Grain refining and modification are common foundry practice for improving properties of cast Al-Si alloys. In general, these types of treatments provide better fluidity, decreased porosity, higher yield strength and ductility. However, in practice, there are still some discrepancies on the reproducibility of the results from grain refining and effect of the refiner’s additions. Several factors include the fading effect of grain refinement and modifiers, inhomogeneous dendritic structure and non-uniform eutectic modification. In this study, standard ALCAN test was used by considering Taguchi’s experimental design techniques to evaluate grain refinement and modification efficiency. The effects of five casting parameters on the grain size have been investigated for A357 casting alloy. The results showed that the addition of the grain refiner was the most effective factor on the grain size. It was found that holding time, casting temperature, alloy type and modification with Sr were less effective over grain refinement.
This paper presents a study of the effect of the modification and cooling rate on the grain count α(Al) in the Al-5Cu alloy. Research was
performed on castings with walls thickness between 3 mm and 25 mm. Cooling curves were recorded to determine the cooling rate and the
degree of undercooling at the beginning of solidification. It has been shown that cooling rate increases exponentially as the wall thickness
of casting decreases. Moreover it has been demonstrated that the cooling rate of castings changes within a wide range (21ºC/s - 1ºC/s)
when the wall thickness changes from 3 up to 25 mm. Metallographic examinations revealed primary grains (primary α(Al) grains). The
paper show that the relationship between the grain count and the degree of undercooling (for non-modified and modified alloys) can be
represented by the equation N = Nv = np·exp(-b/ΔTα), based on the Weibull's distribution of the size of nucleation sites.
The study attempts to investigate the influence of severe plastic deformation (SPD in the hydrostatic extrusion (HE) process on the anisotropy of the structure and mechanical properties of the AA 6060 alloy. Material in isotropic condition was subjected to a single round of hydrostatic extrusion with three different degrees of deformation (ε = 1.23, 1.57, 2.28). They allowed the grain size to be fragmented to the nanocrystalline level. Mechanical properties of the AA 6060 alloy, examined on mini-samples, showed an increase in ultimate tensile strength (UTS) and yield strength (YS) as compared to the initial material. Significant strengthening of the material results from high grain refinement in transverse section, from »220 μm in the initial material to »300 nm following the HE process. The material was characterized by the occurrence of structure anisotropy, which may determine the potential use of the material. Static tensile tests of mini-samples showed »10% anisotropy of properties between longitudinal and transverse cross-sections. In the AA6060 alloy, impact anisotropy was found depending on the direction of its testing. Higher impact toughness was observed in the cross-section parallel to the HE direction. The results obtained allow to analyze the characteristic structure created during the HE process and result in more efficient use of the AA 6060 alloy in applications.
It is well-known that the better the control of the liquid aluminium allows obtaining of better properties. One of the most important defects
that is held responsible for lower properties has been the presence of porosity. Porosity has always been associated with the amount of
dissolved hydrogen in the liquid. However, it was shown that hydrogen was not the major source but only a contributor the porosity. The
most important defect that causes porosity is the presence of bifilms. These defects are surface entrained mainly due to turbulence and
uncontrolled melt transfer. In this work, a cylindrical mould was designed (Ø30 x 300 mm) both from sand and die. Moulds were produced
both from sand and die. Water cooled copper chill was placed at the bottom of the mould in order to generate a directional solidification.
After the melt was prepared, prior to casting of the DC cast samples, reduced pressure test sample was taken to measure the melt quality
(i.e. bifilm index). The cast parts were then sectioned into regions and longitudinal and transverse areas were investigated
metallographically. Pore size, shape and distribution was measured by image analysis. The formation of porosity was evaluated by means
of bifilm content, size and distribution in A356 alloy.
In the present study, the corrosion behaviour of A356 (Al-7Si-0.3Mg) alloy in 3.5% NaCl solution has been evaluated using
cyclic/potentiodynamic polarization tests. The alloy was provided in the unmodified form and it was then modified with AlTi5B1 for grain
refinement and with AlSr15 for Si modifications. These modifications yield to better mechanical properties. Tensile tests were performed.
