GB2307049A - Filtration apparatus and method - Google Patents

Abstract

A method of characterising molten materials comprising the steps of effecting flow of the material through means 24 for filtering out inclusions within the material, monitoring the flow of the molten material through the filtering means, and comparing the measured flow with a predetermined flow characteristic to enable determination of the character of the material. The molten material may be a metal such as aluminium. The filter 24 may be in the base of a crucible 22. Monitoring may be of the flow rate or rate of change of flow of the molten material. The comparisons may be of curves of filtrate weight against filtration time (eg. figs. 8 and 13).

Description

FILTRATION APPARATUS AND METHOD The invention relates to apparatus and method for the characterisation of fluids such as molten metals in determining the quality of the fluid. In particular, the invention relates to characterising molten aluminium by determining the nature concentration of inclusions such as oxides and borides.

It is known to provide devices for filtering a sample of molten metal on line in a foundry. The filtration process allows the molten metal to pass through the device whilst, at the same time, concentrating any inclusions above a filter in the device.

By filtering a known quantity of molten metal, it is possible to quantify the concentration of inclusions present in the foundry process. This quantification is achieved in the known art by metallographic techniques conducted on the solidified residue left above the filter after filtration. For example, analysis can be formed by cutting through the filter and residue in order to preform optical analysis and quantitative metallography of the inclusions which have built up on the filter surface during filtration.

However, such analysis is a laboratory process requiring skilled technicians at some costs and time involvement. Accordingly, characterisation of the foundry process according to the known art is slow and does not give immediate characterisation of the molten metal to enable quick modification.

An object of the invention is to avoid or at least mitigate the problems of the prior art. Accordingly, the invention provides a method and apparatus for on-line filtering and characterisation of fluids such as molten metals. One aspect of the invention provides a method of characterising molten materials comprising the steps of effecting flow of the material through means for filtering out inclusions within the material, monitoring the flow of the molten material through the filtering means, and comparing the measured flow with a predetermined flow characteristic to determine the character of the material. Beneficially impurity contents in the region of 0.001 to 50 mm2/kg can be determined.

Preferably the monitoring step comprises determining the rate of flow or change of rate of flow of molten material for example by determining the amount of material to flow through the filtering means in a given time. The amount of molten material is determined for example by weighing the mass of filtered molten material, either directly or by inference from a known mass of initial molten material or both.

Preferably the method comprises a step of ensuring a minimum amount of material has passed through the filtering means before characterising the sample. Also, the method can comprise the step of stopping measurements when an upper amount of material has passed through the filtering means or an upper time limit is reached.

Beneficially, if a minimum amount of material has not been filtered within the upper time limit, then no characterisation need take place.

Preferably, the method further comprises a calibration step for determining the flow characteristics which can be based on filtering one or more samples from a given foundry process and possibly then analysing the sample or samples using metallographic techniques such as optical microscopy to determine the metallurgical cleanliness which effected the measured flow characteristics for one or more the different samples under known conditions. Alternatively, one or more samples having known constituents, added in preparation of the sample from precharacterised and/or commercially available chemicals or metals, can be used for calibration. Alternatively the method comprises the step of comparing one sample with another for basic comparison of the relative metallurgical quality of the samples.Beneficially this can be used to compare the quality of samples before and after a metallurgical treatment process such as rotary degassing. Standard conditions for calibration or comparison preferably includes one or more of the following: uniform material and construction of filter, standard orientation of filter, uniform size and/or shape of filter surface presented to the molten material, standard initial temperature of molten material before filtration begins, and/or known pressures.

Alternatively, the flow characteristic can be determined over a range of the above parameters, such as different temperatures of sample, thereby enabling the predetermined flow characteristic to be defined depending of the particular parameters in any given characterisation run of a sample. The parameters can be detected automatically using sensors, such as pressure sensors, or input by and controlled by an operator.

Beneficially, a profile of acceptable flow characteristics can be determined for a given foundry in order to determine if the foundry process is running acceptably.

Also, it is possible to determine from the flow characteristic measured for a given sample if for example too little or too great an amount of a certain type of inclusion such as oxide or boride is included in the foundry process, thereby enabling the foundryman to adjust the process without requiring metallographic analysis of further samples.

Preferably, the method comprises pressurisation of the molten material which can be in a two stage process incorporating a priming stage to effect wetting of the filter. Preferably, the initial pressure is in the region of 138000 to 345000 Pa (20 to 50 psi) and preferably a lower processing pressure is about 34500 to 138000 Pa (5 to 20 psi) Another aspect of the invention provides apparatus for characterising molten materials comprising a crucible having a filter for restraining the flow of inclusions in the molten material, means for effecting flow of the molten material through the filter, and means for monitoring the flow of the material through the filter.

