The MetalHealth K-Mould as a Monitoring Tool for Molten Metal Quality. Contents. Abstract. Introduction. Discussion. > Abstract.

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1 Contents > Abstract > Techniques > Introduction > Method > Results and Discussion > Conclusions The MetalHealth K-Mould as a Monitoring Tool for Molten Metal Quality Abstract This technical note discusses the use of the MetalHealth K-Mould to monitor the molten metal quality at various stages of production in an aluminium high pressure die casting plant. (BS1490: LM24 alloy) Prefil testing was also undertaken as part of the study. Issues raised include inconsistent metal quality, and degradation in quality at different process stages. The K-Mould was proven to be an effective tool for detecting coarse inclusions (greater than 500µm) quickly and easily. Contains micrographs. Introduction The alloy under investigation is an aluminium 12% silicon alloy. (BS1490: LM24 alloy) LM24 is essentially a pressure die casting alloy; it has excellent casting characteristics and is generally easier to die cast that the high silicon containing alloys. A die casting operation commissioned N-Tec to provide a molten metal quality of their entire operation. Both the melting system and each of the die casting machines were audited using two techniques for inclusion measurement. The K-mould was used as a first order technique to detect coarse inclusions, and as a discriminator to determine which samples would be subjected to the second order Prefil testing.

2 Techniques Prefil Testing N-Tec uses the Prefil (Pressure Filtration) test to give an on-line quantitative measurement of oxide films and other inclusions. The flow-rate of molten metal through a micro filter at constant temperature and pressure is monitored and used to plot a graph of weight filtered vs. time. Inclusions in the metal, such as oxide films, quickly build-up on the filter surface during a test, reducing the flow-rate through the filter. Therefore the slope and overall shape of the weight filtered vs. time curve indicates the level of inclusions present in the metal. Oxide films affect the initial slope of the curve (20-30 seconds). They result in straight lines, with a slope that decreases as the number of oxide films increases. Fine particulate inclusions such as TiB 2, fine Al 2 O 3 or carbides cause the curve in the Prefil test to deviate from a straight line. The loading of fine particles can be inferred from the point at which the curve begins to deviate from the initial slope. Acceptable cleanliness can therefore be defined by using Upper and Lower bound curves. If a test curve falls between these bounds the metal tested is acceptable. N-Tec has identified a number of Prefil World-Class Production Windows for particular products, such as safety-critical extrusions, rolled sheet and foil, premium quality ingot and T-bar and high quality automotive and aerospace castings (e.g. safety-critical suspension components). Regular Prefil testing and reference to a suitable production window allows the metal quality to be monitored and remedial action to be taken is the quality drops. In addition to the filtration curve, metallographic analysis of the residue that is retained on the filter after a Prefil test allows identification and quantification of the types of inclusions present in metal sample to be carried out. Once the inclusion profile of a product line has been characterised it is easier to select the most appropriate monitoring and control procedures for maintaining consistency. K-Mould The K-Mould test measures coarse inclusions and gas bubbles (statistically). The test die has a series of notches that force the sample to fracture in pre-determined positions, allowing a statistically significant measure of inclusions to be obtained. Visible defects on the fracture surfaces are counted, averaged over a set of three castings and recorded as a cleanliness index or K-Factor. This gives a quantitative measure of defects per unit area.

3 Method Over 1 day in the plant, K-Mould and slug samples were taken from the melter, before and after the filter, after the degasser and from the dip-well of 9 die casting machines. A total of 26 sets of samples were taken. It was decided to initially fracture and examine all of the K-mould samples, to determine if there were any particularly bad samples, or any trends that could be identified. After an initial visual examination of each of these, seven samples were identified to undergo Prefil testing, using N-Tec s proprietary rapid induction remelting technique. The dirtiest K-mould sample was also assessed metallographically, to determine the exact nature of the inclusions present. Results and Discussion K-Mould Audit Most samples were relatively clean, with between 0 and 5 small inclusions across the fracture face, but some were considerably dirtier, and so were obvious candidates for Prefil testing. It was also felt that an audit of the melting system would be of benefit. The tables on the following pages show the fracture surfaces from the seven samples chosen to undergo further assessment. As can be clearly seen, the worst K-mould fractures were observed on samples from holder 8. The samples from holder 2 also contained a proportion of darker regions, whilst the remaining samples exhibited only low numbers of inclusions visible with the naked eye. The figure below illustrates graphically the K-factor results, and also indicates the areas where these results would be expected to lie. Held Treated In Melter Expected Melted Entering Filter After Filter After Degassing LM24 Holder 2 Holder K-Factor Holder 8 The results are good for melted material, they show a degree of scatter for treated material, and are both scattered and skewed to the right for held material.

