Accelerated method for testing soldering tendency of core pins Q. Han*1, H. Xu2, P. P. Ried3 and P. Olson4 An accelerated method for testing die soldering has been proposed and tested. High intensity ultrasonic vibration has been applied through a core pin to molten aluminium in order to simulate service conditions under die casting. Such conditions include high pressure and high impingement speed of molten metal on the pin. Soldering tendency of H-13 steel pins with or without commercial coatings was tested using this accelerated method. The experimental results indicate that soldering occurs within a few minutes of testing using this new method, much faster than that using the conventional methods. The coating failure mechanism identified in this new method is identical to that observed in the conventional methods, suggesting that the new method is suitable for testing soldering tendency of core pins under die casting conditions. Keywords: Die soldering, Die casting, Aluminium alloys, Ultrasonic vibration, Coatings
Introduction The soldering tendency of a coated pin is affected by the type of the coating material, bonding between the coating and the steel, interaction between the coating and the molten metal, impingement of molten metal on the coated pin, the pressure variation during die casting operations, and the heating/cooling process of the pin during service. Interaction with the molten metal tends to erode the coating. The impingement of molten metal on the coated pin may wash out the coating. The pressure variation leads to a stress variation in the pin. The heating and cooling process generates thermal stresses in the pin. Since both the pressure and the heating/cooling are alternating in nature due to the repeated use of the pin during die casting, they cause cycling stresses in the coated pin, which may cause fatigue cracks at the coating/steel interface.1–5 Two methods are widely used to test soldering tendency of coated pins under die casting conditions. One method uses a die casting machine and tests the pins in the cavity of a metal die.6–8 This method provides reliable data but it is costly since hundreds or thousands of castings have to be cast before soldering occurs on the coated pins. The other method is termed as dip test.8–10 A coated pin is repeatedly dipped in molten aluminium alloy for a certain amount of time, removed from the molten metal, and cooled in lubricant in order to simulate the service conditions of a pin during die
1
Department of Mechanical Engineering Technology, Purdue University, North Grant Street West Lafayette IN 47907 2021, USA Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 3 Ried, Engineering, 1210 Princess Anne Street, Fredericksburg, VA 22401, USA 4 Balzers, Inc, 2511 Technology Drive, Elgin, IL 60124-7832, USA 2
*Corresponding author, email
[email protected]
296
ß 2010 W. S. Maney & Son Ltd. Received 2 January 2007; accepted 11 February 2010 DOI 10.1179/136404610X12693537270217
casting operation. The dip and cooling process has to be repeated for thousands of times to get some soldering of the pin. Also for each dip, the pin has to stay in molten metal for an extended time, much longer than the time required for making a die casting, in order for soldering to occur. Furthermore, the dip test cannot simulate the high pressure and the high impingement speed of molten metal on the pin during die casting operations. As a result, the soldering tendency evaluated using the dip test has limited application. This article describes an accelerated method for testing soldering tendency of the coated pins. The method is similar to the dip test except that high intensity ultrasonic vibrations are applied to the coated pin. The idea is to use the coated pin as the ultrasonic radiator/wave guide that transmits ultrasonic vibration into molten metal. Because the radiator is vibrating at a high frequency, the relative velocity between the end of the coated pin and the molten metal is high. Also ultrasonic vibration induces high alternating pressure in the molten metal. The instantaneous pressure can be so high that cavitations in molten metal are induced. The collapse of cavitation bubbles results in a high instantaneous compressive pressure on the pin. Thus, both high relative speed and high alternating pressure are generated on the pin using high intensity ultrasonic vibration. It is expected that the use of high intensity ultrasonic vibration can create conditions that are comparable to that of the high pressure die casting process. For a typical die casting process, as documented in the ‘Metals handbook’,11 the gate velocity is in the range of 0?5– 10 m s21 (20–400 inch s21), and the metal pressure under intensification can exceed 69 MPa (10 ksi).
