doktorska disertacija
Stojana Veskovič Bukudur (Author), Milan Bizjak (Mentor), Damjan Klobčar (Co-mentor)

Abstract

Zakonodaja predpisuje proizvajalcem avtomobilov vse strožje omejitve glede izpustov škodljivih emisij, zato je razvoj motorjev z notranjim zgorevanjem usmerjen predvsem v zmanjšanje porabe goriva pa tudi njegovega popolnega zgorevanja v vsakem trenutku delovanja motorja. Ena izmed danes splošno sprejetih tehnologij, ki omogoča do 10 % zmanjšanje porabe goriva, zlasti pri vožnji v urbanem okolju, je tehnologija start-stop. Sistem start-stop zmanjša porabo goriva s samodejnim izklopom motorja z notranjim zgorevanjem med čakanjem pred semaforjem ali v zastojih in s tem skrajša čas, ko motor deluje v prostem teku. Po drugi strani pa uporaba te tehnologije dodatno obremenjuje ostale elemente avtomobilskega zagonskega sistema, kot so akumulator, zaganjalnik in vžigalne ali grelne svečke itd., ker bistveno poveča število zagonov motorja. V raziskavi sem se usmerila v razvoj grelnih svečk dizelskih motorjev. Žarilne svečke služijo kot pomoč pri zagonu hladnega dizelskega motorja. Njihova naloga je, da predgrejejo zgorevalni prostor dizelskega motorja in s tem olajšajo njegov zagon. Poleg tega žarilne svečke segrevajo zgorevalni prostor tudi še nekaj časa po zagonu hladnega motorja in s tem prispevajo k popolnejšemu zgorevanju dizelskega goriva in k manjšim emisijam trdnih delcev (saj) in škodljivih izpušnih plinov. Ker mora biti dizelski motor pripravljen na zagon v relativno kratkem času in ne sme proizvajati škodljivih emisij v času hladnega delovanja, se mora žarilna svečka segreti na visoko temperaturo, podaljšal pa se je tudi čas njenega delovanja. Proizvajalci dizelskih motorjev zato zahtevajo vse višje temperature delovanja grelnih svečk ob njihovi hkratni podaljšani življenjski dobi. Najbolj občutljiv element grelnih svečk predstavlja grelni element, izdelan iz zlitin Fe-Cr-Al. Te zlitine so znane po svoji odlični oksidacijski odpornosti pri visokih temperaturah in se zelo pogosto uporabljajo kot grelni elementi. Njihova odpornost proti oksidaciji je povezana z nastankom termodinamično stabilne in za kisik neprepustne zaščitne plasti aluminijevega oksida (Al2O3) na površini zlitine, ki zavira njeno nadaljnjo oksidacijo. Kljub odličnim antioksidativnim lastnostim teh zlitin pa je njihova oksidacijska odpornost omejena, če so izpostavljene termičnemu cikliranju nad 1050 °C, kar se zgodi pri delovanju grelnih svečk med zagonom hladnega dizelskega motorja. Razlog za to je pokanje in luščenje zaščitne plasti aluminijevega oksida in poraba aluminija v podpovršinski plasti zlitine zaradi nenehnega ponovnega nastajanja plasti aluminijevega oksida. Ko se vsebnost aluminija v podpovršinski plasti zlitine zmanjša pod kritično koncentracijo (< 3 mas. %), zaščitna plast aluminijevega oksida ne more več nastati, kar vodi v oksidacijo železa in kroma ter nastanek