Derya Dispinar

Microstructure–bifilm interaction and its relation with mechanical properties in A356

Cooling conditions and inclusions are of the most important factors that affect mechanical properties of cast aluminium alloys. This study investigated the effect of secondary dendrite arm spacing (SDAS) on the shape of pores (i.e. bifilm unravelling) and the mechanical properties of cast A356 alloy. Different cooling conditions were established by electrolytic copper chill, H13 steel chill and insulated ceramic that was placed in the mould cavity. SDAS and shape of pores were investigated by optical microscopy. The fracture surfaces of tensile test samples were analysed by SEM and EDX. Weibull two-parameter statistical method was used to assess the tensile properties. Results show that mechanical properties were dominantly affected by pore morphology that was formed by bifilms. Increasing the cooling rate (i.e. decreasing SDAS), the unravelling of bifilms delayed which decreased porosity formation significantly.

Reduced Pressure Test (RPT) For Bifilm Assessment

The assessment of the cleanness of liquid aluminum alloys has traditionally been viewed as difficult, but has been tackled by a number of techniques, all of which have been comparatively expensive and complicated. In the past the presence of entrained defects known as bifilms in metals has not been realised because they are often only a few molecules thick, and so remain invisible, or at best, difficult to detect. The reduced pressure test (RPT) is sensitive to the size and number of entrained oxides which appear to be the major defects affecting metal quality in terms of workability, mechanical properties and corrosion. In addition, the test is a confirmation of the fact that such oxides are double, and thus conveniently called ‘bifilms’. The RPT is a simple, low cost, but fundamentally appropriate technique for the assessment of (a) the number and (b) the size of bifilms in a melt. This paper outlines an initial draft procedure. Future users will be welcome to suggest improvements.

QUALITY COMPARISON BETWEEN MOLTEN METAL FROM REMELTED SHEETS; MILL FINISH AND COATED

Deterioration of metal quality caused by contamination (e.g. coatings) is an issue in remelting of aluminium scrap. Therefore molten metal quality from remelting sheet material with and without coating is compared. In the experiments the crucibles are placed inside a resistance furnace to ensure that the charges are melted under the same conditions at the same time, measuring temperature and hydrogen. The melts was subjected to (i) settling overnight (ii) blowing air through a porous plug for 2.5 min to generate oxides (iii) settle overnight and adding turnings and (iv) adding carbon. The bifilm index is used as a measure of metal quality. The results show a statistically significant difference in the quality of the samples comparing coated and non-coated for all the melt treatments (i) – (iv), even if the charge material only contained approximately 0.4 % coat.

Freezing Range, Melt Quality, and Hot Tearing in Al-Si Alloys

In this study, three different aluminum-silicon alloys (A356, A413, and A380) that have different solidification morphology and solidification ranges were examined with an aim to evaluate the hot tearing susceptibility. T-shape mold and Constrained Rod Casting (CRC) mold were used for the characterization. Reduced Pressure Test (RPT) was used to quantify the casting quality by measuring bifilm index. It was found that bifilm index and solidification range have an important role on the hot tearing formation. As it is known, bifilms can cause porosity and in this case, it was shown that porosity formed by bifilms decreased hot tearing tendency. As the freezing range of alloy increases, bifilms find the time to unravel that reduces hot tearing. However, for eutectic alloy (A413), due to zero freezing range, regardless of bifilm content, hot tearing was never observed. A380.1 alloy had the highest tendency for hot tearing due to having the highest freezing range among the alloys investigated in this work.

Effect of Degassing and Grain Refinement on Hot Tearing Tendency in Al8Si3Cu Alloy

The effect of melt quality on hot tearing susceptibility of Al8Si3Cu alloy was examined under six different conditions, by using a traditional T-shaped mold. Grain refinement was carried out by two different modifiers: AlTi5B1 and Al3B. For each test, samples were cast before and after degassing of melt. Therefore, a new hot tearing tendency index was developed by both bifilm calculations and porosity that occurred in the middle of T-zone of casting parts. Results indicated that hot tearing of cast aluminum alloys was a complex phenomenon, and bifilms play a major role, especially by compensating for shrinkage and consequently contributing to the inconsistencies in results.

Effects of Casting Conditions on End Product Defects in Direct Chill Casted Hot Rolling Ingots

Direct chill casting is a reliable casting process for almost any wrought aluminum alloy for subsequent deformation via hot rolling to supply vital industries such as aerospace, automotive, construction, packaging and maritime. While some defects occur during casting, like hot tearing, some others like surface defect causing blisters, appear after hot rolling process or annealing after final cold rolling steps. It was found that some of these defects are caused by melt impurities formed from entrained folded aluminum oxides or bifilms. A study in a hot rolling casting facility was carried out with different melt cleaning practices, launder and molten metal transferring designs. Bifilm index and reduced pressure test were used for determining melt cleanliness measurement. It was found that porous plug gas diffusons for degassing are more effective than lance type degassers and a design towards less turbulent molten metal flow from furnace to mould cavity are necessary for reducing defects caused by bifilms.