Preferably, the apparatus further comprises means for comparing the measured flow with a predetermined flow characteristic to determine the character of the molten material. The monitoring means can comprise a timer and/or means for determining the amount of material to flow through the filter, for example, a weighing device.

Preferably, the comparison means comprises stored data from known samples used to calibrate the apparatus.

Preferably, the apparatus further comprises means for determining if a minimum amount of material has passed through the filter before any characterisation of the sample takes place. Preferably, there is also provided means for stopping the monitoring process if an upper amount of material has flowed through the filter and/or an upper time limit is reached.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a front elevation view of a first embodiment of apparatus according to the invention; Figure 2 is a side elevation view of the apparatus shown in figure 1; Figure 3 is a plan view of the apparatus shown in figures 1 and 2; Figure 4 is a schematic cross-sectional view of a second embodiment of apparatus according to the invention; Figure 5 is a sectional view of the bottom of the crucible shown in figure 4; Figure 6 is a schematic sectional view of a third embodiment of apparatus according to the invention.

Figure 7 is a schematic block diagram of a control system for apparatus according to the invention; Figure 8 is a schematic graph of filtrate weight versus time, obtained using apparatus according to the invention: Figures 9 to 12 are graphs of filtrate weight versus time under different experimental conditions for molten aluminium; Figure 13 is a schematic graph of filtrate weight versus time used to provide a profile of or characterise metal typical for an on-line process according to the invention; Figure 14 is a schematic graph of the range of total inclusion content occurring over a range of different aluminium foundries; Figure 15 is a schematic block diagram of a second control system for the apparatus according to the invention; Figure 16 is a schematic drawing of a pneumatic control system for part of the apparatus according to the invention;; Figure 17 is a schematic drawing of a temperature control test before initiating a filtration sequence; Figure 18 is a schematic side elevation view of a second catchment tray according to the invention; Figure 19 is a filtration weight versus time graph defining a profile region; Figure 20 is a second filtration weight versus time graph for a second profile region; Figures 21 a and 21b are micrographs at forty times the enlargement for curves A and B shown in figure 19; Figure 22 shows a series of flow curves from a foundry audit; Figure 23 is a further filtration weight versus time graph for two different points in a furnace; Figure 24a is a filtration weight versus time graph for a comparison between three sets of pairs of treated and untreated molten material;; Figure 24b is a filtration weight versus time graph of two samples having equal boride content but of different cleanliness; Figure 25 is a filtration weight versus time graph for three sets of curves having different types of treatment; and Figure 26 is a further filtration weight versus time graph for a further three sets of curves under different treatments.

Referring to figures 1 to 3 there is shown apparatus 10 according to the invention which comprises a container 12 in which a crucible containing molten metal is positioned in use. The container 12 is closed using a lid 14 which in this embodiment is rotatable away from the lid but can be clamped in position using clamp 16 thereby to seal the upper end of container 12. Apparatus 10 further comprises a collecting tray 18 for molten metal filtered through the crucible. There is also provided a control panel 20 which can for example comprise a visual display and user input means such as a keyboard and/or disc drive for example.

Referring to figure 4, there is shown a slightly different embodiment of part of an apparatus 10 according to the invention. A crucible 22 is shown housed within container 12 with lid 14 clamped in position using clamp 16. Crucible 22 comprises a filter 24 which allows molten metal contained within the crucible 22 to pass into collection tray 18 when pressure is applied within container 12. The filter collects inclusions within the molten metal above the filter. The apparatus further comprises a weighing device 26 on which collection tray 18 rests thereby enabling the amount of material collected in tray 18 to be quantified.

Apparatus 10 further comprises a valve 28 which controls the pressure of gas presented above material held within crucible 22. An inlet hose to valve 28 can be connected to a pressurised gas cylinder in use such that on opening valve 28 the gas passes into container 12 via fluid communication means such as a hose between valve 28 and inlet 31 in lid 14 of the container. There is also provided in container 12, a pressure gauge 32 for monitoring the amount of pressure within the system.

For example, the valve 28 can be used to control the pressure in container 12 to obtain an initial pressure, in the order of 207,000 Pa (30 psi) for example (i.e. above ambient atmospheric pressure). By regulating the pressure relief valve 33, it is possible to regulate the pressure in the chamber thereby enabling an initial high pressure or priming pressure to wet filter 24 with the molten material prior to reducing the pressure to a second operating pressure, for example in the order of 83,000 (12 psi).

There can also be provided a probe 30 such as a thermocouple measuring the temperature of material within crucible 22 during the filtration process.

Alternatively the temperature of the sample can be tested using a probe 30 remote from container 12 for example by providing a hand-held probe having a sufficiently long lead between the probe and the control panel 20 or independent of control 20 Also. a flow sensor 34 can be provided to detect if there is excessive leakage of gas through container 12 into the lower region containing attachment tray 18.