kromovih in aluminijevih nitridov in posledično v njen propad. Da bi podaljšali življenjsko dobo grelnih svečk, smo žarilne elemente iz zlitine Fe-Cr-Al obogatili z aluminijem v površinski plasti in s tem povečali njegovo zalogo, ki je potrebna za tvorjenje in ohranjanje oksidne Al2O3 plasti. Uporabili smo različne postopke nanašanja aluminija: PVD, aluminiziranje s kasnejšo termično obdelavo in ALD-postopek. S postopkom fizikalnega nanašanja iz parne faze (Physical vapour deposition – PVD) smo na površino uporov nanesli 2 µm debelo plast čistega aluminija. Z aluminiziranjem v trdnem (aluminizing – powder pack) se je odvisno od sestave zasipa tvorila plast različnih aluminidov ali z aluminijem obogatena plast ?-(Fe, Cr, Al). S postopkom nanašanja atomskih plasti (atomic layer deposition – ALD) smo na površino nanesli tanko plast (nekaj nm) Al2O3, ki naj bi ščitil material pred nadaljnjo oksidacijo. Cilj raziskave je študija različnih fizikalno-kemijskih procesov, ki se odvijajo med delovanjem žarilne svečke. Narejene so bile obsežne mikrostrukturne raziskave prečnih in vzdolžnih prerezov grelnih svečk po trajnostnih testih na laboratorijskih napravah v podjetju Hidria d.o.o., kot tudi po delovanju v realnem motorju pri potencialnih kupcih. Z raziskavami temeljnih fizikalno-kemijskih pojavov pri visokotemperaturni oksidaciji je pojasnjen in ovrednoten vpliv izhodnih mikrostruktur različnih materialov na mehanizem in kinetiko njihove oksidacije pri različnih pogojih. V začetni fazi raziskave smo s termogravimetričnimi analizami primerjali oksidacijske lastnosti komercialnih materialov na osnovi zlitin Fe-Cr-Al, ki so trenutno na tržišču. Preiskovane zlitine so se razlikovale po postopku izdelave, količini in vrsti reaktivnih elementov, količini aluminija in kroma v zlitini ter mikrostrukturnih in mehanskih lastnostih. V naslednji fazi smo na žarilne upore s PVD-postopkom nanesli plast aluminija in z metodo merjenja električne upornosti ugotavljali vpliv temperature in časa na difuzijske procese v zlitini. Pridobljene rezultate smo nato uporabili pri izbiri tehnoloških parametrov aluminiziranja v trdnem, ki smo ga razvili, da bi povečali količino aluminija v površinski plasti in s tem zagotovili hitrejšo oksidacijo zlitine v začetni fazi segrevanja in povečali število možnih obnovitev zaščitne površinske plasti aluminijevega oksida. Zaradi spremenjene sestave grelnih elementov je bilo potrebno prilagoditi tudi metodo spajanja le-teh z drugimi materiali v grelnem elementu. Sprememba tehnoloških parametrov klasičnega obločnega varjenja, ki se je do sedaj uporabljalo v proizvodnji, ne omogoča uspešnega spajanja, saj povečana energija obloka negativno vpliva na ostale elemente žarilne svečke. Za uspešno spajanje teh elementov smo določili tehnološke parametre laserskega varjenja, ki omogočajo ustrezno kvaliteto spoja in s tem daljšo obratovalno dobo žarilne čepne svečke.

