1. Introduction
Technology of high pressure die casting (HPDC) represents a special type of die casting method in modern technology of metal processing, which requires minimum or even null finishing operations in the case of parts production. Owing to its high productivity, HPDC is widespread in different industrial sectors [
1]. Casts produced in the die casting process are characterized by high geometrical precision, good mechanical properties, and low price. Mechanical properties of casts are closely related to formation of fine-grained structure in the case of fast melt cooling with the mould face [
2]; however, defects such as porosity, primarily caused by air entrapment by the melt during the filling phase, significantly influence the quality of casts [
3].
Due to increased porosity occurrence and different share of air entrapment in the cavities, which occur as the result of shrinkage or of gas entrapment in the mould cavity, the porosity type identification in the HPDC casts appears to be complicated. In general, occurrence of porosity in the internal structure of casts is ascribed to change in the state of matter (liquid/solid matter). The cavities, with size ranging from microns to millimetres, depend on the type of metal alloy and on solidification process of casts. Formation of cavities in the structure of casts originates in the melt shrinkage during solidification, in distribution of gasses present directly in the melt, or in combination of these factors, including gaseous porosity caused by gas entrapment in the course of the cycle of high pressure die casting (varying plunger speed, venting, and gating system design) [
4,
5,
6].
To produce a high quality and reliable cast during the HPDC process, synchronization of several process parameters is required. Therefore, the HPDC process must be considered as a reciprocally correlating mechanism of structure of a die casting machine, mould designs, and setup of technological parameters of the casting process [
7,
8]. Therefore, it is important to focus on the issue of reduction of porosity formation in the volume of casts partly in the individual design phases and to find the most adequate intersection of the individual solutions [
9,
10].
Currently, the factor influencing the reduction and compression of pores most discussed is holding pressure. It has been proved that increased values of holding pressure positively affect the distribution and size of pores by means of which they influence the tightness of pores. After solidification of gates, the volume of pores slightly increases, which is caused by the melt shrinkage during solidification. On the other hand, increase of holding pressure shortens the service life of a mould [
7,
11,
12].
Out of different process variables, a significant factor influencing gas entrapment in the volume of the cast is the melt speed in the gate. The speed directly determines the filling mode of the mould-shaping cavity. The melt speed in the gate is directly proportional to the speed of the pressing plunger in the filling chamber. Higher pressing speed changes the character of the flow in the runner from laminar–planar to turbulent–non-planar, which causes discontinuous melt flow. By means of the pressing speed reduction, the melt flow can be tranquillized, which results in a continuous and regular face of the melt flow in the cross-section of the runner. On the other hand, extreme prolongation of the casting cycle at low pressing speed leads to a decrease of the melt temperature, which may result in occurrence of defects such as incomplete topping up, cold laps, and weld lines [
13,
14,
15,
16].
Apart from establishing the technological parameters of the die casting, a crucial factor assuring the quality of castings is the design of the gating system [
3]. In general, the design of the gating system should allow for fast filling of the mould-shaping cavity by the liquid melt so that the metal flows through the mould cavity along straight trajectories without abrupt changes in the melt flow at wide angles [
17,
18,
19,
20]. Air entrapment by the melt during the filling phase represents one of the basic reasons for porosity of casts. Correct design of the gating system can influence the filling mode of the mould cavity filling, which is influenced by a number of factors including shape and massiveness of the cast, weight ratio between the cast and the mould, gating system disposition, shape and area of the gate, mould cavity volume, and the area and arrangement of venting holes. It has been proved that approximately 90% of defects of casts produced in the die casting process are caused by the errors in the gating system design [
16,
21,
22,
23].
A rather significant yet often underestimated factor in the case of die casting of metals is correctly setting the batch weight/volume of material required for a single operation. The weight is determined by a sum of weights of a clean casting in a single-cavity mould and, in case the weight is given by weight of all casts, by weight of the fins, runners, and residues in the filling chamber. In total, it is weight/volume of metal poured into a shot chamber per single operation [
24].
