Livarski vestnik 53 / 2006 Nr. 2
Celal Cingi, Jukka Vainola in Juhani Orkas:
Mold-Metal Reactions in Investment Cast AZ91E Magnesium Alloy
F. Zupanič, T. Bončina, A. Križman, B. Šušteršič, S. Spaić, B. Markoli:
Rapid Solidifi cation of Al-Mn- Be Alloys and Formation of Quasicrystals
Celal Cingi, Jukka Vainola in Juhani Orkas:
Mold-Metal Reactions in Investment Cast AZ91E Magnesium Alloy
Abstract
Mold-metal reactions can be encountered during the investment casting of magnesium alloys. This study was made a) to investigate the degree of reactions between the various refractory materials and the magnesium alloy AZ91E; b) to find new techniques to reduce these reactions.
Investment casting molds containing multiple test pieces 25x25x60mm in dimension
were used. The wax pattern of each test piece was coated with a different ceramic face coat to see the resulting differences on cast metal surfaces. Backup layers of the shell were the same for all test pieces. Fused alumina, fused silica, molochite, zircon, yttria, zirconia and fused magnesia as face-coat ceramics were the studied ceramics.
Digital pictures of the cast surfaces were taken and then an image analyzer was used to assess reacted areas quantitatively. According to the results, fused magnesia and yttria were the best to resist molten magnesium. Fused alumina and zircon were the next. Molochite and zirconia ranked moderate to poor in resisting reactions. Strong mold-metal reactions were seen in case of fused silica. The excellent reaction resistance of magnesia and yttria can be explained by the very negative Gibb’s free energy of formation of these materials.
Analysis with EDS indicates that reaction layers are mostly oxides.
HSC software was used to calculate theoretically the free energy of reaction for the refractories investigated in this work. The results obtained are compared to experimental values.
It was demonstrated that the atmosphere surrounding the shell also affected the
reactions. In molds containing both normal and reduced permeability shells around the test pieces, the surfaces cast in reduced permeability shells showed large decreases in reactions. Mold permeability data are given. It is suggested that reactions in investment casting molds occur in two stages: 1. during pouring and fi lling, 2.after the filling is complete by the effect of external oxygen.
An attempt was made to incorporate the inhibitor KBF4 in the mold structure in the
form of first coat stucco. Rough cast surfaces were obtained, indicating the high sintering temperature used was not compatible with this inhibitor. Similarly, when a preheated investment mold was placed in a bed of KBF4 before casting, only small reductions in reacted areas were obtained.
Investment casting molds containing multiple test pieces 25x25x60mm in dimension
were used. The wax pattern of each test piece was coated with a different ceramic face coat to see the resulting differences on cast metal surfaces. Backup layers of the shell were the same for all test pieces. Fused alumina, fused silica, molochite, zircon, yttria, zirconia and fused magnesia as face-coat ceramics were the studied ceramics.
Digital pictures of the cast surfaces were taken and then an image analyzer was used to assess reacted areas quantitatively. According to the results, fused magnesia and yttria were the best to resist molten magnesium. Fused alumina and zircon were the next. Molochite and zirconia ranked moderate to poor in resisting reactions. Strong mold-metal reactions were seen in case of fused silica. The excellent reaction resistance of magnesia and yttria can be explained by the very negative Gibb’s free energy of formation of these materials.
Analysis with EDS indicates that reaction layers are mostly oxides.
HSC software was used to calculate theoretically the free energy of reaction for the refractories investigated in this work. The results obtained are compared to experimental values.
It was demonstrated that the atmosphere surrounding the shell also affected the
reactions. In molds containing both normal and reduced permeability shells around the test pieces, the surfaces cast in reduced permeability shells showed large decreases in reactions. Mold permeability data are given. It is suggested that reactions in investment casting molds occur in two stages: 1. during pouring and fi lling, 2.after the filling is complete by the effect of external oxygen.
An attempt was made to incorporate the inhibitor KBF4 in the mold structure in the
form of first coat stucco. Rough cast surfaces were obtained, indicating the high sintering temperature used was not compatible with this inhibitor. Similarly, when a preheated investment mold was placed in a bed of KBF4 before casting, only small reductions in reacted areas were obtained.
F. Zupanič, T. Bončina, A. Križman, B. Šušteršič, S. Spaić, B. Markoli:
Rapid Solidifi cation of Al-Mn-Be Alloys and Formation of Quasicrystals
Abstract
We investigated the infl uence of casting parameters at melt spinning on microstructure of AlMn5Be4 (M5) and AlMn10Be4 (M10) alloys. Our main goal was to create conditions for achieving a uniform distribution of quasicrystalline particles in Al-matrix.
In the initial state (after vacuum melting and casting) both alloys contained aluminiumrich solid solution ‹Al and two intermetallic phases: Be4AlMn and Al10Mn3. Because of very large solidifi cation range of both alloys melt spinning had started before the melt temperature exceeded the liquidus temperature at the crucible hole diameter of 1,8 mm. Decreasing the hole diameter to 1 mm enabled heating of the melt above the liquidus temperature before the commencing of casting. In these conditions the microstructure of melt-spun ribbons consisted of quasicrystalline particles distributed in ‹Al. The ribbon thickness was mainly controlled by the wheel speed; the higher was the wheel speed, the thinner was the ribbon. Thicker ribbons (≈150 ?m) consisted of three distinct layers. At the bottom directional solidifi cation took place: at the beginning with the plane growth morphology that latter changed to cellular/dendritic one. The top of ribbons consisted of equiaxed dendritic grains. In thinner ribbons (40-60 ?m) only the fi rst layer was observed. Quasicrystalline particles were the most uniformly distributed in ‹Al in the region of plane growth, otherwise they were accumulated in the intercellular/interdendritic spaces. Appropriate selection of melt spinning parameters resulted in a quite uniform distribution of quasicrystalline particles in Al that is the main prerequisite for using melt-spun ribbons as a starting material for
producing massive parts.
Key words: rapid solidifi cation, quasicrystal, aluminium alloys, metallographic ana
In the initial state (after vacuum melting and casting) both alloys contained aluminiumrich solid solution ‹Al and two intermetallic phases: Be4AlMn and Al10Mn3. Because of very large solidifi cation range of both alloys melt spinning had started before the melt temperature exceeded the liquidus temperature at the crucible hole diameter of 1,8 mm. Decreasing the hole diameter to 1 mm enabled heating of the melt above the liquidus temperature before the commencing of casting. In these conditions the microstructure of melt-spun ribbons consisted of quasicrystalline particles distributed in ‹Al. The ribbon thickness was mainly controlled by the wheel speed; the higher was the wheel speed, the thinner was the ribbon. Thicker ribbons (≈150 ?m) consisted of three distinct layers. At the bottom directional solidifi cation took place: at the beginning with the plane growth morphology that latter changed to cellular/dendritic one. The top of ribbons consisted of equiaxed dendritic grains. In thinner ribbons (40-60 ?m) only the fi rst layer was observed. Quasicrystalline particles were the most uniformly distributed in ‹Al in the region of plane growth, otherwise they were accumulated in the intercellular/interdendritic spaces. Appropriate selection of melt spinning parameters resulted in a quite uniform distribution of quasicrystalline particles in Al that is the main prerequisite for using melt-spun ribbons as a starting material for
producing massive parts.
Key words: rapid solidifi cation, quasicrystal, aluminium alloys, metallographic ana