Technical Note Ref: MH002/01 March 2005

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1 Contents > Abstract > Techniques > Introduction > Method > Results and Discussion > Conclusions Root Cause Analysis of Hot Tearing Using the MetalHealth System Abstract This technical note discusses the use of the MetalHealth Hot Tearing test mould to identify the root cause for differences in hot tearing characteristic between two batches of aluminium 5% Zn ingots, produced by different manufacturing routes. This was achieved through metallographic and compositional analyses, as well as Prefil testing to assess the molten metal cleanliness of the two batches. A series of tests was carried out using the MetalHealth Hot Tearing test mould, and both the MetalHealth Tatur Test mould and the Phaser Thermal Analysis System were also used during the investigation. Significant differences in the behaviour of the two batches have been identified, and the issue resolved with the use of effective grain refinement. Contains micrographs. Introduction The alloy under investigation is a naturally aging aluminium 5% zinc alloy. In this type of alloys the coherency point occurs at a low fraction solid, making them very prone to hot tearing. A customer has reported that two different processing routes used to achieve this alloy composition lead to different hot tearing responses in the final alloy. In order to resolve this issue, it is important to first understand the root causes for these differences. This is the aim of this work, coupled with suggestions for resolution of the problem where applicable. This will be achieved using a number of techniques from the N-Tec MetalHealth suite of tests and protocols. As one might expect, the MetalHealth Hot Tearing test mould will be used extensively in these investigations. Updated picture of hot tearing mould to go here.

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. Tatur Test Mould This mould is based on the Tatur test which measures the feeding characteristics of an alloy. Metal is poured into a wide based cone die and freezes with no enhanced feeding. As a result the casting suffers from surface slumping, internal micro-porosity and large, well defined pipe. Qualitative and quantitative measurements can be taken to record the individual feeding characteristics of the metal and to compare one alloy with another. The test is particularly good for comparing the effect of grain refinement, modification and gas content on porosity formation and distribution in a wide range of casting alloys.

3 Insert Tatur picture Phaser Thermal Analysis System The Phaser Thermal Analysis System is a powerful non-destructive tool that measures the cooling curve generated by a sample of metal solidified at a controlled rate. Through further analysis, features on the cooling curve can be correlated with the type and volume of phases present in the cast microstructure. Phaser is able to detect secondary reactions that can dramatically influence mechanical and casting properties. Phaser is capable of generating a complete database of reference curves to allow the prediction and therefore control of the cast structure. Method The first phase of the work involved the characterisation of the two different ingot batches; one from each process route. This was done using a combination of metallographic assessment and compositional analysis using OES. Prefil was used to assess the molten metal quality of the ingots. A series of testing utilising the MetalHealth Hot Tearing and Tatur test moulds and the Phaser Thermal Analysis System was then carried out. This included testing at different mould temperatures in the Hot tearing mould, and with or without grain refiner additions in the various conditions. Results and Discussion The ingots were cut, polished and etched to reveal the grain structure, as shown below. The ingot grain size for batch 7 was 300µm, where as for batch 8 it was 1000µm. The difference in the grain size was also reflected in the greater amount of shrinkage pipe exhibited by ingots from batch 8. 2mm 2mm Batch 7

4 The microstructures of the two ingots showed no obvious difference in the phases present. Spectrographic analyses were taken from the two different batches of ingot, to see if there were any differences. Titanium and boron analyses indicated slightly more grain refiner was present in batch 7. No other elements showed a significant difference between the two casts. Prefil Testing of Ingots Samples from the two batches of ingots were rapidly re-melted using a proprietary induction melting technique and Prefil tested. The resulting curves are compared to the Prefil World Class Production Window below Upper Bound Weight (kg) A Cast 2777 Cast Low er Bound Time (s) Batch 7, which has a finer grain size, had a faster Prefil flow rate curve than batch 8, indicating a lower level of coarse inclusions. Both plots are straight, indicating a low level of fine inclusions. Residues from the Prefil tests were sectioned and polished to show the inclusions present, and the microstructures are shown below (the Prefil filter is shown at the bottom of the pictures). The main inclusions are oxide films, which appear as branched, irregular dark lines. 63µm 63µm Oxide Films Prefil Filter Batch 7

5 Quantitative analysis of the Prefil residues is shown in the table below. The greatest difference is in the number of oxide films present; batch 7 having 50 films/kg and batch Inclusion Batch 7 Total Inclusion Content (mm 2 /kg) Oxide Films (number/kg) Spinel (mm 2 /kg) Refractory (mm 2 /kg) Titanium Boride (mm 2 /kg) Trace Trace Alumina Needles (mm 2 /kg) - Trace Aluminium Carbide (mm 2 /kg) - Trace 8 having 233 films/kg. This explains the lower Prefil curve for batch 8. Very few titanium diboride (TiB 2 ) grain refiner particles were found in either sample. Hot Tearing Tests Hot tearing tests were cast with a mould temperature of 400 C to give the best discrimination between metal from the two batches. When ingot from batch 7 was cast, two fingers cracked where as four fingers cracked with ingot from batch 8. These were the tests that showed the greatest difference in hot cracking behaviour. The results are shown below. Batch 7 2 Cracks 4 Cracks A comparison of the microstructures of the cracked fingers for the two different batches of ingot (second longest finger) was carried out. It can be seen in the micrographs overleaf that the crack in the finger for batch 7 is irregular and has numerous areas of tearing behind the main crack surface. In comparison the failure crack for batch 8 is much less irregular and has substantially fewer tears behind the main crack surface.

