1 INTRODUCTION

High-pressure die casting using machines with horizontal cold chambers (currently the most common process for manufacturing near-net shape cast components) allows very high production rates with close dimensional tolerance and a good surface finish. A description of the injection process in a machine of this type and of the stages into which this process is usually divided can be found in a companion paper (Zamora et al. [1]). The amount of trapped air during the initial slow injection stage may represent a considerable contribution to the total mass of trapped air which gives rise to porosity in the manufactured part. This is particularly so when inappropriate operating parameters are used during injection. Previous investigations aimed at determining adequate values of these parameters were mostly based either on analytical [2,3] or numerical [4,5,6] approaches, or on experiments carried out using water as the working fluid [7,8]. However, there are few experimental studies in real casting machines [9,10,11], and those that exist do not especially focus on the slow injection stage.

Air entrapment mechanisms in the injection chamber are related to the behavior of the surface wave of molten metal caused by the plunger motion. As explained by Tszeng and Chu [2], if the plunger reaches a speed higher than a certain optimum value, the wave will reflect against the chamber ceiling and its forward face might roll over, causing air entrapment, as shown in figure 1(a). On the other hand, if the plunger speed does not reach this optimum value, the wave might reflect against the end wall of the chamber and trap air in front of the plunger and along the chamber ceiling, as shown in figure 1(b). The optimum maximum plunger speed has been studied experimentally in a real die casting machine, as described in the mentioned companion paper [1]. Obviously, the dynamics of the free surface of molten metal and air entrapment effects also depend on the plunger acceleration law. The main objective of this work was to investigate experimentally, in a die casting machine and real operating conditions, and for a given plunger acceleration law, the influence of the maximum plunger speed during the slow injection stage on the spatial distribution of entrapped air at the end of this stage. Experiments involving different initial filling

ABSTRACT: The influence of the maximum plunger speed on the occurrence of different air entrapment mechanisms during the slow injection stage in horizontal die casting injection chambers is investigated experimentally. The experiments were carried out for different operating conditions in a real high-pressure die casting machine. In order to identify the different zones in the injection chamber where, depending on the operating conditions, the air may be trapped, the injection process was stopped at the end of the slow stage. Porosity was measured in different regions of the casting obtained with different plunger speeds and initial filling fractions. Prevailing spatial distributions of trapped air for given operating conditions in some casting cuts are shown.

Key words: High-pressure die casting, porosity, air entrapment mechanisms, injection chamber.

Experimental Investigation of Air Entrapment Effects in Die Casting Injection Chambers

R. Zamora1, J. Sanes1, F. Faura1, J. López1, J. Hernández2

1Dept. de Ingeniería de Materiales y Fabricación, ETS de Ingeniería Industrial, Universidad Politécnica de Cartagena, c/ Doctor Fleming s/n, E-30202 Cartagena, Spain. URL: www.upct.es e-mail: rosendo.zamora@upct.es, pepe.sanes@upct.es,

felix.faura@upct.es, joaquin.lopez@upct.es 2Dept. de Mecánica, ETS de Ingenieros Industriales, UNED, Ciudad Universitaria, E-28040 Madrid, Spain. URL: www.uned.es e-mail: jhernandez@ind.uned.es

fractions and different maximum plunger speeds were carried out. In order to analyze the distribution of entrapped air in the chamber during the slow stage, the injection shot finished when the chamber was completely filled with metal. The resulting castings were transversally divided into a number of equally sized parts. It was expected that the analysis of the results would allow us to relate the measured porosity distributions to the different air entrapment mechanisms that may occur for operating conditions far from the optimal.

(a)

(b)

Fig. 1. Typical free surface profiles when the plunger speed is (a) higher and (b) lower than the optimal speed.

2 EXPERIMENTAL PROCEDURE

2.1 Equipment and instrumentation

The die casting machine, the instrumentation and the composition of the aluminum alloy used in the experiments are described in a companion paper [1]. The measurement of porosity in the castings was carried out using the classic Archimedes method.

2.2 Description of experiments

As already mentioned, in all the experiments the injection shot was stopped when the chamber was almost completely filled, except for the space occupied by the trapped air (the final pressure in the chamber was approximately the same in all the experiments). Figure 2 shows a schematic representation of the injection chamber at this instant. The same series of manufactured castings described in [1] were analyzed in the present work. Figure 3 shows the castings of different series corresponding

Fig. 2. Schematic representation of the injection chamber at the

end of the injection shot.

to an initial filling fraction f = 25.2 %. The castings for which the measured plunger speed law failed to reproduce the desired law closely enough, or those with an excessive error in weight, were discarded using the same criteria as in [1].

