Lasersko reparaturno zavarivanje alata s različitim oblicima impulsa

Repair welding of cold-work tool steels with conventional methods is very difficult due to cracking during remelting or cladding and is generally performed with preheating. As an alternative, repair welding with laser technology has recently been used. This paper presents the influence of different pulse shapes on welding of such tools with the pulsed Nd:YAG laser. Repair welding tests were carried out on AISI D2 tool steel, quenched and tempered to hardness of 56 HRc, followed by microstructural analysis and investigation of defects with scanning electron microscopy. Test results suggest that it is possible to obtain sound welds without preheating, with the right selection of welding parameters and appropriate pulse shape.Reparaturno zavarivanje alatnih čelika za rad na hladno konvencionalnim metodama je vrlo teško zbog pukotina nastalih tokom pretaljivanja i navarivanja i općenito vrši se uz predgrijavanje. U posljednje vrijeme se kao alternativna metoda koristi reparaturno zavarivanje upotrebom laserske tehnologije. Ovaj rad predstavlja utjecaj različitih oblika impulsa na zavarivanje alatnih čelika s pulsnim laserom Nd:YAG. Reparaturno zavarivanje testirano je na alatnom čeliku AISI D2, termički obrađenim na tvrdoću 56 HRc, poslije čega je provedena analiza mikrostrukture i traženje defekata skeniranjem elektronskim mikroskopom. Rezultati testiranja sugeriraju da je moguće izvesti dobre zavare bez predgrijavanja pravilnim izborom parametara zavarivanja i odgovarajućeg oblika impulsa

Pukotine na kalupima (alatima) za tlačno lijevanje uslijed toplinskog umora

Die-casting dies are exposed to high thermal and mechanical loads. Thermal fatigue cracking of dies due to thermal cycling may importantly shorten the life-time of the die. Cracks degrade the surface quality of dies and consequently the surface of castings. In this study, thermal fatigue cracking of dies was analyzed during the process of die casting aluminium alloys. During the process cracks were observed and measured and their location and size were determined. Thermal and mechanical loads cause high local stresses and consequently surface cracks. First cracks occur as early as after 2000 cycles and propagate progressively with cycles.Kalupi (alati) za lijevanje pod tlakom su izloženi velikim toplinskim i mehaničkim opterećenjima. Pukotine uslijed toplinskog umora su zbog izmjene toplinskih ciklusa važan ograničavajući mehanizam životnoga vijeka kalupa. Pukotine smanjuju (degradiraju) kvalitetu površine kalupa i posljedično površinu odljevka. U ovom radu su analizirane pukotine nastale na kalupu zbog toplinskog umora tokom tlačnog lijevanja aluminija. Tokom procesa su promatranjem i mjerenjem utvrđene lokacije i veličine nastalih pukotina na kalupu. Toplinska i mehanička opterećenja uzrokuju velika lokalna naprezanja i posljedično pukotine na površini kalupa. Prva pukotina pojavljuje se već nakon 2000 ciklusa i progresivno se širi povećanjem broja ciklusa

Lasersko reparaturno zavarivanje alata s različitim oblicima impulsa

Repair welding of cold-work tool steels with conventional methods is very difficult due to cracking during remelting or cladding and is generally performed with preheating. As an alternative, repair welding with laser technology has recently been used. This paper presents the influence of different pulse shapes on welding of such tools with the pulsed Nd:YAG laser. Repair welding tests were carried out on AISI D2 tool steel, quenched and tempered to hardness of 56 HRc, followed by microstructural analysis and investigation of defects with scanning electron microscopy. Test results suggest that it is possible to obtain sound welds without preheating, with the right selection of welding parameters and appropriate pulse shape.Reparaturno zavarivanje alatnih čelika za rad na hladno konvencionalnim metodama je vrlo teško zbog pukotina nastalih tokom pretaljivanja i navarivanja i općenito vrši se uz predgrijavanje. U posljednje vrijeme se kao alternativna metoda koristi reparaturno zavarivanje upotrebom laserske tehnologije. Ovaj rad predstavlja utjecaj različitih oblika impulsa na zavarivanje alatnih čelika s pulsnim laserom Nd:YAG. Reparaturno zavarivanje testirano je na alatnom čeliku AISI D2, termički obrađenim na tvrdoću 56 HRc, poslije čega je provedena analiza mikrostrukture i traženje defekata skeniranjem elektronskim mikroskopom. Rezultati testiranja sugeriraju da je moguće izvesti dobre zavare bez predgrijavanja pravilnim izborom parametara zavarivanja i odgovarajućeg oblika impulsa

Astronomische Grundlagen einiger frühmittelalterlichen Kultstellen in PrahaAstronomske osnove nekaterih zgodnjesrednjeveških kultnih točk v Pragi

The authors present a structure of cult points in Prague which was used during the Early Middle Ages. The structure was composed on the basis of astronomical and ritual principles. The former present a sun calendar. The latter principles led the authors to determine the use of a ritual angle, ritual measuring units and their multiples. The entire structure is thus also an ideogram, which supposedly also aided in sustaining equilibrium among the natural forces

The weld-pool solidification mode of ferritic stainless steels

The paper presents the analysis of solidification mode at gas tungsten arc welding (GTAW) of AISI 430 ferritic stainless steel. Two solidification modes (epitaxial and equiaxial) were discovered, which have a major influence on a weld tensile strength. The optimal welding parameters for ferritic stainless steels were found in a narrow range. They should be selected according to mechanical strength of the welded joints and not only according to their visual appearance

Problems in repair-welding of duplex-treated tool steels

The present paper addresses problems in laser welding of die-cast tools used for aluminum pressure die-castings and plastic moulds. To extend life cycle of tools various surface improvements are used. These surface improvements significantly reduce weldability of the material. This paper presents development of defects in repair welding of duplex-treated tool steel. The procedure is aimed at reduction of defects by the newly developed repair laser welding techniques. Effects of different repair welding process parameters and techniques are considered. A microstructural analysis is conducted to detect defect formation and reveal the best laser welding method for duplex-treated tools