Detector 34 can be provided in-line with valve 28. Additionally, a sensor 35 can be used to detect whether or not lid 14 is secured in position. Preferably all sensors 30, 32. 34. 35 and control valve 33 together with an output from weighing device 26 are connected to a central control system, possibly housed in control panel 20.

Turning to figure 5, filter 24 is seated in a recess on a ledge 80 at the bottom of crucible 22. Filter 24 is held in position using a cement 82. Additionally, the crucible comprises a mask 84 to prevent flow of molten material around the filter 24. The mask 84 can be made of a mica paste containing platelets of mica, used to define the size and shape of the exposed surface of filter 24 inside crucible 22.

Additionally, crucible 22 comprises an outer layer 86 of material such as a cement paste to restrict airflow through the bottom of crucible 22.

Preferably, crucible 22 is an alumino silicate fibre crucible such as a procal six crucible available from Foseco. Filter 24 is preferably a ceramic filter preferably held in position using an adhesive such as Fraxbond 715 adhesive available from the Carborundum Company Limited. The internal mask 84 for the filter can be used such as Microfine Mica (potassium aluminium silicate).

Preferably, container 12 is a steel casing attached to base 23. Additionally, there is held within container 12 a gasket at both the upper end and lower end of crucible 22 to restrict the out flow of gas from the container for example between container 12 and lid 14 or through aperture 25 in base 23. Preferably, crucible 22 is made of fibrous ceramic such as alumino-silicate and filter 24 is preferably made of alumina granules bonded with silica, or silica carbide or sintered metal such as stainless steel. Such a crucible and filter are described in detail our pending UK patent application having the same filing date as this application. The teachings of that application are incorporated herein.

Figure 6 shows a third embodiment of a crucible 22 and part of a second filtration apparatus 10 according to the invention. Apparatus 10 comprises a connector 31 such as a C/A quick release connector for connecting the system to a pressurised gas source. The upper part or lid 14 of system 10 is hingeably connected by hinge 15 such as a floating hinge, to a body comprising wall 12 and lower gasket and base plate 23.

The lid 14 is clamped into place using a swing clamp 16 to seal the pressurised gas inlet at connector 31 against an upper gasket 19 which is positioned on top of a plate 27 placed over the top of crucible 22. Plate 27 and crucible 22 comprise pressure equalising holes I which enable pressurised gas from inlet connector 31 equalise about crucible 22 whilst at the same time forcing molten material within the crucible to pass through filter 24 and outlet 25 due to the relatively high pressure level above the molten fluid.

Apparatus 10 in figure 6 further comprises a furnace 48 which can be an induction heater comprising an induction coil 49 for induction heating of the material in crucible 22. Accordingly, a solidified sample already contained within crucible 22 can be inserted in container 12 and heat it up to a requisite temperature. The temperature of the molten sample can be tested using a probe similar to probe 30 described in relation to figure 4.

Accordingly, as shown in figure 7, apparatus can be operated substantially automatically using a computerised control system such a controller 36 comprising a central processing unit together with requisite interface devices to enable controller 36 to monitor the conditions at various sensors and to control devices such as valve 28.

Accordingly, figure 7 shows controller 36 connected for input from thermocouple 30, pressure gauge 32, flow meter 34 and lid sensor 35. The system further comprises a memory 38 for storing data, for example, a user input 40 such as a keyboard or disk drive for example. Additionally, there is provided a display 42 such as a liquid crystal display unit. The system can further comprise a power supply 44 which can comprise a battery and a re-charging device to recharge the battery from a mains supply. The power supply 44 could comprise a mains connector and transformer. Further, there can be provided an output 46 which could be a printer, or data logger for example.

In use, molten material can be ladled into crucible 22 which is then inserted into container 12. Lid 14 is secured in position by clamp 16 and the container 12 pressurised by opening valve 28 until a pre-determined pressure is detected by gauge 22. Molten metal is then caused to pass through the filter 24 and aperture 25 into catchment tray 18. Figure 8 shows schematically different types of filtration rate behaviour which occur depending on the nature of the molten material. Clean molten material tends to generate a linear variation of filtrate weight versus time, whilst impurity content or inclusion content tends to produce a parabolic or more complex shaped curve such as the lower truncated curve. For example, oxide films within a sample generate a curve which curves away from the linear curve produced by clean material after a certain period of time.

Whilst the original slope or rate of filtration might not distinguish this material from a metallurgically clean one eventually, the curve is characteristically different.

Similarly, particulate inclusions in molten metal or more complex mixtures of inclusions generate more significant deviation from the relatively linear response of a clean material and are quantifiable for example by calibration using samples of known inclusion concentrations and mixtures.

Figure 9 shows the distinction which can be achieved depending on the type of filter used in the crucible. Specification one represents a filter having a more porous constituency or more unfinished surfaces compared to specification two for example.

