the formation of bainitic sheaves (BS), i.e. by suppressing the formation of ferrite-pearlite and acicular ferrite (FP-AF). Cr at the level used in this work is not that effective. Presence and packet size of bainitic sheaves decrease the Charpy V-notch impact energy at 20 °C (CVN20) in comparison to ferrite-pearlite and acicular ferrite microstructures.

SHORT-CIRCUIT OXYGEN DIFFUSION IN THERMALLY GROWN SILICA LAYER

Amorphous polymer-derived Si-C-N ceramics can be doped with different elements (Al, B etc.) through various pre-ceramic polymer routes. Thus, controlling of the high temperature oxidation resistance can be achieved on an atomic level. An important factor for silica layer growth is oxygen diffusion in protective thermally grown layers. In order to get insight of the oxygen diffusion mechanism, analysis should include both, bulk and short-circuit diffusion. XRD measurements of oxidized Si-C-N and SiC revealed the possibility that oxide layers were fully crystallized and are composed of nano-sized cristobalite-like grains. Secondary ion mass spectrometry depth profile analysis after 18O2-16O2 isotope exchange experiments on oxidized SiC indicated that short-circuit diffusion is probably grain boundary diffusion of molecular oxygen.

Influence of the initial length of the Cu bars on the structure and micro-hardness of friction-welded and open-die forged Al/Cu bimetallic bars

When the testing of large quantities of bimetallic joints of aluminium (Al) and copper (Cu) is considered, consumption of Cu, as a raw material, is of significant importance. For tensile testing of Al/Cu joints, it is recommended to produce joints by using a longer initial length of Cu bars, whereas for the examination of micro-structure or other properties of these joints, Cu bars of shorter lengths can be used. In order to introduce the lower consumption of Cu for the testing of larger quantities of Al/Cu joints, it is necessary to determine whether the usage of different initial lengths of Cu bars produces the same micro-structural effects in joints obtained by using the same processing parameters or not. In this study, micro-structures and micro-hardness profiles of Al/Cu bimetallic joints produced by using continuous drive friction welding and noticeably different initial length of Cu bars will be analyzed. Also, mentioned properties of these joints in the open-die forged condition will be examined.

Cleavage Fracture in Continuously Cooled V-Microalloyed Medium Carbon Steel

Medium carbon V-microalloyed steel continuously cooled from the austenitization temperature at still air, with predominantly acicular ferrite structure, has been investigated by means of four-point bending of notched Griffiths–Owens’s type specimens at liquid nitrogen temperature. Local fracture stress and plastic strain were determined by using finite element analysis and fracture surface examination using scanning electron microscope. It was revealed that cleavage fracture initiation, which takes place close to the notch root in the narrow zone of high plastic strains, is not related to any broken coarse second phase particles. It was assumed that microcracks nucleate by strain induced fracture of pearlite nodules. Two effective surface energy values of 24 and 42 J/m2 were estimated according to the Griffith’s equation, indicating the influence of crystallographic orientation between neighboring grains at the origin of fracture. Lower value was attributed to fracture of coarse ferrite–pearlite units with similar crystallographic orientation and higher value to fracture propagation through fine acicular ferrite matrix.

Influence of V and N on Transformation Behavior and Mechanical Properties of Medium Carbon Forging Steels

The influence of vanadium and nitrogen on microstructure and mechanical properties of medium-carbon steels has been studied by means of metallography and mechanical testing. Vanadium addition to the low nitrogen steel suppresses the formation of ferrite-pearlite following the low reheating temperatures and microstructure consists of bainitic sheaves. Increasing nitrogen at the same vanadium level promotes the acicular ferrite formation. For high reheating temperatures, dominantly acicular ferrite structure in both the low nitrogen and the high nitrogen vanadium steels is obtained. The results suggest that vanadium in solid solution promotes the formation of bainite, whereas the effect of nitrogen is related to the precipitation of VN particles in austenite with high potency for intragranular nucleation of acicular ferrite and to the precipitation of V(C,N) particles in ferrite with high potency for precipitation strengthening. Addition of both vanadium and nitrogen considerably increases the strength level, while CVN20 impact energy increases on changing the microstructure from bainitic ferrite to the fine ferrite-pearlite and acicular ferrite.

Mechanisms of Fracture in Medium Carbon Vanadium Microalloyed Steels

The relationship between microstructural parameters and cleavage fracture has been studied in (950–1300°C) air cooled vanadium microalloyed steels by means of impact testing, light microscopy and scanning electron microscopy. Large variations in impact energy are obtained as a function of test temperature and microstructure. The results show that accicular ferrite (AF) and a multiphase structure consisting of ferrite-perlite (FP) and 30–70% AF posses room temperature toughness superior to that of classical bainitic sheaves (BS) as revealed by impact energy level. However, AF is superior to FP-AF in term of energy transition temperature.