Evaluation of spark plasma sintering and forging of aluminum based powders for near-net shapes
Başlık çevirisi mevcut değil.
- Tez No: 540060
- Danışmanlar: Prof. MATHIEU BROCHU
- Tez Türü: Doktora
- Konular: Maden Mühendisliği ve Madencilik, Metalurji Mühendisliği, Mining Engineering and Mining, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2018
- Dil: İngilizce
- Üniversite: McGill University
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 220
Özet
Özet yok.
Özet (Çeviri)
Al powder metallurgy (P/M) offers several advantages over traditional manufacturing methods, such as near-net-shape production and eliminating or reducing material waste. However, the native oxide layer of Al prevents the sintering of Al particles and must be eliminated, but the stable character makes physical means of reducing the layer ineffective. Some strategies to disrupt the oxide layer are liquid phase sintering or forging of sintered compact. Forging is more attractive in terms of near-net-shape processing, as heterogeneous shrinkage that can arise during liquid phase sintering can lead to dimensional inaccuracies. Another alternative, spark plasma sintering (SPS), is a pressure and electrical current-assisted sintering process and considered as an effective way to reduce the oxide layer problem because of its ability to breakdown the oxide layer and clean particle surfaces which can eliminate the need for a forging step. SPS of inert gas atomized pure Al powder was investigated by monitoring electrical resistance curves of the process under different pressures. Resistance curves showed an enhanced descent in resistance after 200 °C. This decrease is more pronounced with an increase in loading pressure. The hardness ratio of alumina to Al increases with increasing temperature, becoming more pronounced after 200 °C, and softening of the Al leads to the easier breakdown of the surface oxide. Three-point bending tests and fracture surface analyses of the samples were correlated with the resistance curves. Samples with more enhanced descent in resistance results in fracture surfaces showing more dimples which is an indication of metal to metal contact. Alumix 431D powder, the P/M version of Al alloy 7075, was sintered by SPS and upset forging by SPS was applied to the sintered sample to eliminate the particle bonding problem encountered in this alloy after sintering. Significant improvement in the flexural strength and strain were observed after the SPS forging process and the fracture surface analysis indicates better particle bonding compared to the as-sintered sample. The ductility of both samples was significantly reduced after the T6 temper. To further isolate the source of the bonding issue with this alloy, the Alumix 431D powder was modified by replacing the native air atomized pure Al particles with gas atomized pure Al. SPS of this modified Alumix 431D powder, followed by a three-point bending test and fracture surface analysis shows that the particle bonding problem continued after this modification. Application of the SPS forging on the sintered samples improved particle bonding, leading to the conclusion that the particle bonding problem is not coming from the air-atomized pure Al originally present in the powder mixture, but that it is the composite type surface oxide of Alumix 431D that requires a secondary process such as forging. The tool stresses during the isothermal SPS forging of a modified Alumix 431D compact into near-net shapes were analyzed for different punch geometries of axisymmetric shapes using finite element analysis (FEA) simulations. Results reveal that graphite, the most commonly used tool material for SPS, is insufficient for use during isothermal SPS forging of an Al-Zn-Mg-Cu alloy sintered compact into certain geometries due to the formation of stresses that reach the critical value for compression or tensile strength. Overall, SPS is an effective process to obtain good particle bonding for the sintered compacts of inert gas atomized pure Al powder, whereas the additional step of forging is required when using Alumix 431D powder. Additionally, it is necessary to be aware of the limitations of graphite as a tool material during the forging of sintered Alumix 431D compacts into near-net shapes.
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