Figure 10 shows the variation of weight of filtered material over time, depending on the temperature of the molten material. Figure 11 shows the variation of filtered weight over time depending on the content of oxide films.

Figure 12 shows a set of curves from metal containing different levels of oxide and titanium diboride particles. A truncated curve is obtained for dense inclusions of both titanium diboride and oxide.

Accordingly, by calibrating the system by using a known type of filter, operating pressure and temperature, it is possible to quantify the nature of the filtration over time to characterise certain flow rates or rates of change of flow rate produced for certain types of molten metal depending on the nature of inclusions within the metal. The invention provides characterisation of molten metal by monitoring the rate or rate of change of flow of material through a filter without conducting complex metallographic analysis of the residue of the filtration. Moreover, the invention allows a simple on-line testing of molten material to ensure that it meets specified criteria such as cleanliness or oxide content thereby ensuring that the foundry process is operating according to requirements.As shown in figure 14 a typical foundry tends to produce metal having a cleanliness or inclusion content which varies within a band B. Depending on the type of process used in producing the metal, the mean value of band B for a given foundry is found to lie anywhere along the curve shown in figure 14 and can of course be very wide depending on the consistency with which the process is carried out. However, in order to ensure that the mean value of the total inclusion content of the output of the foundry is consistently low or within an acceptable commercial criterion and that the variation in quality of the product, i.e. the width of band B is kept to a minimum. The system according to the invention can be used to monitor the quality of output from the foundry in a real time manner.Accordingly, controller 36 can be used to monitor the variation in quality between given samples and enable adjustments to the foundry process to be made to correct any deviation.

Figure 13 shows a pre-determined profile region 50 which might be deemed as an acceptable window of filtration rate behaviour for samples from a given foundry.

Profile region 50 is defined by a lower curve 52 having a pre-determined initial slope 53 and plateau region 54. Region 50 is defined on its upper side by a substantially linear line 55 representing clean material. For the purposes of operation of the automatic system, an upper time limit 58 and upper weight limit 62 can be specified in order to determine when measurements of a given sample are finished. A lower weight limit 60 can also be specified in order to determine if any measurement run on a given sample is to be taken as validly representing the nature of the sample. Accordingly, if insufficient material passes through the filter to tray 18, then the sample run is deemed invalid.In the event of a successful measurement generating a filtrate weight versus filtration time curve which falls within region 50 then that sample has met the predetermined profile characteristics necessary for the given foundry process and is deemed acceptable. If the filtrate weight versus filtration time curve falls outside region 50 then the sample is not deemed acceptable. Characterisation of earlier samples might then allow a foundry man to determine from the shape of curve of a failed sample what is wrong and address or correct the foundry process accordingly before taking further measurements.

Of course, in the event that it is desired to have certain inclusion content, for example in an aluminium sample containing an inoculant or grain refiner, then upper curve 55 determining the upper bound of region 50 need not be a straight line but representative of a certain level of inclusion content for example. Other possible levels of characterisation of a sample are possible, such as monitoring the initial rate of filtration which might be compared to a given minimum value which is represented by line 53 in figure 13. Also, it is possible to monitor the rate of change of the flow rate to compare this with a given second derivative of a calibration curve, thus enabling comparison between a sample behaviour and a preset region between curves 53 and 54 of line 52, for example.

The system is applicable to molten materials other than aluminium, especially, iron, nickel. zinc and magnesium based alloys.

Referring to figures 15, there is shown a schematic block diagram of a second electronic system 70 for use with the apparatus according to the invention.

Components in common with the system shown in figure 7 are labelled with the same two digit reference and a letter 'a' suffix. In this example an interface unit 72 is connected to an input port of a personal computer such as a 486 PC, 36a. The interface unit 72 communicates with a type K thermocouple labelled 30a, pressure sensor 32a, and load cell 26a via an amplifier 26b, at an analog to digital conversion input.

Digital inputs are received at unit 72 from user buttons A, B & C, air flow sensor 34a and lid sensor 35a. A digital output is provided to a shoot bolt valve 74 which locks the lid 14 in position during operation. Digital output also goes to an audible sounder 78 and a main valve 28a. An analogue to digital output communicates with pressure regulator 76 which controls the pressure in the crucible container 12a.

Referring to figure 16 there is shown a pneumatic circuit for controlling the pressure in container 12a as well as the pneumatic shoot bolt 75 via valve 74. The pneumatic circuit 80 comprises a first air filter 81 such as a 5 micron air filter and a second air filter 82 such as a 0.01 micron air filter. A pressure switch 83 controls flow of air from a pressurized source 84 to the four/two solenoid valve 74 which controls the position of shoot bolt 75 which locks the lid 14a in position above container 12a. Additionally, pressurised gas is supplied to the electromagnetic regulator 76 and goes on to the three/two direct acting solenoid valves 28a which in turn communicates with crucible container 12a.