4 Melting System Sample ID Macrograph of Fracture K-Factor In tower melter 0.5 Entering filter 0.25 After filter 0 After degasser 0.75

5 Holding Furnaces Sample ID Macrograph of Fracture K-Factor In holder 2 1 In holder In holder

6 Metallographic assessment was carried out on samples from holder 8 to determine the exact nature of the inclusions present. The macrographs from samples taken from holder 8 are shown in the figures below. The inclusions observed were a mixture of oxide films and spinel particles. Sample from holder 8 x 40 showing a) spinel and b) long oxide film Sample from holder 8 x 160 showing detail of above

7 Prefil Testing Prefil tests were run on all of the above samples, and the results can be seen on the following pages. Melting System The figure below shows the metal quality in the melting system Clean Reference 1 Melter Weight (g) Before filter After filter 0.4 After degasser Low er Bound Time (s) All of the curves were straight lines in the upper portion of the World Class Window, indicating good quality metal, with few fine inclusions. 1.4kg of metal from the melting furnace flowed through the filter in 104 seconds. Leaving the melter/ entering the filter it took 95 seconds, and leaving the filter 91 seconds. After degassing the metal took 109 seconds, showing that this process had a detrimental effect on metal quality, introducing coarse particulate into the melt. However, this is not normal practice for the client; LM24 metal is not normally degassed. It was noted at the time of the trial that the degasser was not being used according to the manufacturer s guidelines. The machine is set up to run a four minute degassing cycle, which in our experience is generally made up of two stages; a flux feed stage, and then a gas purge stage. The machine is only being used for one or two minutes, depending on metal temperature, and so the full benefits are not being achieved, and the purge cycle may not even be activated. This is one area where further work could be beneficial to metal quality. However, further experience tells us that metal quality immediately after degassing is often reduced, but that a dwell of minutes can lead to much improved results, in excess of starting quality. Since there is no dwell built into the system, and there is a very limited possibility of introducing one into the system, degasser optimisation is crucial. The above points are general to all degassing cycles.

8 Holding Furnaces The figure below shows the metal quality measured in the holding furnaces Clean Reference 1 Holder 2 Weight (g) Holder 5 Holder Low er Bound Time (s) There are wide variations in metal quality between the three furnaces sampled, and although they are all within the World Class Window, the sample from holder 5 was a straight line of 86 seconds, samples from holder 2 and holder 8 showed considerable curvature, indicating a higher proportion of particulate, both coarse and fine. 1.4kg of metal flowed through the filter in 110 and 138 seconds respectively. Referring back to the details of the tests, it was noted that machine 8 was running; machine 2 had been stopped for 15 minutes; and machine 5 for 40 minutes. This has had a substantial effect on metal quality. This is a recognised trend, and illustrates the beneficial effect of settling time on dirty furnaces. However, build-up at the bottom of the furnace can still be a real issue, and regular furnace cleaning is essential under these circumstances. It is worth stressing again that the furnace still contains large numbers of inclusions, and this will have two main problems: 1. Every time the furnace is recharged, the settled inclusion layer will be disturbed, releasing inclusions back into the melt, and lowering the bulk metal quality. 2. If the furnaces are not cleaned regularly to remove the inclusion layer it will continue to build and will eventually cause compounded metal quality problems in the bulk metal. Clean furnaces and clean metal can go a long way towards resolving this issue. Most flux manufacturers produce fluxes designed for use in holding furnaces such as these, and further work in this area may be of benefit.

9 Conclusions The use of the K-Mould as a first order test for inclusions, and as a discriminator to determine which Prefil samples to run has worked extremely well. The data shows that the metal quality sampled during the audit was not as good, or as consistent, as it should be. Subsequent, selective Prefil testing showed further details of this, as can be seen in the plots and accompanying text. Details of the findings are noted below: The molten metal quality in the melting system is relatively good, but was degraded by degassing. Degassing is an area where further work is recommended in order to optimise performance. The molten metal quality in the running furnaces is significantly lower than that of the as-transferred metal. Settling times have played a big effect in cleaning up the furnaces, although this is not a solution to the problem. Further work is suggested to more fully characterise the molten metal quality in the holding furnaces. Cleaning practices should also be examined, perhaps considering a flux operation for the holding furnaces. In summary, the K-mould has provided a quick and easy method for assessment of molten metal quality in terms of coarse inclusions. Prefil has provided further information about the metal quality in terms of both coarse and fine inclusions, and the trend remains the same. For more information about anything covered in this case study, please contact N-Tec at: Thornhill Road North Moons Moat Redditch B98 9ND ENGLAND Tel: +44(0) Fax: +44(0) julia.pickering@ntecgroup.com For more information about purchasing a system, please contact Metaullics at: Aurora Road Solon Ohio USA Tel: Fax: dvneff@metaullics.com This document and its contents are copyright N-Tec 2005 and may not be reproduced without permission