Hiixocastiiig
Chapter 3 Test and Experiment Methods
3.3 Evaluation and Analysis of the Cast Compnents
When the billet reaches the desired temperature, the dies must be clamped together by a hydraulic piston acting on the moving half of the die, so clamping it up against the fixed half of the die with 50tons of force. The billet is then pushed out of the furnace on the set of rails by a pneumatic ram and lines up the cradle carrying the billet with the shot sleeve below it. The cradle holding the billet is rotated 180° and the billet drops out and falls 300mm and into the shot sleeve below. The billet must be quickly checked to see that it is indeed in the shot sleeve and that it is sitting flat in the shot sleeve. The machine inject button must now be pressed. The machine injects the billet into the die by the plunger on the end of the hydraulic ram moving forward inside the shot sleeve controlled by the computer in real time according to the velocity time profile previously set up.
After the plunger on the end of the Hydraulic ram has moved forward as much as it can until the metal stops it moving since the die cavity is full of metal and there is nowhere further for the metal to move to, the dies must remain clamped together and the plunger under pressure for ten seconds to ensure the billet has fully solidified. After ten seconds the dies are opened and the plunger which is still under pressure pushes the biscuit out of the shot sleeve and the cast component is pushed into and kept in the moving half of the die cavity.
The part is then removed from the moving half die cavity. This first casting is discarded since the PA111 paste is detrimental to the casting causing excessive porosity and discolouration'91 of the part.
Another billet is placed in the resistance furnace for heating and the same procedure is followed to obtain another part. This part is then evaluated and analysed.
die, increase or modify the flow volume through the die by using overflows. The Literature Survey Chapter has an in depth discussion of this defect type. The brief explanation above is simply to re- iterate the defect type for ease of reading.
The part is examined internally by using an X-ray radiographing machine. The machine must be set to 67 KV and focal distance 100 and power of 8mAs. The part is X-ray radiographed in both planes.
The x-ray radiographs are examined. Dark and light areas correspond to less dense and more dense areas respectively. Also areas which are darker correspond to thinner sections of the part and vice versa. Areas where there is an inhomogeneous patch of darkness, that does not correspond to a geometry change to a thinner section, represents undesirable internal integrity of the cast metal (it is less dense here, either due to porosity, cold shuts or both).
The casting must now be sectioned where the inhomogeneous area is. To locate the same area on the casting the scale of the x-ray radiograph to the casting must be calculated. A simple scale is calculated by dividing the known casting measurement by the same measurement on the X-ray radiograph. The scale for each X-ray radiograph must be measured and calculated. The next step to locate the correct area is to measure the position on both x-ray radiograph planes. The measurements from the x-ray radiograph are converted using the scale and then scribed onto the part to mark the position of the inhomogeneity. A sectioning procedure then reveals the inhomogeneous section and the section is mounted in Bakalite powder and then placed in a mounting press and held for the preset time, pressure and temperature to produce a sample suitable for mounting on the Scanning Electron Microscope (SEM) and for performing Energy Dispersive Spectroscopy (EDS). The microscopy is viewed to analyse the inhomogeneous section. By looking at the microscopy of the section it is possible to determine what type of defect it could be, cold shut, hot tear, macro or micro porosity. Then further analysis of the section further may need to be done by performing Energy Dispersive Spectroscopy (EDS) across the inhomogeneous section to check for high concentrations of oxides indicating cold shuts or oxide entrapment. The edges of the defects are of interest to identify the defect type, jagged edges indicate hot tears, round globular pores indicate entrapped gas porosity, jagged edged small pores indicate shrinkage porosity. It is often the case that the defect is a combination of the different types.
It is now necessary to postulate how the defects were created during the casting process. The investigation will concentrate on two main defect classes. The first class of defects are shortfill and
Cold Shuts and the second class is porosity. Porosity is further divided into shrinkage porosity and gas entrapment porosity.
Shrinkage porosity comes about when a section of the casting is not fed with new liquid metal. This can happen if the section is surrounded by solidified metal that is it is cut off from any liquid metal.
Since the metal was under high pressure when it was injected into the die and the die surface is steel and relative to the metal is cold, the metal will solidify against the surface of the die. As the metal solidifies further it shrinks and if no new metal is drawn in to take up the void created during shrinkage the void becomes permanent and is called shrinkage porosity. This explanation allows one to check if the porosity was caused by shrinkage, that is if a section of casting is cut off from new metal and there is porosity in mat section it is likely that porosity was caused due to shrinkage. To be sure it is shrinkage porosity the casting must be sectioned, mounted in resin and polished with 6um fluid. The sample is then analysed under an optical microscope at low-medium magnification, x5 is normally sufficient. The geometry of the porosity pores are examined and if they have jagged edges then the porosity is caused primarily by shrinkage' .
Gas entrapment porosity occurs when gas is trapped inside the metal. This can happen during filling the cavity or from when the metal was in the furnace or when the SSM slug was made. Upon solidification the entrapped gas will come out of solution/suspension in the metal and will form pores in the metal. These pores are porosity. The casting is likely to be filled under laminar conditions since the metal is in the thixotropic state which has an extremely high viscosity, it is unlikely that gas is entrapped during this period. While the metal is in the furnace it is solid or thixotropic, below the liquidus temperature and undisturbed so it is unlikely to absorb gas during this period. However when the slug was made gas could have been entrapped. To ascertain between the porosity type (gas or shrinkage) the casting must be examined as described for the case of shrinkage porosity. Since these (gas entrapment) pores are formed by gas coming out of solution/suspension they will be rounded in shape and will have smooth edges'13'.
The samples cast are X-ray radiographed to determine the position of the porosity then sectioned and analysed as above to determine if the porosity is caused by shrinkage or entrapped gas.
If the pores are in fact more like cracks, not rounded in shape and longer than 3mm then it must be considered that this could be a cold shut. To further ascertain this, die casting must be sectioned, mounted and polished as described above so they mat may be examined on a Scanning Electron
Microscope (SEM) equipped with an Energy Dispersive Spectroscopy (EDS) unit. The SEM's EDS unit must scan across the crack and special attention must be paid to the level of oxygen at the edge of the crack. The oxygen level is important since if the crack is caused by a cold shut then this would imply two flow fronts meeting. These flow fronts would have oxidized since they are in contact with the air in the cavity. If both edges of the cracks show a high concentration of oxygen then this crack is likely to have been caused by two flow fronts meeting that were too far below the liquidus temperature to fuse together. During further solidification these flow fronts break apart further as stress in the part increases as it cools and shrinks.
A cold shut can be avoided by casting the metal at a higher temperature and by using overflows to move the cold shut into an area outside of the casting. The shrinkage porosity can be avoided by feeding the area using a subgate and or heating or cooling sections of die sufficiently to modify the directional solidification to ensure all sections are fed with liquid metal as they solidify. Entrapped gas porosity can be reduced by reducing the injection speed, reducing the amount of die lube applied and reducing the amount of entrapped gas and or hydrogen content of the initial Al slug through better slug manufacture practice. The entrapped gas porosity can also be moved by using overflows.
113,141 . However before any of these steps are taken it is necessary to fully understand the nature of the defect formation. The next section investigates this.