The effect of swarf addition to the melt and its melting sequence during slurry preparation in a high pressure die-casting

Hooman Hadian1, a *,Mohsen Haddad Sabzevar1,band Mohammad Mazinani1,c

1Department of Materials and Metallurgical Engineering, Faculty of Engineering,

Ferdowsi University of Mashhad, Mashhad, Irana ho.hadian@stu.um.ac.ir, b haddadm@um.ac.ir, c mazinani@um.ac.ir

Keywords: slurry, swarf, AS9U3 alloy, solid fraction, dendrite, globular

Abstract. An internal cooling agent is used in rapid slurry forming (RSF) process to produce a high solid fraction slurry for a short period of time. In the process used in the present investigation, the metallic swarf which is known to be a low enthalpy material was employed as the internal cooling agent and was added to the melt. During the processing of slurry, the swarf started to melt and then a high solid fraction slurry was formed. The process was described by exchanging the enthalpies between the low and high enthalpy materials during slurry preparation process. A commercial Al-Si-Cu alloy, i.e.AS9U3aluminium alloy, was selected for this investigation. The examination of the microstructure showed that the Al-Si eutectic colonies start to melt resulting in the formation of globular Alpha-Al particles during the melting process of swarf material due to the multiplication of secondary dendrites arms. The breakage of the dendrite arms and the spheroidization of broken particles were suggested to be the origin of the non-dendritic globular particles in the final microstructure. The amount of primary globular Alpha-phase was measured by the image analysis software and the results showed that around 35 percent of the solid fraction has been formed at the temperature of 580 ºC for the case of AS9U3 Aluminum alloy in a high pressure die-casting process using the specimens 4mm in thickness.

1. Introduction

The unique behavior of semi-solid Aluminum alloys has always made this method as one of the main ways to produce parts close to their final shape [1].The purpose of this casting method is to change the dendritic structure of the alloy to non-dendritic one in order to improve its microstructure and mechanical properties. An important disadvantage of semi-solid casting processes is the duration of the process. New researches have led to the development of a variety of techniques that allow the production of non-dendritic structures in the shortest possible times [2]. In these methods, the non-dendritic structure is produced by introducing a localized internal cooling zone associated with stirring the molten alloy [3-4].

The main reason for the use of 356 aluminum alloy in the production of semisolid castings is the existence of an extremely freezing range of this alloy. In other words, because of the high freezing zone, the temperature sensitivity of the alloy is low, and this lowers the amount of solid fraction [5]. Therefore, it is possible to produce semi-solid slurry of this alloy. AS9U3 Aluminum alloy is the most popular and most widely used high-pressure die cast alloys with the engineering application, such as engine parts, radiators and gas regulators. This alloy will have very high temperature sensitivity, due to its low of freezing rang, which will change with the smallest changes in the temperature of the solid fraction. In the present study, the addition of a swarf surface in the vicinity of the molten metal by changing the instantaneous melt enthalpy in AS9U3 Aluminum alloy will provide the possibility for the production of semi-solid slurry by the compensation the temperature sensitivity of the alloy [6].

The relationship between the melting stages and the formation of a semi-solid -solid solution of the Aluminum alloy using enthalpy exchanger materials at various times using the RheoMetal business method has been investigated by Sledge et al[7]. Microstructural studies with regard to this

method have shown that three different regions, i.e. single free zone layers, liquid and solid phases, and an initial melting zone are formed in the early stages of the process. By increasing the process time, most of the Al-Si eutectic phases begin to melt and spherical particles of-aluminum is formed.

2. Materials and methods

In this phase of the study, the effectiveness of adding swarf to the melt prepared to be injected into a high-pressure die casting machine was studied and tested. In the first method, the swarfs were added from top to the melt was surface and stirred for about 20 seconds. In the next method, the swarf were placed on a pre-heated surface with the temperature of 200 °C and the melt was then stirred for about 20 seconds.