In addition, bifilm index and SDAS values were calculated and microstructure of the samples was investigated. As a result of the corrosion
test, the Ecorr values for all conditions were determined approximately equal, and the samples were pitted rapidly. The degassing of the
melt decreased the bifilm index (i.e. higher melt quality) and thereby the corrosion resistance was increased. The lowest corrosion rate was
founded at degassing and as-received condition (3.9x10-3 mm/year). However, additive elements do not show the effect which degassing
process shows.
In this work, T-shaped mould design was used to generate hot spot and the effect of Sr and B on the hot tearing susceptibility of A356 was investigated. The die temperature was kept at 250o C and the pouring was carried out at 740o C. The amonut of Sr and B additions were 30 and 10 ppm, respectively. One of the most important defects that may exist in cast aluminium is the presence of bifilms. Bifilms can form by the surface turbulence of liquid metal. During such an action, two unbonded surfaces of oxides fold over each other which act as a crack. Therefore, this defect cause many problems in the cast part. In this work, it was found that bifilms have significant effect over the hot tearing of A356 alloy. When the alloy solidifies directionally, the structure consists of elongated dendritic structure. In the absence of equiaxed dendrites, the growing tips of the dendrites pushed the bifilms to open up and unravel. Thus, leading to enlarged surface of oxide to become more harmful. In this case, it was found that these bifilms initiate hot tearing.
The paper is devoted to grain-refinement of the medium-aluminium zinc based alloys (MAl-Zn). The system examined was sand cast Zn10
wt. %. Al binary alloy (Zn-10Al) doped with commercial Al-3 wt. % Ti – 0.15 wt. % C grain refiner (Al-3Ti-0.15C GR). Basing on the
measured attenuation coefficient of ultrasonic wave it was stated that together with significantly increased structure fineness damping
decreases only by about 10 – 20%. The following examinations should establish the influence of the mentioned grain-refinement on
strength and ductility of MAl-Zn cast alloys.
The article presents research aimed at determining the effect of adding rare earth elements to near-eutectic Al-Si and Al-Si-Ni alloys on the microstructure and mechanical properties of the obtained products. Material for the research was prepared using a melt spinner – a device used for rapid crystallization, casting thin ribbons, which were then subjected in subsequent stages to fragmentation, consolidation and plastic working. The ribbons and extruded rods cast were described in terms of their structure and their strength properties were determined at different measurement temperatures. It was shown that the lightweight materials produced from aluminium alloys using the rapid solidification process have an ultra-fine structure and good strength properties.
Analysis under a microscope confirmed that the addition of rare earth alloys Al-Si and Al-Si-Ni causes fragmentation of the microstructure in the tapes produced. The presence of rare earth elements in the alloys tested has an impact on the type and the morphology of the particles of the microstructure’s individual components. In addition to the change in particle morphology, the phenomenon of the separation of numerous nanometric particles of intermetallic phases containing rare earth elements was also observed. The change in microstructure caused by the addition of rare earth elements in the form of a mischmetal increases the mechanical properties.
The paper presents results of measuring attenuation coefficient of the Al-20 wt.% Zn alloy (AlZn20) inoculated with different grain
refiners. During experiments the melted alloys were doped with Al-Ti3-C0.15 refining master alloy. Basing on measurements performed
by Krautkramer USLT2000 device with 1MHz ultrasound wave frequency it was stated that grain refinement reduces the attenuation
coefficient by about 20-25%. However, the examined alloys can be still classified as the high-damping ones of attenuation greater than 150
dB/m.