Pressure transducer 32a is provided in communication with the internal pressure of container 12a. Accordingly, the pneumatic system can be used to provide the requisite priming and processing pressure above the molten metal sample held within a crucible in container 1 2a and to monitor that pressure.

Beneficially, the system 70 can detect errors in the system including no air pressure, broken thermocouple, lid not closed, excessive time taken to reach temperature, metal temperature failing to reach test temperature, excess time taken for metal flow, weigh ladle or tray 18 not in place, tray 18 not empty. For example. the test conditions can be pre-set such as maximum test time of 5 minutes, minimum weight change of collection tray 18, 20 grammes or say 1% of weight of sample, high pressure application time 5 seconds, low pressure application time 2 seconds, number of high/low pressure cycles before aborting test 3, maximum test weight 1.4 kg. Having preset these values, system 70 can be run by loading a crucible containing 1.4 kg of metal into container 1 2a and inserting a thermocouple into the test molten metal sample.The system 70 measures the sample temperature periodically, for example every 3 seconds, firstly to ensure that an initial temperature. Ti, of say 100"C, is reached. Thereafter. system 70 ensures that the sample temperature goes on to exceed a chosen operating or test temperature Tt of say 700"C. The system then ensures that the temperature of the sample indicated by the solid line shown in figure 17 reaches a maximum in excess of Tt. The maximum temperature is taken as being as one where the temperature change is less than 10"C between successive temperature readings. The system then monitors the temperature until it comes back down to the test temperature Tt when the filtration can begin by applying a priming pressure and operating pressure to the sample as previously described.

Referring to figure 18 there is shown a modified catching tray 118 comprising a handle 120 connected to a body portion 122 which is formed from a sleeve of expanded metal mesh. The sleeve is perforated with holes of approximately 1 cm diameter. The sleeve carries a receptacle 124 which is shaped so as not to protrude through the bottom of sleeve 122. In use, only the bottom of sleeve 122 contacts the weighing device 26, or load cell, therefore, the temperature of the molten metal caught in receptacle 122 is not directly conducted to the weighing device 26. This feature mitigates against having to re-calibrate the weighing device due to excessive heating and thermal expansion thereof. Accordingly, means is provided for preventing conductive contact between the molten metal sample and the weighing device.Beneficially, the apertures provide convective cooling of the molten metal as well.

Further examples of the use of the invention will now be given.

EXAMPLE 1 - DEVELOPMENT OF REFERENCE CURVES AND BENCHMARKING Our studies have shown that the flow behaviour of a given alloy through a given filter at constant temperature and pressure is a unique function of alloy composition and inclusion content. For each alloy there exists a range of flow rates that represent the behaviour of the material under normal industrial processing conditions. From a database of flow rate curves all generated under the same conditions it is therefore possible to describe a unique "window" or profile region in a weight-time diagram that is defined by an upper boundary curve which would be typical of very clean metal, and a lower boundary curve which would represent metal with a high level of oxide or other inclusions.

This profile region can be used as a datum, against which to characterise of "Benchmark" an unknown material. The closer the flow characteristic of the unknown material lies to the upper boundary curve then the cleaner the material.

One such profile region for aluminium alloy A356 is shown in figure 19 where curve A defines the upper boundary and curve B defines a lower boundary.

In this case the alloy is un-grain-refined and does not contain any added grain refiner such as titanium diboride. The data has been obtained from material sampled at 730"C and filtered under a pressure of 12 psi (over atmospheric pressure), using a sintered alumina granule filter with a porosity of 70 llm.

Figure 20 is a second window obtained from filtration of A3 19 alloy under the same conditions. It is clear that the two results are characteristically different and reflect the different flow behaviour of the materials.

Figure 21A and 21B are a pair of micrographs showing the build up of inclusions on the surface of the filter taken from samples with flow characteristics defined by curve A and curve B respectively of figure 19. It is clear that material with a flow characteristic similar to curve A has a low level of inclusions in the metal, whilst material with a flow characteristic similar to curve B has a higher level of inclusions. In fact curve A is formed from alloy having an inclusion content of less than or about 0.01 mm2\kg and curve B by content of greater than or about 1 mm2\1cg It is possible, using standard quantitative metallographic procedures to count up and quantify the number of inclusions present in each of those (and any other) residues.

In this way a direct correlation between inclusion content and flow characteristics can be realised.

Figure 22 shows a series of 26 curves sampled under identical conditions as those used to generate the acceptable profile region between curves A and B of figure 19.

By comparison of figures 19 and 22, it can be seen that with the exception of one result, all curves lie inside the expected region of the weight-time diagram.