The reference sample casting operation was carried out at 590 °C to investigate the microstructural changes. Homogenization process was conducted at three temperatures of 570, 580, 590 ° C. The production of metallic chips with desirable sizes from the aluminum ingot required for the process was performed using the milling machine. After washing and separation of the iron particles from the mix, the specimens were dried and classified with the mesh size 4. The amount of solid fraction in microstructures was calculated by the Scheil equation for each alloy[8].In order to prepare the samples for metallography, they were first polished using the diamond paste and were then etched with a keller solution (50ml H2O, 15ml HCL, 25ml HNO3 and 10ml HF) for the optical microscopic examination. The etching time was 15 seconds. Size and shape factor of the phases were determined using MIP image analysis software. The shape factor was calculated using Eq. 1[2].  In this equation A is the area and P is the perimeter of the phase in the microstructure.

Fshape = 4πA/P2 (1)

3. Results and discussion

3.1. Chemical compositionThe chemical composition of the cast samples is given in Table 1.

Table 1.The chemical composition of test samples in this investigation (in weight percent)

Element Silicon Iron Copper Mangenese magnesium Nickel Zinc Lead Tin

Percentage 9.38 0.8 1.86 0.33 0.11 0.43 2.3 0.42

0.19

3.2. Evaluation of the microstructure of reference sample (No swarf).As it is observable in the microstructure of the reference sample at 590 ° C, as in Fig. 2, the

growth of the dendritic initial -phase with the eutectic mixture has happened.Due to the high speed of injection of the melt in the high-pressure die casting machine as well as relatively low thickness of the components produced in this method, the freezing rate was very high and the average diameter of the dendrites was measured to be about 32 m.

Figure 2: The microstructure of the reference material sample at 590 ° C.The flake like phase can be seen in the microstructure of this sample which is probably silicon rich phase and since the alloy has about 2% copper, the copper particles and intermetallic compounds might be formed during freezing as in Fig. 3

Figure 3: Metallography image of silicon and copper phases in the alloy at 590 ° C.3.3. Evaluation of the microstructure of sample containing swarf. By the addition of swarf,

the primary -phase has become spherical in shape (Fig. 4).One can observe oxide phase and inclusions and in some areas of the microstructure the primary

-phase are coagulated and collided, all of which confirm the effect of scattering, melting and re-freezing. In addition, the number and size of solid -phase in the sample are increased. The uniformity of non-dendritic structure of the sample indicates the role of swarf in increasing and lengthening the primary -phase.

Figure 4.Metallographic image by adding 8 percent of weight with swarf and stirring 20 seconds, at590 ° C.

The results of image analysis of reference samples solidified at 590 °C with or without swarf are given in Table 2. Due to the uniformity of the melting temperature, the mean diameter of primary

Oxide shells and impurities

Growth of secondary Alpha phase

Growth and spherically of primary Alpha phase

-phase in the sample containing the swarf is increased. This may be attributed to the presence of swarf and its localized heat exchange with the melt by creating cold regions. Furthermore, the increase in the shape factor of -phase indicates its distribution and spheroidization in the microstructure which have occurred by the addition of swarf and stirring the melt.

Examples adding Swarf

Reference sampleCharacteristic

94.748.1Initial -solid phase average (μm)89742307The average area of primary -solid phase (μm2)0.4050.206Initial -Phase Shape Factor

Table2. The results of image analysis of microstructures for samples produced at 590 ° C.

After casting the specimens at three temperatures of 570, 580 and 590 ° C with the addition of 8% wt. swarf to the melt and stirring it for 20 seconds, the image analysis of the microstructures using the optical microscope was carried out. The microstructures of the specimens produced at 580 and 590 °C by the addition of swarf from the surface before injecting it to the casting machine show that the primary -phase at the temperature of 590 °C is somewhat globular with the shape factor of 0.405, however, the shape factor is 0.643 for the case of the specimen cast at 580 °C suggesting that the specimens produced at 580 °C are more globular than other specimens. The microstructures of the specimens produced at the same temperatures by the addition of swarf from the bottom indicate that the primary -phase in the specimen cast at 590 °C is somewhat globular with the shape factor of 0.551 whereas this is 0.623 for the specimens cast at the temperature of 580 °C and hence, one may clearly see that the specimens produced at 580 °C have become more globular.