Melt Cleanliness Comparison of Chlorine Fluxing and Ar Degassing of Secondary Al-4Cu

The treatment of liquid aluminum prior to casting typically consists of purging gas and/or fluxes through the melt. By the use of several chemicals during these operations, several environmental problems can occur. Therefore, in this study, the melt cleanliness of Al-4Cu secondary alloy was investigated by comparing the use of argon degassing with or without chlorine fluxing. Reduced pressure test was used to assess the melt quality. Highest quality melt was obtained by Ar degassing with preheated graphite lance without the need to use any chemicals.

ETİAL 195 Alaşımında Sr Modifikasyonu ve Tutma Süresinin Si Morfolojisi ve Mekanik Özelliklere Etkisi

Otektik ustu Si iceren Al-Si alasimlari tipik olarak asinmaya karsi direnc arzu edilen parcalarda uygulama alani bulmaktadir. Asinma direncini arttiran en temel parametre, Si morfolojisidir. Otektik ustu alasimlarda, Si yapisi dokum sicakligindan etkin bir sekilde etkilenmektedir. Ancak, dokum sirasinda olusabilecek bifilm hatalari Si icin potansiyel heterojen cekirdek kaynagi olabilmektedirler. Bu dogrultuda bu calismada, 800 °C ergitme sicakligin farkli bekleme sureleri sonrasinda dokumler yapilarak, sivi metal icerisinde var olabilecek bifilmlerin Si morfolojisi uzerine etkisi incelenmistir. Bifilm miktari, sivi metalden alinan vakum altinda katilasma testi sonrasinda elde edilen numunelerin kesit alanindaki goruntu analizi ile tespit edilmistir. Uretilen numunelerin mikroyapilari uzerinde de goruntu analizi yapilarak Si morfolojisindeki degisim incelenmistir. Ayni zamanda her parametre icin cekme testi yapilarak Si morfolojisi ile mekanik ozellikler arasi iliski ortaya cikartilmistir Hypereutectic Al-Si alloys are typically used in applications where wear resistance is required. The primary factor that determines the wear properties is the morphology of Si in these types of alloys. Pouring temperature is one of the ways to control Si size and shape. On the other hand, bifilm defects that may be present in the liquid can significantly alter the morphology of Si acting as a heterogeneous nucleant. Therefore in this work, castings were made at 800 °C with varying holding time and the effects of bifilm on the Si morphology was studied. Bifilm content was measured by Reduced Pressure Test and image analysis was used to characterize Si morphology. In addition, tensile tests were carried out to correlate Si morphology with mechanical properties.

Wear properties of squeeze cast in situ Mg2Si–A380 alloy

Abstract In situ composites have been studied mainly to eliminate wettability, porosity and non-uniform distribution of particulate problems in composite materials. Al–Mg2Si alloy are preferred choice of such group of alloys owing to the combination of light weight and improved mechanical properties. The morphology of Mg2Si is one of the key parameters for the wear properties. In this work, squeezed casting method was used to produce samples where porosity was aimed to be reduced and morphology of Mg2Si was altered. In addition, the wear properties of in situ A380–Mg2Si were investigated. It was found that as squeeze pressure was increased, porosity and size of Mg2Si was decreased and wear rate was increased.

Corrosion Behavior of B and Ti Grain-Refined Sr-Modified A356

Corrosion behavior of A356 aluminum alloy with different grain refiners was evaluated. For this purpose, the Sr-modified A356 alloy was grain refined with Al5Ti1B and Al3B, and then, the corrosion characteristics were determined. For the mechanical properties, impact and Vickers hardness tests were carried out. To determine the corrosion behavior, both the electrochemical impedance spectroscopy technique and by potentiodynamic polarization plots were carried out in a 3.5 wt.% NaCl solution at 25 °C. The corrosion resistance of A356 alloy was dramatically improved in sequence with the following modifications compared to the unmodified castings: Sr, Ti and B additions. This situation applies to both different cooling rates, namely die and sand casting. The increase in corrosion rate was more than 80% for all alterations of the microstructure. Additionally, charge transfer resistance was increased ten times in die cast samples, while this value was 2.5 for sand cast samples compared to the unmodified alloy. It is important to note that sand casting of the unmodified alloy was improved four times with regard to the die cast unmodified A356.