Utilization of a batch is given by the ratio of the sheer weight of the cast and batch weight, which is usually 25–30% in the case of small and medium size casts and 50–60% in the case of heavy casts, reaching even 80% in the case of very heavy casts. The metal residues in a filling chamber have the greatest weight share in the batch weight. As the batch weight significantly influences the price of the cast, the returnable material such as runners, fins, and residues in the chamber must be reduced to a minimum amount technologically required for production of high-quality casts [
25].
In the case of a horizontal filling chamber, the die casting ratios are more advantageous than in the vertical case. If the pressing plunger reaches the mould joint, the biscuit height in the filling chamber can be minimal, provided that the metal dose per single operation is correct [
26,
27].
This paper deals with the issue of how height influences the porosity of castings in a runner residue in the filling plunger. The sets of casts were produced with variable biscuit height Z in the case when the biscuit height was selected with values of Z1 = 10, Z2 = 20, and Z3 = 30 mm. Based on evaluation, it was detected that increasing height of biscuit Z causes decreasing values of cast porosity. Consequently, simulations of the pouring cycle were conducted by means of the programme MagmaSoft 5.4 with the variable height of biscuit Z, which were used to identify the main causes of changes in porosity share in casts. The monitored values were temperature of the melt in the filling chamber, temperature of the melt in the area of the gates, and the mode of the melt flow in the runners. Based on simulations, it was proved that increasing height of biscuit Z, which is closely connected to batch volume, resulted in an increase of the melt temperature in the areas of the measuring spots. This also influences the period of the melt solidification in the area of the gates, which directly conditions the period duration of the holding pressure effect and pore reduction in the cast volume. Concurrently, with the lower height of the biscuits, the ratio of the metal and gas volume in the gating system changes as well, by means of which the mode of the melt flow in the runners is affected. Correlation of these factors, i.e., lower temperature of the melt under the concurrent change of the mode of melt flow, results in an increase of porosity share in the volume of casts with a decreasing biscuit height Z.
2. Materials and Methods
Evaluation of biscuit height influence on the porosity values of casts and consequent cause analysis of changes were conducted with the cast of the electromotor flange with the relevant gating system (
Figure 1).
Porosity share in the castings was examined at the points in which further mechanical machining of the casts is performed (
Figure 1—porosity monitoring locations). The points were evaluated as crucial to the possibility of revealing cavities in the cast volume in machining of the holes.
The influence of the biscuit height on porosity share was monitored. The biscuit height was conditioned by batch volume or by metal dose volume per single operation.
Table 1 shows the basic volume characteristics of the gating system together with variable values of batch volume and biscuit height Z.
To assure relevant results that demonstrate the influence of biscuit height on porosity values in the monitored areas, the individual series of casts were die cast with a consistent setup of technological parameters of the die casting cycle. The values of technological parameters used in the setup are shown in
Table 2.
Inner homogeneity of the analysed casts was primarily assessed by means of a nondestructive method in the RTG laboratory through evaluation of the images generated by RTG VX1000D (North Star Imaging, Marlborough, Marlborough, MA, USA) equipment. Consequently, the share of porosity in the selected points was evaluated as percentage share of pores in the scratch pattern of the monitored point. Porosity analysis of scratch patterns of the specimens was performed using an OLYMPUS GX51 (Tokyo, Japan) microscope at 100× consequently, the analysis was processed by the programme ImageJ.
Possible causes of change of porosity share in casts with regard to change of biscuit height were examined by means of the programme Magmasoft MAGMA 5.4.1—HPDC module. Setup of input parameters for the simulation was identical to the setup of technological parameters of the die casting cycle for the case in which die casting of testing casts was performed (
Table 2). To improve accuracy of simulation and to obtain a more detailed description of the target entity, a grid with high fineness and efficiency of generation was formed. In the case of the die casting simulations, a fine grid with 89,464,224 cells was used. However, the gating system consisted of 1,671,742 cells.