Fig. 3. Casting series corresponding to an initial filling fraction of 25.2 %.

In order to analyze the porosity distribution, each casting was divided into a number of equally sized parts, depending on the size of the casting. For a given chamber, this size is obviously determined by the initial filling fraction. For the three filling fractions, for which results are presented below, the castings were divided into two (for f = 25.2 %) and three parts (for f = 37.4 % and 50 %), as can be seen in figure 4, corresponding to the zones close to the plunger and to the chamber end wall, and, in the latter case, also to the middle zone. Possible pores existing at the surface of any part were covered with adhesive plastic tape.

Fig. 4. Division of castings into parts in order to measure porosity.

Die Halves

Pouring hole

Injection chamber

Plunger motion

Casting

Molten metal

3 RESULTS AND DISCUSSION

Figure 5 shows some of the results obtained for the mean value of the porosity measured and the 95 % confidence interval (based on a Student's t distribution) in each part of the casting as a function of the maximum plunger speed, for three different initial filling fractions and a 50 mm diameter chamber. The porosity in each part was calculated as (ρ0-ρp)/ρ0, where ρ0 is the alloy density (2.68 g cm-3) and ρp is the part density. It is worth noting that the cut of the casting into various parts may cause the division of existing air cavities, which in turn makes the uncertainty interval for the porosity of each part to be higher than that for the total casting porosity. It can be observed from figure 5 that the measured porosity tends to be higher in the middle sections of the chamber or near the plunger for low plunger speeds, whereas for high plunger speeds it tends to be accumulated in the sections close to the end wall. Notice that the porosity measured in part A, which is adjacent to the chamber end wall, is low and roughly constant for low plunger speeds and tends to increase with plunger speed for sufficiently high values of this speed. This increase can be attributed to the effects of wave breaking caused by the impact of molten metal against the chamber ceiling. For parts B and C, porosity variations with plunger speed are less pronounced and, in general, non-monotonic. A flow pattern similar to that of figure 1(b) is expected to be responsible for the relatively high porosity level in parts B and C at moderately low plunger speeds, for which porosity in part A is low. Figure 6 shows photographs of longitudinal sections of two castings obtained with f =37.4 % and two plunger speeds lower and higher, respectively, than the optimal, and figures 7 and 8 show a transversal X-ray photograph and a standard photograph, respectively, of two different castings obtained for f = 50 % and a high plunger speed. In the case of figure 8, the plunger was stopped when the molten metal filled the runner. The locations of the zones of trapped air in all these figures are consistent with the results of figure 5.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0

2.0

4.0

6.0

8.0

Poro

sity

(%)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0

2.0

4.0

6.0

8.0

Poro

sity

(%)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0

2.0

4.0

6.0

8.0

Poro

sity

(%)

Plunger speed (m s-1)

f = 25.2 %

f = 37.4 %

f = 50 %

AB

ABC

ABC

Fig. 5. Porosity measured in parts A, B and C, for initial filling fractions of 25.2 %, 37.4 % and 50 %.

(a)

(b)

Fig. 6. Longitudinal sections of two different castings for f = 37.4 % and plunger speeds (a) lower and (b) higher than the

optimal.

Fig. 7. X-ray photograph of a casting obtained for f = 50 % and

a high plunger speed.

Fig. 8. Photograph of a casting obtained for f = 50 % and a high plunger speed.

4 CONCLUSIONS

Experiments in a real high-pressure die casting machine were conducted to investigate the dependence of the spatial distribution of trapped air in the molten metal during the slow injection stage on the maximum plunger speed. Despite the limitations of the injection control system to reproduce accurately the desired plunger motion law and final chamber pressure, the uncertainties introduced by the manual pouring of molten metal and shrinkage effects, the results for porosity distributions in different zones of the injection chamber are consistent with the air entrapment mechanisms that are expected to occur for the corresponding plunger speed ranges. In future studies, we will try to overcome the mentioned limitations, and the influence of the plunger acceleration law will be studied.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support of the Spanish Ministerio de Ciencia y Tecnología (MCYT) under grants DPI2001-1390-C02 and PB98-0007, and the MCYT and the European Commission under grant 1FD97-2333. We would also like to thank Mr. Pedro Belmonte for assisting with the experiments, and to Mr. Sebastián Gallardo for his advice and help in the arrangement of the experimental set-up.

REFERENCES

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