Experimental Figure 1 illustrates the apparatus built for the accelerated die soldering test. It consisted of an ultrasonic
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1: ultrasonic generator; 2: controller for electric furnace; 3: melt temperature indicator; 4: pneumatically operated device; 5: air inlet; 6: transducer; 7: booster; 8: horn; 9: core pin; 10: furnace 1 Experimental system for accelerated die soldering test
generator, a transducer, an ultrasonic horn, an ultrasonic radiator/wave guide to transmit ultrasonic vibration into aluminium melt, and an electric resistance furnace. The transducer was capable of converting up to 1?5 kW of electric energy at a resonant frequency of 20 kHz. The ultrasonic radiator, 3/4 inch diameter and 5 inch long (Fig. 2), was the pin to be tested. High intensity ultrasonic vibrations were applied axially to the pin made from H-13 steel. The amplitude of ultrasonic vibration at the end of the pin was 16 mm. The relative speed between the ultrasonic radiator and the molten metal in this study can be calculated. The displacement at the radiator tip is given by X ~AM cos (2pft)
(1)
where AM is the amplitude, f is the acoustic frequency, and t is time. The relative speed V between the radiator and the molten metal is given by dX (2) ~{2pfAM sin (2pft) dt Thus the maximum relative velocity between the radiator and the molten metal is VM ~2pfAM ~2|106 mm s{1 (3) ~2 m s{1 ~79 inch s{1 V~
This relative speed, ,2 m s21 or 79 inch s21, is close to the speed of metal flow during die casting operations. For conventional die casting processes, the gate velocity ranges from 0?5 to 10 m s21 (20–400 inch s21).11 The metal flow velocity inside the die is location dependent but usually slower than the gate velocity. The alternating pressure that can be generated near the ultrasonic radiator is difficult to estimate because the theory on the formation of cavitation has not been well developed. Literature data indicate that the threshold
Accelerated method for testing soldering tendency of core pins
2 Core pin used in accelerated die soldering testing
pressure of cavitation in molten metals is in the range of 0?2–100 MPa depending on alloy type, existing bubble size and oxide content in molten alloy, and the solid/ liquid interfaces.12 The intensity of ultrasonic vibration used in this experiment was so high that cavitation was generated in molten metal.13 The amplitude of the instantaneous pressure produced in the authors’ experiment exceeded the threshold pressure since cavitation occurred in molten metal. It is reasonable to assume that the amplitude of the instantaneous pressure generated using high intensity ultrasonic vibration is lower than, but comparable to, the pressure associated with die casting processes. For soldering testing, H-13 steel was used for making the ultrasonic radiator. The chemical composition of the premium quality AISI H-13 is given in Table 1. The as received material was soft annealed and the maximum hardness was 235 HB. The pins were then heat treated by a commercial heat treating company according to the procedure described by the ‘NADCA recommended procedures for H-13 tool steel’ (NADCA publication no. 229). The final hardness of the pins was in the range of 44–46 HRC. Heat treated pins were coated with four types of physical vapour deposited hard coatings by Balzers Inc. (Table 2). The coating microstructure was inspected using optical microscopy. Figure 3 shows the crosssectional microstructure of the coatings on the H-13 steel substrate. The thickness of the Balinit Lumena coating was ,7 mm. The thickness of all other three coatings was ,2 mm. A380 aluminium alloy was held in a graphite crucible and melted in the electric furnace. The temperature of the melt was controlled at 665¡15uC. During the testing, the radiator/pin was dipped in the melt for a certain period of time, and was then quenched with compressed air. After the dip testing, the interface between the soldered aluminium and the end of the pin was examined using optical microscopy and SEM.
Table 1 Chemical composition of H-13 steel Element
C
Si
P
S
Cr
Mo
V
Fe
wt-%
0.40
1.00
0.020 max.
0.003 max.