Furthermore and by way of example, the inclusion content at two locations in a casting furnace have been measured as: Total Inclusion Number of Content Oxide Films Chargewell 0.90 mm2/kg 640 per kg Dipwell 0.23 mm2/kg 88 per kg This is the breakdown of inclusions present in the two curves shown in figure 23.

It is clear that there is a good correlation between the inclusion content and flow characteristics.

The position of the flow characteristic with respect to the reference curves is therefore a good indicator of metal cleanliness for this material.

Once alloy behaviour under pre-determined filtration conditions has been correlated with inclusion contact and the relevant upper and lower boundary curves have been established it is then possible to quantify all unknown material of the same composition. by reference to other position in the weight-time window. This provides a rapid on-line audit and benchmarking procedure for molten metal cleanliness.

EXAMPLE 2 - THE USE OF CURVE SHAPE TO DISCRIMINATE INCLUSION TYPE AND MONITOR MOLTEN METAL TREATMENT PRACTICES.

Figure 19 shows the footprint for ungrain refined aluminium alloy A356 generated under standard conditions of 730"C, 30 psi prime and 12 psi running pressures as described earlier. Also shown in figure 24a are 3 pairs of curves relating to metal sampled from a 1 tonne transfer ladle before and after treatment by the addition of grain refiner and strontium modifier.

Three different A356 melts were prepared in a 5 tonne induction furnace. The melts were made from charges comprising slightly different scrap to virgin ingot mixes in the approximate ratio of 30%:70% and poured into a one tonne transfer ladle.

2 kg liquid metal samples were taken from each of the three different transfer ladles and the flow characteristics plotted using the procedure described above.

Each transfer ladle was then rotary treated using a Foseco rotary degassing station according to a standard procedure which involved the simultaneous addition of Aluminium-Titanium-Boron, rod (Al-STi-lB) to grain refine the metal and Aluminium Strontium (Al-lOSr) to modify the metal, After treatment of each ladle, a 2 kg sample was taken and the flow characteristic replotted. It is clear from the figure that the treatment procedure has resulted in a significant change to the flow characteristic which reflects the introduction of fine titanium diboride particles from the Al-STi-lB rod.

Titanium diboride particles and clusters thereof are typically in the size range 1-10 Mm but because of the pore size of the filter (e.g. 90 Mm) such inclusions (when present at the normal metallurgical levels required for grain refinement) cause the filter to partially block up by a bridging mechanism. Bridging mechanisms typically operate in filtration systems when the pore size to inclusion size ratio is in the range of between 10 and 20:1, and therefore the filters used in the present invention are extremely sensitive to the presence of TiB2. Filters of the type used to clean molten metal during casting have pore sizes in the range 600-2500 llm and are consequently relatively insensitive to the presence of TiB2 in the metal.Indeed, they are especially chosen to allow TiB2 to pass through them without blocking.

It can further be seen from figure 24a that the initial quality of the untreated metal is reflected in the treated metal flow characteristics, thereby giving a clear indication of molten metal cleanliness of TiB2 treated metal. From this data it is possible to define a cleanliness profile region for "treated" metal against which all unknown molten metal of the same composition can be compared. The profile region is defined as that proximal the upper of the three solid curves in figure 24a. It is further possible to monitor the level of TiB2 addition by measurement of the position of the flow characteristic for a sample by reference to a treated metal calibrated by quantitive measurement of TiB2 content in material representing the upper bound and lower bond of the treated A356 profile region respectively.

A further example of the affect of Ti and metal cleanliness on the flow characteristic of a sample are shown in figure 24b. The solid and dotted lines are the flow curves for a "clean" metal and "dirty" metal respectively as quantified below: Overall Metal Cleanliness Solid curve Dotted curve "clean " "dirty Total Inclusion Content 0.36 0.71 Oxide Film Content 30 112 Breakdown [mm2/Kg] Inclusion Band Area 3.6 19.3 Oxide Films No/Kg 30 112 Intercepts 58 1574 Length Short Long Alumina Film Area [mm2/Kg] Alumina needles MgO 0.11 Irregular Oxides Spinels 0.15 Borides 0.36 0.38 Carbides Nitrides Other 0.06 It can be seen that in both cases the TiB2 content is approximately the same 0.36 and 0.38 which figure is derived from 0.02wt% being added in the form of Al5TiB grain refiner rod.The sample consisted of 2kg of LM25 control material rapidly induction melted and stabilised to 730"C. One sample was treated directly whilst the other was turbulently mixed for five minutes to simulate the formulation of oxide and other inclusion prior to testing.

Accordingly, the user can define a lower limit for an acceptable profile or characterisation curve representing an acceptable level of cleanliness and boride content of molten metal. Such a line could be positioned by a user intermediate the dotted and solid line shown in figure 24b. In order to ensure that there is an appropriate level of boride content, an upper limit to the profile region could be defined somewhere above the solid curve of figure 24b but below a continuous straight line curve or other curve representative of a pure metal having no boride content.