The paper gives an introduction to nanostructuring techniques used for industrial fabrication of bulk nanocrystalline metals – basic
materials utilized in shaping nanoscale structures. Nanostructured metals, called nanometals, can be produced by severe plastic deformation (SPD). We give an expert coverage of current achievements in all important SPD methods and present future industry developments and research directions including both batch and continuous processes. In the laboratories of both WUT and UOS we have developed industry standard equipment and machinery for nanometals processing. Utilizing the latest examples from our research, we provide a concise introduction to the field of mass production of nanometals for nanotechnology.
The presented results describe the effect of severe plastic deformation on the structure and mechanical properties of AA5083 and AA5754 alloys. Both materials were subjected to single hydrostatic extrusion (HE) and cumulative hydrostatic extrusion in the case of AA5083 and a combination of plastic deformation by equal-channel angular pressing (ECAP) with the next HE for AA5754. After the deformation, both alloys featured a homogeneous and finely divided microstructure with average grain size deq = 140 nm and 125 nm for AA5083 and AA5754, respectively. The selection of plastic forming parameters enabled a significant increase in the UTS tensile strength and YS yield stress in both alloys – UTS = 510 MPa and YS = 500 MPa for alloy AA5083 after cumulative HE, and 450 MPa and 440 MPa for alloy AA5754 after the combination of ECAP and HE, respectively. It has been shown on the example of AA5083 alloy that after the deformation the threads of the fasteners made of this material are more accurate and workable at lower cutting speeds, which saves the cutting tools. The resultant properties of AA5083 and AA5754 alloys match the minimum requirements for the strongest Al-Zn alloys of the 7xxx series, which, however, due to the considerably lower corrosion resistance, can be replaced in many responsible structures by the AA5xxx series Al-Mg alloys presented in this paper.
This paper presents results of a study of the effect of inoculation of yttrium on the microstructure of AZ91 alloy. The concentration of the inoculant was increased in samples in the range from 0.1% up to 0.6%. The influence of Y on the thermal effects resulting from the phase transformations occurring during the crystallisation at different inoculant concentrations were examined with the use of Derivative and Thermal Analysis (DTA). The microstructures of the samples were examined with the use of an optical microscope; and an image analysis with a statistical analysis were also carried out. Those analyses aimed at examining oh the effect of inoculation of the Y on the differences between the grain diameters of phase αMg and eutectic αMg + γ(Mg17Al12) in the prepared examined material as well as the average size of each type of grain by way of measuring their perimeters.
Ferrtic/martensitic and ODS steels were fabricated by the mechanical alloying process, and their microstructures and mechanical properties were investigated. The 9Cr-1W and 9Cr-1W-0.3Ti-0.35Y2O3 (in wt.%) steels were prepared by the same fabrication process such as mechanical alloying, hot isostatic pressing, and hot rolling processes. A microstructural observation of these steels indicated that the Ti and Y2O3 additions to 9Cr-1W steel were significantly effective to refine the grain size and form nano-sized Y-Ti-O oxide particles. As a result, the tensile strengths at room and elevated temperatures were considerably enhanced. Considerable improvement of the creep resistances at 700°C was also evaluated. It is thus concluded that 9Cr-1W ODS steel with Ti and Y2O3 additions would be very effective in improving the mechanical properties especially at elevated temperatures.
In this study, the effects of grain size refiner addition and various pre-heating mold temperatures on AlSi9 cast alloy microstructure and solidification have been evaluated. For different process conditions, thermal analysis was performed for all samples and cooling curves were established. Important parameters in liquidus and eutectic Si-phase regions have been calculated using the first derivative cooling curves. Secondary Dendrite Arm Spacing (SDAS) variation was also determined. Experimental results question the effectiveness of cooling curve parameters in providing the microstructure data as a function of refinement. The present work shows that the effect of grain refiner addition on the value of SDAS was higher when the solidification time was lower. It indicated that the solidification parameters such as nucleation temperatures of α-Al phase, undercooling temperature and total solidification time were affected by grain refinement. It has been found that the addition of grain refiner affect the eutectic phase formation time. However, it has no effect on the eutectic phase morphology.