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Influence of cooling rate on microstructure and mechanical properties of castings from magnesium alloys AZ91, WE43B and Elektron21 produced by investment casting
Jakubcová, Eliška ; Klakurková, Lenka (referee) ; Juliš, Martin (advisor)
This master's thesis deals with the effects of the cooling rate on the grain size of magnesium alloys based on Mg–Al (AZ91E) and Mg–Zr–RE (WE43B and Elektron21). The effects of the cooling rate are analyzed on samples of varying thickness for different cooling methods (forced–air–cooling, cooling into polymer, natural air cooling). We also evaluated the final microstructure, porosity, hardness, and mechanical properties for investment castings. Based on the evaluated grain sizes, we demonstrated a significant difference in the cooling rate influence on the grain size for the alloy AZ91E. Compared to Zirconium-based alloys (WE43B and Elektron21), for which the zirconium content influences grain size the most. For the alloys based on Mg–Zr–RE, it is possible to prefer casting conditions, without negative enlarging of the grain size.
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Welding of Al-Si coated high-strength steel 22MnB5
Šebestová, Hana ; Horník, Petr ; Mikmeková, Šárka ; Novotný, Jan ; Mrňa, Libor
Al-Si-based coatings are widely used as surface protection of steel sheets against high-temperature oxidation during the hot stamping process. Laser welding is a suitable technology for hardened components joining because it does not produce a wide heat-affected zone with a degraded microstructure. The highly concentrated energy of the laser beam is responsible for rapid heating and subsequent rapid cooling of material which results in insufficient homogenization of the weld metal. In Al-rich areas (Al originates from the coating), ferritic bands are formed in the dominantly martensitic matrix. These bands decrease the strength of laser weld by more than one third compared to the base metal. The low-power electric arc placed immediately behind the laser beam can help to achieve a slower cooling of the weld metal, reduce the ferritic bands and increase the strength of weld joints up to 90 % of base metal tensile strength.
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Influence of cooling rate on microstructure and mechanical properties of castings from magnesium alloys AZ91, WE43B and Elektron21 produced by investment casting
Jakubcová, Eliška ; Klakurková, Lenka (referee) ; Juliš, Martin (advisor)
This master's thesis deals with the effects of the cooling rate on the grain size of magnesium alloys based on Mg–Al (AZ91E) and Mg–Zr–RE (WE43B and Elektron21). The effects of the cooling rate are analyzed on samples of varying thickness for different cooling methods (forced–air–cooling, cooling into polymer, natural air cooling). We also evaluated the final microstructure, porosity, hardness, and mechanical properties for investment castings. Based on the evaluated grain sizes, we demonstrated a significant difference in the cooling rate influence on the grain size for the alloy AZ91E. Compared to Zirconium-based alloys (WE43B and Elektron21), for which the zirconium content influences grain size the most. For the alloys based on Mg–Zr–RE, it is possible to prefer casting conditions, without negative enlarging of the grain size.
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Synergic effects of laser – TIG welding
Kubíček, J. ; Doležal, P. ; Mrňa, Libor ; Šebestová, Hana ; Horník, Petr
Laser welding compared to conventional arc methods is accompanied by rapid cooling rates. It can lead to the formation of undesirable microstructure of the weld metal and heat affected zone of some steel grades which has a negative impact on their mechanical properties. The preheating is quite problematic because of high speed of laser welding. The possibility of preheating by the heat of the arc discharge of the associated TIG burner has been tested. Precise holder has been fabricated to fix the burner to the laser welding head allowing TIG electrode positioning towards the laser beam and the workpiece. A number of high strength steel S460MC and S700MC welding experiments was performed. The effect of TIG current on microstructural changes has been proven. Some welding experiments were compared to the SYSWELD welding simulations.\n
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