Members of the original SSM research group, Fred Cooper, Alex Banach, Clive Goodman, Krithinath Sharma, Belinda Verijenko for pioneering SSM in South Africa and being the first ever to install an SSM manufacturing cell and subsequent SSM castings in produced the country. . Reynolds number, modified for Non-Newtonian flow behavior 25 Diagram 2-17: Schematic geometry of the two-dimensional cavity.
Introduction
Introduction to Semi-Solid Metal Behaviour
The diagram shows the sensitivity of the solids fraction of the metal at a specific temperature to changes in the concentration of alloying elements. Forming semi-solid metal requires a shear force to change the apparent viscosity of the metal and in this way the metal can be induced to flow [24,1].
Introduction to High Pressure Die Casting Process
An existing method of achieving a rate of shear stress on a liquid to mold it into a specific shape is the High Pressure Die Casting (HPDC) process. Equally important is the method's ease of retrofitting a system to feed semi-solid metal into the machine instead of the conventional liquid metal.
Rheocasting
Hiixocastiiig
Comparison of Semi-Solid HPDC to Conventional HPDC Process
The shear of the fluid occurs at the interface of the metal with the mold cavity surface. The metal can (in the case of aluminum casting) have a temperature of up to 720ºC and the internal equilibrium temperature of the mold can be 250°C.
Selection of Components
These stresses ultimately cause the surface of the mold to form small cracks that slowly grow larger with each cycle. The North American Die Casting Association (NADCA) has developed a useful set of standards and specifications for SSM Die Casting1401.
Die Casting Machine Parameters
The temperature of the die affects the rate of heat flow from the metal in the die. From the discussion in section 2.3, the lifetime of the die in terms of heat control is dependent on the temperature difference between the die and the metal.
Die Design
The overflow must be placed in the area of the die where a flow modification is required. The metal pressure is limited by the machine's clamping force and the cross-sectional area of the die.
Machine Type
Defects, Types and Causes
Solidification porosity is distinguished from entrapped gas porosity in casting samples by its jagged shape, which is associated with the formation of voids (pores) due to shrinkage. Solidification porosity should be avoided by incorporating directional solidification techniques into the die design. The oxide that was on the surface of the flow front is trapped and further destroys the homogeneous metallographic structure.
Another method by which a new flow front can be formed is if the die geometry splits the flow into two or more flow paths. If the metal of both current fronts is too cold, the probability of cold arrests is even greater and the severity of cold arrests is greater. The reason for incomplete filling may be due to high-pressure gas trapped in the extreme part of the die, thus preventing metal from entering this extreme part.
Another reason for short filling may be due to the formation of a severe cold closure and its witness can be seen on the surface of the casting. To reduce high-pressure gas pockets, venting should be sufficient at die extremities.
Identification of Key Variables in SSM High Pressure Die Casting of Components
Test and Experiment Methods
- Selection of Component
- Method .1 Split Line
- Evaluation and Analysis of the Cast Compnents
- Set up of simplification dies to separate flow into two, two dimensional planar models
- Casting the two planar model parts Method
- Evaluation and analysis of parts made from two planar model dies - identification of defects formulate reasons/causes of the defects
- Formulation of solutions to above defects
- Implementation of solutions onto part's original die All the modifications must be considered and calculated
- Heating and cooling channels
- Experimental Apparatus and Procedure
- Analyse, Evaluate and Rate Components from section 3.9
- Rating Method
- Selection of the Optimum Level Settings
The first part of the examination is to decide on the parting line of the mold (mold). The parting line should be through the center of the taper to allow the cover to open. As the billet heats up, the casting parameters of the high pressure casting machine are set.
To locate the same area on the cast, the scale of the x-ray radiograph to the cast must be calculated. Shrinkage porosity occurs when a portion of the casting is not fed with new liquid metal. The die of the short arm, (part of a spacer adapter assembly used in high voltage overhead mast cable bundles).
Piston diameter affects metal flow rate, cookie modulus and metal pressure. The second grading system uses component density as a medium for quantifying casting porosity.
Results and Discussion of Results
- Initial design of Component die and Initial castings produced
- Die Design
- First Run of Component and Results .1 Short Shot
The two faces are both circular rings and one face has the steering wheel bolted to itself and the other face is attached to a square section on the steering shaft. As a result, the driving boss scores a weighting factor for this factor of one tenth (0.1). As a result, the Short Arm is given one (1) and the Lead is given one tenth (0.1) for this weighting factor.
The rudder scores eight tenuis (0.8) and the short arm scores one (1) for this weighting factor.