EXAMPLE 3 - OPTIMIZATION OF MOLTEN METAL TREATMENT PLANT The present invention has been used to measure and optimize the performance of a rotary flux injection machine for the treatment and cleansing of aluminium alloy.

It is well known to use the process of rotary degassing to assist in the removal of dissolved hydrogen and oxide films from molten aluminium alloys.

Rotary degassing is well known in the industry and involves the simultaneous stirring and injection of gas into a bath of molten metal in such a way that a fine dispersion of bubbles causes the diffusion of hydrogen into the bubbles followed by its subsequent removal by flotation of the bubbles to the free surface. At the same time, oxides and other insoluble inclusions such as are commonly found in aluminium alloys are physically intercepted by the bubbles and carried to the top of the bath where they can be removed by skimming or some other operation.

The gas typically used for this process is normally an inert gas such as argon or in some cases nitrogen and it is also known to sometimes add a small proportion of reactive gas such as chlorine or freon into the main body of inert gas in order to enhance the capture process by means of favourable chemical reactions. It is also known to add a solid or liquid flux at the same time as rotary treatment.

The present invention allows the effect of different rotary treatment practices to be quantified.

A 50 kg charge of virgin aluminium ingot of A356 was melted in the normal way in a small electric bale-out furnace. Once melted the alloy was brought to a temperature of 730"C and a 2 kg sample filtered to produce a flow characteristic of 'as-melted" metal curve A as shown in figure 25.

The metal was then treated according to standard practices using a Hepworth Minerals and Chemicals rotary flux injection unit known as a Heproject, to degas and clean the metal using nitrogen injection with no flux addition. Following the treatment the temperature was re-established at 730"C and a 2 kg sample was taken and its flow characteristics determined. This is curve B shown in figure 25.

The metal was then further treated according to the same procedure but with the addition of 20 grams of HMC-137 treatment flux in the nitrogen flow. Again the melt was equilibrated at 730"C and the flow characteristic of the material was measured. The result is shown as curve C of figure 25. It is clear from the figure that little change in flow characteristics occurred after the initial gas only treatment but that a significant change in the flow characteristic occurs after flux treatment.

The experiment was then repeated using a scrap ingot charge to deliberately increase the inclusion content (oxide film) of the metal. The three flow characteristics are given in figure 26. Curve A is "as melted", B degassed without flux and C degassed with flux. In this case it can be seen that the gas only treatment has improved the quality of the metal somewhat but that the flux treatment has further cleaned the metal to a level similar to the earlier result and in line with the upper bound of the A356 window.

Accordingly, the proposed system and method of metal analysis determines metal quality through flow rate or continuous analysis of flow infiltration. Analysis can therefore be very quick as well as accurate. The apparatus and method allow the ability to discriminate between and quantify alloy chemistry as a result of fluid viscosity differences, unwanted or deleterious inclusion such as oxide or intermetallic particles, deliberate beneficial additions such as titanium diboride particles, and the intergradient dispersion of different types of inclusion.

Claims (38)

1. A method of characterising molten materials comprising the steps of effecting flow of the material through means for filtering out inclusions within the material, monitoring the flow of the molten material through the filtering means, and comparing the measured flow with a predetermined flow characteristic to enable determination of the character of the material.

2. The method of claim 1 wherein the monitoring step comprises determining the rate of flow or change of rate of flow of molten material for example by determining the amount of material to flow through the filtering means in a given time.

3. The method of claim 1 or 2 wherein the amount of molten material is determined by weighing the mass of filtered molten material, either directly or by inference from a known mass of initial molten material or both.

4. The method of claim 1, 2 or 3 comprising the step of ensuring a minimum amount of material has passed through the filtering means before characterising the sample.

5. The method of claim 4 wherein the minimum amount is in the order of 10% of the initial sample mass.

6. The method according to any preceding claim comprising the step of stopping measurements when an upper amount of material has passed through the filtering means or an upper time limit is reached.

7. The method of any of claims 1 to 6 further comprising a calibration step for determining the flow characteristics based on filtering one or more samples from a given foundry process using the method of claim 1 and then analysing the sample or samples using a metallographic techniques such as optical microscopy to determine the metallurgical cleanliness which effected the measured flow characteristics for one or more the different samples under known conditions.

8. The method of any of claims 1 to 7 comprising a calibration step using one or more samples having known constituents added in preparation of the sample from pre-characterised and/or commercially available chemicals or metals.

9. The method of any of claims 1 to 8 comprising the step of comparing the flow properties of one sample with another for basic comparison of the relative metallurgical quality of the two samples.

10. The method of claim 9 used to compare the quality of samples before and after a metallurgical treatment process such as rotary degassing.