To clarify the causes of changing porosity under the influence of different biscuit heights, it was necessary to establish the following assumptions:
To a high degree, the size and distribution of pores in casts are influenced by holding pressure. Its influence is conditioned by the action of hydrostatic pressure on the mould-shaping cavity. The action of holding pressure upon the melt in the mould-shaping cavity stops at the moment solidification of the melt is finished in the area of the gates, which is closely connected with its initial temperature when entering the particular point. The following opinion was expressed: the lower dose volume of liquid metal per single operation has lower thermal capacity and the melt has a tendency to premature solidification when passing through the gating system, thus reducing the period of holding pressure acting upon the melt in the mould-shaping cavity.
The change of the metal volume dose per single operation leads to a change of the melt/gas ratio in the gating system. The filling time of the mould cavity filling was set to 16.14 ms, which implies that, with the biscuit height Z1 = 10 mm and within the same time interval, a higher volume of gasses and vapours are inevitably forced out than in the case of the biscuit with a height of Z3 = 30 mm. In such a case, a more striking mixture of melt and gas may occur, which directly increases porosity share in the cast volume.
Verification of the premise (a) was conducted by the application of module HPDC, Results—Filling/Temperature, for the case in which the monitored temperature of the melt in the filling chamber and in the area of the gate is at the points shown in
Figure 1. To determine the solidification period of the melt in the area of the gate and to determine the period of holding pressure action, we used the HPDC module: Results/Solidification and Cooling until Eject/Fraction Solid. Assessment of the melt flow mode in the gates, verification of the premise (b), was conducted by applying the Results —Filling/Temperature module in HPDC concurrently with the evaluation of thermal conditions in the melt.
4. Discussion
The experiments conducted proved the remarkable influence of biscuit height and liquid metal dose per single operation on cast porosity. It was proved that decreasing biscuit height leads to an increase of porosity in the casts. The aim of this paper was not only to describe dependence of the change of porosity of casts on biscuit height, but to clarify the causes of these changes as well.
The first cause of change in cast porosity is the thermal capacity of the metal comprised in the volume of dose per single operation. It was proved (
Table 3) that in the case of a higher volume of metal dose per single operation with a constant pouring temperature of 705 °C, the melt flowing in the filling chamber cooled more slowly. This fact is related to the thermal capacity of the melt volume in the case in which the higher metal volume has, logically, higher thermal capacity.
Insufficient cooling of the melt in the filling chamber correlates with the melt temperature in the area of the gates. Based on
Table 4, it is evident that in the case of a dosing volume of 481.05 cm
3, corresponding to the biscuit height Z3, the amount of heat in the melt volume can compensate for heat loss occurring when the melt passes through the gating system. Due to this aspect, the measuring point—the area of the gate and mould-shaping cavity—are supplied with the “fresh melt“ at a higher temperature, which positively influences porosity elimination.
These observations indicate that the action period of holding pressure is prolonged, which results in diminishing size and distribution of pores in the cast.
A significant factor is also the volume of gases and vapours in the gating system prior to triggering of the pouring cycle. According to
Table 6, the volume of gases in the gating system at biscuit height Z1 is 1307.82 cm
3, which is 76.94 cm
3 more than the 1230.89 cm
3 volume for biscuit height Z3. The volume difference must be forced out of the mould cavity during the same period of time, i.e., 16.14 ms. A more massive mixture of gasses and melt occurs when the melt passes through the gating system, which is shown in
Figure 5. In this situation, with biscuit height Z1 and with the overall filled volume of the gating system reaching 94%, the occurrence of gas in the runners can be observed during concurrent filling of the cast. The gas is consequently transported into the volume of the cast, as shown in
Figure 6, which also explains the large increase of porosity of the casts produced at biscuit height Z1, contrary to casts with biscuit height Z2 and Z3. In simple words, the volume of gasses contained in the gating system with the biscuit height Z1 is incapable of release at the melt flow face, and is therefore mixed with liquid metal.
Thus, it can be concluded that porosity occurrence in casts produced in the die casting process is strongly influenced by the following:
Thermal capacity of the melt volume, which conditions the cooling speed of the melt;
Melt temperature, which influences the phase of holding pressure action;
Ratio of melt and gasses in the gating system of the mould, which conditions dragging of gasses by the melt and their mutual mixture;
Mutual correlation of the aforementioned factors.