Short Shot Profile
Full Shots
Complete castings were made using the following shot profile shown in Table 4-8 and graphically in Diagram 4-8. The end of the stroke is calculated using the volume of the casting and the diameter of the stroke sleeve.
Full Shot Profile
X-Ray Radiographs and Rurther Analysis of the Complete Parts
An X-ray of one of the castings, Diagram 4-10, has clear evidence of a crack on the side of the cylinder farthest from the port. This casting was then appropriately cut to expose the crack as described in Section 3.3 of the Test and Experimental Methods chapter. The crack was then further analyzed using an energy dispersive spectroscopy (EDS) unit on the SEM.
For this purpose, a line is drawn along which the EDS unit scans and then plots the values of the elements of interest. The line along which the EDS unit scanned is shown in Figure 4-13, and the element intensity plot is shown in Figure 4-14. It is important to note that the concentration of aluminum is high and the concentration of oxygen is low in the matrix surrounding the crack (in diagram 4-13 it is this gray color).
It is also important to note that at the border between the crack and the matrix, which is predominantly aluminum, there is a rapid increase in silicon and oxygen. This increase in oxygen indicates that the crack is due to cold closure due to the meeting of two flow fronts.
The Modular Die Results
- Results of Planar Model One
- Results of Planar Model 2
- Visual inspection Results
- X-Ray Radiography results
The current behavior of the original part can be assumed by examining the snapshots in diagram 4-22. Here the matrix geometry of the sand glass section diverges to form an upper cylindrical representation. This section is in the center of the upper thick cylindrical demonstration section and due to the restriction of the sand glass does not add new metal during solidification.
The situation here is emphasized by the very narrow hourglass neck section in the middle of the planar model casting (see diagram 4-22). The porosity is located in the section representing section 6 of the original component (upper cylindrical section) and boxed in diagram 4-23. The other method to aid directional solidification is to vary the temperature of the mold in the areas required.
There was also no metal pressure to force the metal to the ends of the boiler. However, this would adversely affect the steering stiffness as already discussed in the first planar model section.
Discussion of Results from First Original Component and Planar Model Castings
- Cold Shuts
- Porosity
- Improvements to Overcome the Defects
- Implementation of Improvements onto Die
From the results it was seen that the cold closing defects were found in the center of the casting. In one of the original component castings, cold sealing was seen at the interface between the upper cylinder and the end cap. The cold seal is in one part of the cylinder and remains within the circumference of the cylinder as it extends into the end cap.
According to the defects in diagrams 4-31 and 4-32, the cold seals are located in the center of the upper cylinder. The cooling channels will cool the upper part of the mold, which is in contact with the upper cylindrical part of the casting. This is what this research aims to avoid and therefore the cooling channels are strategically placed behind the upper cylindrical section of the casting to drastically cool this section.
The use of oil allows more precise control of the temperature of the matrix in a limited range. There are three cooling channels machined as rough holes in the top of the die.
Mil! 1
Results of Original Components After Die and Process Variable Setting Selection from Taguchi
- Process Variables Recorded
- Die Temperature
- Velocity Profile of the Metal Injection Plunger
- The intensification pressure
- X-Ray Radiography and Density Analysis of Parts
The top of the matrix (where the channels are) is about 200°C cooler if the channels have water flowing through them. The speed of the metal injection piston determines the flow rate of the metal filling the cavity of the die. The second slow step moves the metal to the edge of the casting's gate.
The influence of a consistent velocity profile is clearly shown on the variance of the cavity filling time. There is evidence of a fine crack at the interface of the cylinder head and end cap. Experiment 2 was then performed and X-rays of the two acts were performed.
This is the analysis of the castings using method two, described in the Test and Experimental Methods section. It is clear from the literature review chapter that metal temperature and metal flow rate interact to influence the apparent viscosity of the fluid.
Conclusion and Recommendations
These results were obtained from the analysis of the X-ray radiography of the planar model castings and the component castings themselves. These physical modifications improved the internal integrity of the component as evaluated using X-ray radiography techniques. The process parameter of the die temperature in localized areas was found to be critical to the internal integrity of the part.
The shear rate experienced by the SSM fluid while filling the die cavity is a function of the injection rate. There is an interaction between injection rate and metal temperature due to their combined relationship to the viscosity of the SSM fluid. There is an interaction between localized die cooling and SSM fluid temperature with respect to the internal integrity of the casting.
The desired result of castings with high internal integrity was achieved using a metal temperature of 578°C and local cooling of the die cavity. There is a stronger interaction between metal temperature and injection rate in relation to the internal integrity of the casting.