Casting temperature is one of the most important and effective factors in the process. This parameter has a significant effect on the amount of solid fraction. If this temperature drops significantly, it causes an inappropriate increase of solid fraction resulting in an increase in the average grain size and degree of agglomeration and a decrease in spheroidization. Conversely, at very high casting temperature, the heat that is introduced into the system is increased and solid and semi-solid regions become more limited on the surface and will be gradually disappeared.

Figure 5.Metallographic image casting by a high-pressure die casting machine with swarf and stirring 20 seconds (a) at 590 ° C (b) at 580 ° C.

The solid fraction, which in fact represents the process temperature, has a great influence on the microstructure and the behavior of the semi-solid slurry. By an increase in the solid fraction, the average grain size increases. On the other hand, stirring the melt for a few seconds causes the displacement and distribution of these grains throughout the melt and accordingly, the grains become more spherical. The evaluation of the fraction of phases in the microstructures by metallographic examination of the specimens at different temperatures and different methods of adding the swarf to the melt indicates that the primary solid phase has increased relative to the eutectic phase. This

a b

globularphase

globularphase

Initial -phase Shape Factor

Initi

al

-Pha

se S

hape

Fac

tor

The

ave

rage

are

a of

pri

mar

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-pha

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experimental evidence confirms the effectiveness of the swarf and its melting and re-freezing. The effect of primary solid phase fraction on its grain size (average area) and spheroidicity is shown in Fig. 6. It can be said that by increasing the primary -phase, the average grain size (average area) increases. This seems reasonable since the amount of frozen material and the freezing time at lower temperatures have been increased. It can also be seen that by increasing the solid fraction, the spheroidicity increases up to the temperature of 580 °C and then decreases beyond this temperature. One reason for the reduction of spheroidicity is that the grains are enlarged and more collision occurs for these large grains.

Figure 6: The effect of solid -phase fraction on the area and the shape factor. According to the results of Scheil equation and the MIP picture analysis software for each alloy, the solid fraction decreases with an increase in the casting temperature. The solid phase at 580 °C was measured to be around 35%.

4. Conclusion

In this study, the effect of adding swarf during semi-solid casting (pressure casting) of the aluminum alloy AS9U3 was examined and the microstructure of the cast alloy was investigated. The swarf particles were added to the melt at three temperatures of 570, 580, 590 °C from the surface and bottom of the melt was then stirred for 20 seconds for its homogenization. The results of this research can be summarized as follows:

1- According to the previous findings, aluminum alloy 356 is suitable for semi-solid casting due to its wide range of freezing and low temperature sensitivity. However, the experimental results with regard to other aluminum alloys having a low freezing range and high temperature sensitivity show that by adding swarf to the melt and consequent sudden changes in enthalpy within the melt before injection into a high pressure die casting machine, it is possible to change the dendritic structure to globular.

2- Due to the freezing range between 603 °C and 548 °C, the AS9U3 alloy showed an increase in the number of globular solid -phase at the melting temperature of 580 °C.

3- Decreasing the injection temperature from 590 °C to 580 °C or increasing the primary solid fraction can change to some extent from dendritic to spherical structure. However, injection process at the temperatures lower than 580 °C causes coarser microstructures with less grains spheroidicity.

4- The condition under which the spheroidicity of 64.3% was obtained for the AS9U3 alloy at the temperature of 580 °C was considered to be the most suitable condition for changing dendritic to globular structure using the solid content of about 35%.

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