6 Batch 7 63µm 63µm 125µm 125µm The figure below shows the grain structures near to the main crack for tests on the two batches (second longest finger). Although not as pronounced as found in the ingots, the grain structure of the fingers for batch 7 is still finer than for batch mm 2.5mm Grain refinement of this alloy increases the fraction solid at which the solidifying metal reaches coherency. This means that grain-refined metal should accommodate the stresses imposed during the hot tearing test for longer, resulting in less cracking. Thus, further tests were carried out in which grain refiner additions were made to metal from batch 8. Two levels of addition of Al-5Ti-1B rod were made; 1 part grain refiner per 1000 of melt (50ppm Ti, 10ppm B) and 1 part per 5000 (10ppm Ti, 2ppm B). The results are shown in the figure overleaf. The lower grain refiner addition (10ppm Ti, 2ppm B) reduced the number of cracked fingers from four down to only two fingers. The higher grain refiner addition (50ppm Ti, 10ppm B) resulted in no fingers cracking across completely, with only small cracks in the longest finger.

7 4 Cracks GR Addition of 10ppm Ti, 2ppm Boron 2 Cracks GR Addition of 50ppm Ti, 10ppm Boron 1 Partially Cracked A further test with the same addition of Al-5Ti-1B rod (50ppm Ti, 10ppm B) was carried out on ingot from batch 7. The result was the same; there were only small cracks in the longest finger. Sections were taken from the second longest finger of the hot tearing tests, approximately 10cm from the bottom of the finger, and were polished and etched to reveal the grain structure. The table below gives the approximate grain size for all fully filled hot tearing tests. Test ID Ingot Batch No. of Cracks Grain Size (µm) HT HT HT HT HT HT HT HT HT HT HT HT HT HT HT

8 The figure below is a plot of grain size vs. number of cracked fingers in the hot tearing test. The results show that to reduce the hot cracking significantly the grain size needs to be less than 500µm. They also show that the difference in cracking behaviour between similar tests is due to variations in grain size. For example, test HT6 which had only two cracked fingers showed a grain size of 450µm, whereas test HT7 cracked across four fingers and had a grain size of 800 µm Grain Size (µm) Batch Number of Cracks Prefil Testing with Grain Refiner In addition to the Prefil tests on the ingot samples, Prefil tests were carried out to demonstrate the sensitivity of Prefil to grain refiner additions. The baseline curve was that of ingot with no addition (batch 7), which showed a relatively straight line in the upper half of the Prefil Production Window. Additions of grain refiner (5 and 10ppm B) resulted in the Prefil test becoming increasingly curved, which is a typical response to an increased loading of fine particles. This is illustrated in the figure overleaf.

9 Upper Bound Weight (kg) No GR 5ppm B 10 ppm B 0.4 Low er Bound Time (s) Tatur Tests Tatur tests were also taken from the melts used to cast the hot tearing tests. The figure below compares centre-line sections through Tatur test samples from the two batches of ingot and a grain refined melt. The nature of the pipe is different, with both base ingot samples showing a much rougher internal shrinkage pipe surface than the grain-refined sample. The volumes of shrinkage pipe and the calculated density of the solid are given in the table. Batch 7 + GR Rule of Mixtures Shrinkage Apparent Sample Density (g/cc) Pipe (cc) Density (g/cc) Batch GR The region at the base of the shrinkage pipe was sectioned to further study the dispersed porosity present. Overall there was less porosity in the grain-refined sample compared with the as-supplied ingots, although the volume of the shrinkage pipe was greater. The nature of the shrinkage in the Tatur test from batch 7 was different to that from batch 8. Batch 7 had bands of shrinkage below the main area of pores, running parallel to the base of the pipe. Metal from batch 8 did not show these bands below the main area of porosity. In both cases, oxide films were associated with the porosity. This was more marked in the sample from batch 8.

10 Thermal Analysis Thermal analysis was carried out using N-Tec s in-house Phaser system. Small samples were taken from the melts used to cast the hot tearing tests and were subsequently re-melted for analysis. The figure below shows the curves produced Batch (1) 2777(2) (1) Temperature (C) Time (Seconds) The main difference between the analyses from the two batches of ingot was in the cooling rate after the first nucleation of solid. Samples from batch 7 cooled more rapidly than those from batch 8.

11 Conclusions A series of MetalHealth tests and microstructural assessments have been carried out. The as-supplied ingots showed a difference in grain size, with Batch 7 having an average grain size of 300µm and batch 8 a grain size of 1000µm. Prefil testing and analysis showed that although the overall inclusion content of the two batches was quite low, batch 8 did contain a greater number of inclusions. The greatest difference was in the number of oxide films present. Very few titanium diboride (TiB 2 ) grain refiner particles were found in either sample. The degree of hot tearing was consistently worse for batch 8 than for batch 7, and the nature of the crack was found to be different. Correlation of the measured grain sizes from the hot tearing test fingers showed that the grain size was closely related to the number of fingers that cracked in the test. Addition of grain refiner reduced the hot cracking in both batches to virtually zero when the grain size was approximately µm. Tatur tests showed that batch 8 had a greater volume of shrinkage pipe and a higher apparent solid density than batch 7. Grain refinement of batch 8 resulted in a larger volume of shrinkage pipe, less dispersed porosity at the base of the pipe and a smoother internal pipe surface. Thermal analysis of the two batches of ingot showed that there was no significant difference in the α-aluminium nucleation temperature, although there was a difference in the plateau shape and length. In summary, differences in hot tearing characteristic were traced to different (and inconsistent) levels of grain refinement in the ingot stock. Adequate grain refinement almost eliminated hot tearing in both batches. 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