5.3
1.40
1.0
Balance
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a Alcrona (AlCrN); b coating D (CrN); c Futura nano (TiAlN); d Lumena (thicker TiAlN) 3 Thickness of coatings on H-13 steel substrate
the formation of this intermetallic layer is controlled by either the diffusion of aluminium into steel or the diffusion of Fe into molten aluminium depending on which element has a higher diffusion coefficient. It is well known that the thickness of a newly formed phase xI during a diffusion controlled process is given by15
Results and discussion Uncoated H-13 pins in molten A380 alloy Experiments were first carried out to test soldering of the uncoated H-13 pins. The pins were dipped into the A380 melt for different times with or without ultrasonic vibration. The experimental results on the uncoated pins without ultrasonic vibration are illustrated in Fig. 4, where the aluminium microstructure is white coloured and the steel is grey. The dipping times were 0?5, 1 and 5 min respectively. After the pin was dipped in molten aluminium for 30 s, intermetallic phases were observed at some locations on the end surface of the uncoated pin, but there was no metallurgical bond between the aluminium and steel. The presence of a gap indicated that soldering had not yet occurred at this moment.14 Metallurgical bonds between steel and aluminium began to form when the pin was dipped in molten metal for 1 min (Fig. 4b). As the dipping time was increased, the thickness of the reaction layer was also increased. The thickness of the intermetallic layer was ,60 mm, of which ,15 mm near the steel is a dense layer of intermetallic phase and the other 45 near the aluminium contains a mixture of intermetallic phases (grey coloured) and aluminium (white coloured). Obviously,
xI ~2aðDtÞ1=2
(4)
where a is an interfacial constant, D is the diffusion coefficient, and t is the diffusion time. The diffusion coefficient D of solute is in the order of 1029 m2 s21 in liquid and is 10212 m2 s21 in solid.15 Equation (4) can be used to estimate the thickness of the intermetallic layer shown in Fig. 4c. For a diffusion time of 300 s (5 min) assuming a50?114,15 the thickness of the intermetallic layer is 125 mm if the liquid diffusion coefficient is used and is 1?97 mm if the solid diffusion coefficient is used. The intermetallic layer shown in Fig. 4c is most likely due to diffusion of Fe into molten aluminium because (i) the thickness is an order of magnitude larger than calculated using a solid diffusion coefficient (ii) aluminium and intermetallic phase coexist in the layer near the aluminium alloy, which is a typical
Table 2 Details of Balinit die casting coatings provided by Balzers
298
Coating
Alcrona
D
Futura Nano
Lumena
Composition Hardness, HV Thickness, mm Maximum temperature, uC Coating colour Coating type
AlCrN 3200 1–5 1080 Violet grey Monolayer
CrN 1750 1–6 1300 Silver grey Monolayer
TiAlN 3300 1–4 1600 Violet grey Nanolayer
TiAlN 3400 8–15 1650 Violet grey Nanolayer
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4 Microstructure near surface of uncoated pins after dipping in molten A380 alloy (without ultrasonic vibration) for a 30 s, b 1 min and c 5 min
feature of the growth of solid phases into a molten alloy. Figure 5 shows the microstructure of the uncoated pin after it was subjected to high intensity ultrasonic vibration for 1 and 5 s in molten A380 alloy. Soldering has occurred on the pin in both cases. No gap between the steel pin and the A380 alloy was found throughout the entire surface (the end surface of the pin). The SEM image of the uncoated pin subjected to high intensity vibration in molten metal for 1 s is shown in Fig. 6. A uniform layer of intermetallics was formed on the pin surface. Composition analysis using energy dispersive spectrometer (EDS) suggested that the intermetallics contain Al, Fe and Si. These kinds of intermetallics usually occur in the reaction layer between H-13 and A380 alloys.16,17 The thickness of the intermetallic layer is ,1 mm. Calculating using equation (4) yields a thickness of 7?2 mm if the liquid diffusion coefficient is used and is 0?23 mm if the solid diffusion coefficient is used. In fact the diffusion time is much shorter than 1 s because it takes time to heat the surface of the pin from room temperature to elevated temperatures. The use of high intensity ultrasonic vibration produces ultrasonically induced streaming at the pin surface.12 Ultrasonically induced streaming is a turbulent flow starting at the solid/liquid interface towards the molten metal. This makes heat transfer between the pin and the molten metal much faster. It also plays an important role in cleaning the surface of the pin so intimate contact between the pin and the molten metal can be achieved. Any air gap or surface contaminations that separate the pin from the molten metal can be mechanical barriers for the diffusion of elements across the interface. Assuming the diffusion
time is a fraction of 1 s, the thickness of the intermetallic layer shown in Fig. 6 will be more closer to the thickness calculated using the liquid diffusion coefficient. Comparing the microstructure of the uncoated H-13 pin surface subjected to ultrasonic vibration with that without vibration, it can be concluded that soldering occurred 60 times faster for uncoated pins subjected to ultrasonic vibration than without ultrasonic vibration. The results indicate that high intensity ultrasonic vibrations can be used for an accelerated testing of soldering tendency of core pins.
