Synthesis Of The High Strength Aluminum Alloys By Spark Plasma Sintering And Their Structures And Properties

Aluminum is one of the richest elements in the world, and applied extensively. And Al alloys have received considerable attention due to their low density and high specific strength. And its strength can compare beauty with both Ti alloys and super high strength steel after alloying. In present, there are several methods to improve strengthen as follows: 1, Alloying: Via adding Cu, Zn, Mg etc alloy elements to improve alloy intension; 2, Molding; 3, amorphouzation: including amorphous phase or nanometer phase; nano-scale compounds particles or norm crystal phase attached in the α-Al matrix. The highest intensity of Al alloys that obtained by use of alloying methods in the Lab is 740 MPa; the applied highest intensity in practice is 540 MPa. It is very difficult to improve the intensity of Al alloys more using the method of alloying. It is restricted by shape and sizes that using the method of cold distortions to improve the intensity of alloys. In present, it is getting along well with the third method to improve the intensity of alloys and higher intensity Al alloy can be made. In the recent researches, various amorphous alloy with high Al contain (Al≥85at%) shows excellent integration mechanism capability: draught rupture intensity>1000MPa, nicer bend toughness, corrosion resistant, high resistance ratio and low temperature coefficient. But nanocrystalline alloys have high intensity and high toughness, and higher integration mechanism capability than generanl alloys. With to make the amorphous alloys or nanocrystalline alloys can by Rapidly solidification technique (RS) or machine alloying technology (MA). By RS technology con obtain alloys with the powder, foil, silk and ribbon shape, moreover by MA technique only gain powder alloys. In this thesis, we choosed AlLaNi and AlMnCe system with study object, structure and properties of the balk high strength aluminum alloys is prepared by a powder spark plasma sintering technique, its technique was studied. Furthermore,effect of sintering temperature on the room temperature properties of these alloys was discussed. The shapes of the amorphous alloys are, however, limited to ribbon, wire, film and powder, since the synthesis of these alloys requires high cooling rates from the melts. Their small size makes engineering application unlikely. For this reason, hot pressing and/or hot extrusion methods were recently used to fabricate bulk sample. However, a large pressure (>1GPa) and higher pressing temperature are needed. Spark plasma sintering (SPS) is a recent innovation in activated sintering and densification by charging a high-pulsed electric current directly through the powders in a die under externally applied pressure. Furthermore, SPS allows very fast heating and cooling rates, very short holding time, and the possibility to obtain fully dense samples at comparatively low sintering temperatures, typically a few hundred degrees lower than normal hot pressing. Innovative materials have been obtained with metals, ceramics, polymers,semiconductors, bio-, nano-, amorphous-, and structural ceramics and composites. Some of them cannot be prepared without SPS. Pulse electric current sintering is an advanced technology for materials synthesis and processing with potential application. But the sintering mechanism is in dispute. In order to get best sintering effects, the mechanism of pulse electric current sintering was investigated in present dissertation. In the present research, a control sintered softening parameter ζ, which combines the effects of particle size and liquid volume fraction, is proposed to separate densification and distortion events. This sintered parameter has two critical values, ζDENSIF and ζDISTORT, for densification and shape loss,respectively. It must be smaller than ζDISTORT to avoid shape distortion, while for densification, it must be larger than ζDENSIF. As ζDENSIF < ζ<ζDISTORT, there is window of sintering conditions where densification is achieved without accompanying shape loss. 131353(2)(1)SSSSPPRESSSSSSSSLDENSIFACCCσσσζαη?+??= ???????? 131353()(1)SSSSPPRESSSSSSSSLDISTORTBCCCσσσζαη?+??= ???????? By using SPS, a strong electrical field is produced in the small gaps between the particles and the momentary high-temperature field is generated by pulse energy, spark impact pressure and electrical resistance heating at the initially sintered stage. The local high temperature is created in the contacting areas of the particles where aluminum oxide films are ruptured by the spark plasma. Therefore, fresh surfaces of the powder come into direct contact. At the same time, the spark discharges may bring about evaporation and melting of the surface of the powder particles, thus inducing neck formation, growth and contact flattening. As a result, the compact bulk alloy can be obtained. The structure of the Al90Mn8Ce2 alloy consist of α-Al and compound(Al6Mn, AlCe, Al10Mn7Ce2). In the DSC figure, one exothermic peak at 726.5 K arises, which indicates crystallization of minor amount of the amorphous phase in the powders. The X-ray analysis of the Al85La10Ni5 alloy consist of α-Al and compound(La3Al11, Al4La, LaNi3, Al3Ni). The DSC shown two exothermic peaks at 553.6 K and 622.3 K arises, which indicates crystallization of minor amount of the amorphous phase in the powders. Tm = 894.6 K with denotes the melts of α-Al solutions. Alloy has high strength and hardness, due to fine hardening, solid solution hardening, precipitation hardening and dispersion hardening. A high density of Al85La10Ni5 and Al90Mn8Ce2 bulk alloys with porosity lessthan 1.5 % can be prepared by spark plasma sintering at 693-793 K, sintering time of 5min and pressure of 100MPa. The porosity of the alloy pressed at 573 K is large. Above 693 K, the density of the alloy increases with increases in sintered temperature due to a decrease in porosity. The highest hardness and compression strength of the sintered alloy reaches HRB 98 and 810 MPa at sintered temperature 793 K, respectively. However a further increase of sintering temperature leads to drop of hardness of the sintered alloy, which may be induced by the grain growth or coarsening of the intermetallic compounds at high temperature. The wear resistance of the alloy sintering at 723 K is twice as high as that of the conventional A390 alloy. The high wear resistance of the Al85La10Ni5 and Al90Mn8Ce2 bulk alloys is attributed to the existence of the large amount of the intermetallic compounds. The compression ductility is reach 4-6 %.