11. The method of any preceding claim operated under predetermined standard conditions for calibration or comparison.

12. The method of claim 11 wherein the standard conditions includes one or more of the following: uniform material and construction of filter, standard orientation of filter, uniform size and/or shape of filter surface presented to the molten material, standard initial temperature of molten material before filtration begins, and/or known pressures applied to the sample during characterisation.

13. The method of any preceding claim comprising the step of determining a profile or region of acceptable flow characteristics for a given foundry in order to enable simple determination of whether the foundry process is running acceptably by characterising a sample from the foundry to determine if its flow characteristics fall with the profile region.

14. The method of any preceding claim comprising the step of determining a profile region of acceptable flow characteristics for a range of concentrations for a given type of inclusion in the molten metal sample, the profile region having an upper and lower limit representative of a range of concentrations of inclusion.

15. The method of claim 14 comprising the further step of measuring the flow characteristics of a sample and comparing the flow characteristics with the profile region in order to enable determination of whether the sample contains inclusions within the predetermined range of inclusion concentration and/or to ascertain an approximate indication of the absolute value of inclusion concentration by comparison of the flow characteristics of the sample relative to the upper and lower limits of profile region.

16. The method of claim 14 or 15 wherein the flow characteristics at the upper and lower limits of the profile region are representative of known concentrations of inclusion which concentrations are determined by metallographic characterisation of the residue obtained by filtration of a sample, for example by optical techniques.

17. The method of any preceding claim comprising pressurisation of the molten material to effect filtration.

18. The method of claim 17 wherein the pressurisation comprises two stages incorporating a priming stage to effect wetting of the filter.

19. The method of claim 18 wherein the primary or initial pressure is in the region of 138000 to 345000 Pa (20 to 50 psi).

20. The method of claim 18 or 19 wherein a lower processing pressure applied to the sample after priming, which processing pressure is about 34500 to 138000 Pa (5 to 20 psi).

21. Apparatus for characterising molten material comprising a crucible having a filter for restraining the flow of inclusions within the molten material, means for effecting flow of the molten material through the filter, and means for monitoring the flow of the material through the filter.

22. The apparatus of claim 21 further comprising means for comparing the measured flow with a predetermined flow characteristic to determine the character of the molten material.

23. The apparatus of claim 21 or 22 wherein the monitoring means comprises a timer.

24. The apparatus of claim2 1, 22 or 23 wherein the monitoring means comprises means for determining the amount of material to flow through the filter, for example, a weighing device.

25. The apparatus of any of claims 21 to 24, wherein the comparison means comprises stored data from known samples used to calibrate the apparatus and or the foundry from which the material is taken.

26. The apparatus of any of claims 21 to 25 wherein the apparatus further comprises means for determining if a minimum amount of material has passed through the filter before any monitoring of the sample takes place.

27. The apparatus of any of claims 21 to 26 wherein there is also provided means for stopping the monitoring process if an upper amount of material has flowed through the filter and/or an upper time limit is reached.

28. The apparatus of any of claims 21 to 27 or independently thereof for characterising molten materials, comprising means for effecting flow of the material through means for filtering out inclusions within the material, means for monitoring the flow of the molten material through the filtering means, and means for comparing the measured flow with a predetermined flow characteristic to enable determination of the character of the material.

29. The apparatus of any of claims 21 to 28 wherein the monitoring means comprises means for determining the rate of flow or change of rate of flow of molten material for example by determining the amount of material to flow through the filtering means in a given time.

30. The apparatus of any of claims 21 to 29 comprising means for weighing the mass of filtered molten material, either directly or by inference from a known mass of initial molten material or both.

31. The apparatus of any of claims 21 to 30 comprising means for ensuring a minimum amount of material has passed through the filtering means before characterising the sample.

32. The apparatus of any of claims 21 to 31 comprising means for stopping measurements when an upper amount of material has passed through the filtering means or an upper time limit is reached.

33. The apparatus of any of claims 21 to 32 further comprising means for determining the flow characteristics of one or more samples from a given foundry process and means for storing data based on the analysis of the sample or samples using a metallographic techniques such as optical microscopy to determine the metallurgical cleanliness which effected the measured flow characteristics for one or more the different samples under known conditions.

34. The apparatus of any of claims 21 to 33 comprising means for pressurising the molten material to effect filtration.

35. The apparatus of claims 34 comprising means to enable two stages of pressurisation including a priming stage to effect wetting of the filter.

36. The apparatus of any of claims 21 to 35 comprising means for monitoring the temperature of the molten material.

37. The apparatus of claims 36 wherein the temperature monitoring means enables detection of an initial minimum temperature to assess to operability of a temperature measuring device such as a thermocouple.

38. The apparatus of claims 36 or 37 wherein the temperature means stores a preset operating temperature beyond which the sample temperature must exceed in order to enable monitoring of the filtration of the material.