Keywords

žarilne svečke dizelskih motorjev;zlitine Fe-Cr-Al;visokotemperaturna oksidacija;aluminiziranje;difuzijsko aluminiziranje v trdnem;mikrostruktura;lasersko varjenje;

Data

Language: Slovenian
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UL NTF - Faculty of Natural Sciences and Engineering
Publisher: [S. Veskovič Bukudur]
UDC: 669.018.4:621.43.045.6(043.3)
COBISS: 67018499 Link will open in a new window
Views: 367
Downloads: 79
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Other data

Secondary language: English
Secondary title: high temperature oxidation of alloys for glow plugs with extended lifetime
Secondary abstract: Car manufacturers are facing increasingly strict regulations for harmful emissions from combustion engines. The development of internal combustion engines therefore focuses primarily on reducing fuel consumption and on their complete (ideal) combustion at every moment of engine operation. One of the most widely accepted technologies today that can reduce fuel consumption by up to 10%, especially when driving in urban environments, is start-stop technology. The start-stop system reduces fuel consumption by automatically switching off the combustion engine when waiting at traffic lights or in traffic jams, thus reducing engine idling times. On the other hand, the implementation of this technology places an additional load on other elements of the car's starting system, such as the battery, the starter and the spark or glow plugs, etc., as it significantly increases the number of engine starts. In this study I concentrated on the development of glow plugs for diesel engines. The glow plugs are used to support the cold start of diesel engines. Their task is to preheat the combustion chamber of the diesel engine and thus facilitate the start. In addition, glow plugs continue to heat the combustion chamber for some time after the cold engine is started, thus contributing to more complete combustion of the diesel fuel and thus to lower emissions of particulate matter (soot) and harmful exhaust gases. Since the diesel engine must be ready to start in a relatively short time and generate as few harmful emissions as possible in cold operation, the glow plug must both heat up to a high temperature and work at a high temperature for a longer period of time. The manufacturers of diesel engines therefore demand ever higher operating temperatures for glow plugs as well as an extension of their service life. The most sensitive element in the glow plug is a heating coil made of FeCrAl alloys. The FeCrAl alloys are known for their excellent oxidation resistance at elevated temperatures and were primarily used as heating elements. Their superior oxidation resistance is related to the formation of a thermodynamically stable and oxygen-impermeable protective layer of aluminium oxide (Al2O3) on the surface, which prevents further oxidation of the material. Despite the excellent anti-oxidation properties of these alloys, their oxidation resistance is limited when they are exposed to thermal cycles up to 1050 °C, such as in the case of glow plug operation when starting a cold diesel engine. The reason for this is the cracking of the alumina scale due to the difference in temperature expansion compared to the alloy matrix and the consumption of aluminium in the subsurface area of the alloy matrix due to the constant re-formation of the alumina scale. As soon as the aluminium content in the subsurface area is reduced beneath critical concentration (< 3 wt.%), the alloy can no longer reform the continuous alumina scale, which leads to catastrophic breakaway oxidation through the formation of rapidly growing iron and chromium-based oxides and aluminium and chromium nitrides (AlN, Cr2N). To extend the service life of glow plugs, we have enriched the glow resistors made of a FeCrAl alloy with aluminium in the surface/subsurface layer, thus increasing the number of possible re-formation of the protective aluminium oxide layer. Various methods of aluminium deposition were used: PVD, powder pack aluminization with subsequent heat treatment and ALD process. The aim of the research is to investigate physical and chemical processes that take place during the operation of a glow plug. Extensive microstructural investigations of cross-sections and longitudinal cross-sections of glow plugs were carried out after sustainability tests on laboratory equipment at the company Hidria d. o. o. and after operation in real engines at potential customers. The influence of the initial microstructures of various materials on the mechanism and kinetics of their oxidation under various conditions is explained and evaluated by research of basic physical and chemical phenomena during high-temperature oxidation. In the preliminary phase of the research, thermogravimetric analyzes were used to compare the oxidation properties of commercial materials based on FeCrAl alloys. The alloys studied differed in the production process, in the quantity and type of reactive elements, in the content of aluminium and chromium in the alloy, and in the microstructural and mechanical properties. In the next research step, an aluminum layer was applied to the heating resistors made of FeCrAl alloy by the PVD process and the influence of temperature and time on the diffusion processes in the alloy was determined by measuring the electrical resistance. The obtained results were then used to select the technological parameters of powder pack aluminization, which we developed to increase the amount of aluminum in the surface/subsurface layer, thus ensuring faster oxidation of the alloy in the initial heating phase and increasing the number of possible re-formations of protective alumina scale. Due to the changed composition of the heating resistors, it was also necessary to adapt the method of joining them with other materials/parts of the heating element. The change of technological parameters of conventional arc welding, which has been used in production so far, does not allow successful joining, as the increased arc energy has a negative effect on other elements of the glow plug. We have therefore developed a laser welding technology to successfully join these elements.
Secondary keywords: Diesel engine glow plugs;FeCrAl alloys;high temperature oxidation;aluminizing;powder pack aluminizing;microstructure;laser welding;Dizelski motorji;
Type (COBISS): Doctoral dissertation
Study programme: 0
Thesis comment: Univ. v Ljubljani, Naravslovnotehniška fak., Oddelek za materiale in metalurgijo
Pages: XXVIII, 229 str.
ID: 13011151