Selective laser melting of Al and AlSi10Mg: parameter study and creep experiments

Selective laser melted (SLM) aluminum alloys are widely used for many technical applications. However, the application is limited to low temperatures due to their relatively poor creep resistance. The creep resistance and strength could be enhanced by oxide dispersion-strengthening. A hypothesis is that oxygen intake during selective laser melting can lead to formation of fine aluminum oxides and thus strengthen the SLMed part. To elucidate this in more detail, selective laser melted AlSi10Mg was tested in creep experiments at temperatures of 300 °C. Although, in other studies at lower temperatures, a relatively large stress exponent for creep was found, the high temperatures in this work led to a creep exponent of just 7 to 8, indicating no significant dispersion strengthening. Furthermore, for future research, it was necessary to investigate the feasibility of SLM with pure aluminum. For this purpose, a parameter study was carried out and an optimum parameter set for pure aluminum was found. Dense samples with a porosity below 0.2% were produced. Selective laser melting was carried out with a varying oxygen content in the inert-gas atmosphere to elucidate the hypothetic strengthening effects by oxygen intake. However, even at 800 ppm oxygen in the atmosphere, no effect on hardness and microstructure could be observed.

Effect of Ca and Rare Earths on Compressive Creep Behavior of Mg-4Al Alloy

The effect of Ca and rare earths on compressive creep behavior of Mg-4Al alloy was investigated with a special apparatus. The microstructures were analyzed by OM, XRD, SEM and EDS before and after compressive creep test. The results reveal that a small amount of Ca was added into AE41 alloy in order to refine the crystalline structure and improve the creep resistance. Comparing with NdPr rare earths alloy, the creep resistance of LPC rare earths alloy decreases, but is still better than AE41 alloy. The as-cast microstructure of AE41 alloy is mainly composed of α-Mg matrix and Al11Nd3 phase. The acicular Al11Nd3 phase is prone to decompose at high temperature, which leads to the poor creep resistance of AE41 alloy. The acicular Al11RE3 phase is gradually replaced by Al2Ca and Al2RE with Ca addition into AE41. LPC rare-earth mixture is in cluster at grain boundaries so that the creep resistance is worse than that of alloy containing NdPr rare-earth mixture.

Mechanical and Creep Properties of the Cast Magnesium Alloy

Currently most commercial magnesium alloys are based on the Mg-Al system and it is reasonably well developed. Although the Mg-Al based alloy system has excellent castability and adequate ambient temperature mechanical properties, it shows poor creep resistance. Therefore, our group has focused on finding the way to improve the creep properties of Mg alloys. This paper presents a brief summary of the research achievements in this area recently made by AFML(Advance Functional Materials Lab in PNU, Korea). The properties of newly designed Mg alloys in our group are presented and compared with the properties of commercial A356 alloy.

Effects of Initiator Level on Performances of Kraft Fiber Reinforced Unsaturated Polyester Composites

Relative lower strength and poor creep resistance of wood-plastic composites (WPC) restrain their wider applications in building and automotive. A novel wood-polymer composite was prepared using Kraft fiber and unsaturated polyester (UPE), which had much higher strength and better creep resistances than that of traditional thermoplastic WPC. Effects of initiator on the mechanical properties and creep resistance of this novel composite were investigated by tensile evaluation, DMA, SEM and short-term creep test. Test results indicated that initiator level had important effects on mechanical properties and viscoelastic behaviors because of various crosslinking densities of UPE matrix and interface adhesions between Kraft fiber and UPE resin under various initiator levels. With initiator level increased from 0.3% to 1%, the tensile strength and interface adhesion increased at the beginning and then decreased, while the instantaneous strain and maximum strain in the creep test decreased gradually. As for hot molding at 125°C, initiator level shall be less more than 1% and be preferable to 0.5%-0.7%.

Creep Resistance of Thermoplastic Nanocomposites

Relatively poor creep resistance is considered as a deficiency of thermoplastic materials in general. In order to comprehensively and deeply study the effect of nanoparticles incorporated into a polyamide matrix a systematic investigation on various kinds of nanofillers under differed temperatures and stress levels have been performed. It was found that the creep resistance of polyamide 66 can be significantly enhanced due to the incorporation of nanofillers and the behavior of the composites can be easily described by the model of Findley.

Assessment of Microalloying Effects on the High Temperature Fatigue Behavior of NiAl

Binary NiAl suffers from a lack of strength and poor creep properties at and above 1000 K. Poor creep resistance in turn affects low cycle fatigue (LCF) lives at low strain ranges due to the additional interactions of creep damage. One approach for improving these properties involves microalloying with either Zr or N. As an integral part of a much larger alloying program the low cycle fatigue behavior of Zr and N doped nickel aluminides produced by extrusion of prealloyed powders has been investigated. Strain controlled LCF tests were performed in air at 1000 K. The influence of these microalloying additions on the fatigue life and cyclic stress response of polycrystalline NiAl are discussed.

Mechanical Properties and Microstructure of Certain Niaico(Hf) Alloys

Cobalt-modified NiAl alloys are being studied for their potential for room temperature ductility and toughness. An alloy of Ni - 29.3 a/o Al - 36.7 a/o Co has shown improved toughness and ductility with respect to binary NiAl alloys due in part to a stress-induced martensitic transformation. Furthermore, the cobalt additions have altered the slip behavior to {110}<111> type from {110} <001> for binary NiAl alloys. Hafnium was added to improve the alloy's relatively poor creep resistance and high temperature strength. Hf was found to be insoluble in the NiAlCo alloy and formed precipitates with a hexagonal structure. The Hfmodified alloy had improved high temperature strength. In addition, the Hf apparently changed the creep mechanism resulting in poorer creep resistance at low temperatures, but improved creep resistance at higher stresses and temperatures.

Effect of Incompatibility Stress on the Fit of Metal-Ceramic Crowns

The interactive effect of coping thickness and a positive thermal contraction mismatch between metal and porcelain on the fit of metal-ceramic crowns has not yet been experimentally determined. Previous studies have suggested that marginal distortion may be due to contraction differences, although finite element analyses indicate that these distortion effects should be negligible. The marginal gap between metal-ceramic crowns and prepared dies was determined under conditions designed to exaggerate distortion effects. These included the use of thin metal copings (0.1 and 0.2 mm), a chamfer preparation, an alloy with relatively poor creep resistance, and a large thermal contraction mismatch between the alloy and porcelain layers. Gap changes which resulted during porcelain firing cycles were relatively small, but larger marginal discrepancies developed in crowns prepared with a compatible porcelain during grinding and abrasive blasting procedures. This study conclusively demonstrates that incompatibility stress induced by a positive contraction mismatch is not a primary cause of marginal or generalized distortion of metal-ceramic crowns and suggests that external grinding and internal abrasive blasting of crowns are more likely causes of this effect.