Coated H-13 pins in molten A380 alloy Soldering should not occur on the coated pins in the dip test since the coatings are made of line compounds that do not react with molten metal. Indeed, no soldering was observed when the coated pins were dipped into molten aluminium alloys for 8 h. Also, no soldering was observed in a conventional dip test (involving repeatedly dipping a coated pin in molten alloy for 15 s followed by quenching in a water based lubricant for 15 s), up to the point at which the test was interrupted after 2000 cycles. When the coated pins were subjected to the ultrasonic test in molten A380 alloy, soldering occurred in a few minutes, representing a significant reduction in testing times. Figure 7 shows the tip of an H-13 pin with Alcrona coating after 5 s dipping with ultrasonic vibration. At some of the locations on the coating at the end of a pin, interactions of molten aluminium with the steel pin have already started, indicating that the onset of soldering has occurred. Figure 8 shows the SEM image and the EDS result of an intermetallic layer in the soldering area for the H-13 pin with Alcrona coating. Energy dispersive
5 Microstructure near uncoated H-13 pin surface after pins were dipped in to molten A380 alloy for a 1 s and b 5 s under high intensity ultrasonic vibration
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6 Image (SEM) of microstructure on pin surface and EDS analysis results of intermetallic layer on surface of uncoated pin subjected to high intensity vibration in molten A380 alloy for 1 s
y60 s or longer for SAF to reach one for the coated pins. The experimental results are in agreement with industrial observations that coated pins are more resistant to soldering under die casting operations. Of the commercial coatings, Lumena resisted soldering longest in the ultrasonic tests. It took y30 s to reach an SAF value of one for the pins coated with Alcrona or D layers and y60 s for Futura nano, but the SFA has reached only 0?74 after 240 s for the pin with a Lumena coating. These results are in agreement with industrial experience, based on information provided by Balzers.
spectrometer results indicate that intermetallic phases containing Al, Fe and Si have been formed at the locations where soldering has initiated. The testing results on the other three types of coating were similar, the only significant variation being in the initiation time for soldering. It is usually difficult to determine the moment when the onset of soldering has actually started. To evaluate the soldering tendency of a coating, a soldering area fraction (SAF) was defined as the ratio of the area of the soldered region over the total surface area at the end of a pin. Figure 9 shows the plots of the measured SAF against the dipping times when the coated pin was subjected to ultrasonic vibration. Data used in Fig. 9 are listed in Table 3. The SAF value of the uncoated H-13 pin is unity at 1 s, indicating that the end surface of the pin is totally covered by a reaction layer, whereas it took
Mechanism of coating failure under accelerated testing Tests that use a coated pin to transmit ultrasonic vibration into the molten metal create conditions similar to those experienced in die casting. At high frequency (20 k Hz in the present work), the relative velocity between the end of the pin and the melt is sufficient to generate impingement and cavitation in the molten metal, while the vibration induces alternating stresses in the pin. The collapse of the cavitation bubbles during the compression phase of the wave creates shock waves, resulting in a high instantaneous compressive pressure on the pins. Thus, pins dipped in the melt will experience melt impingement, fatigue due to pressure variation, and chemical attack. Failure of coated pins under such severe conditions is likely to be caused by fatigue failure of the coating, producing cracks or other defects that can act as initiation sites for soldering.
7 Tip of H-13 pin with Alcrona coating after being dipped into molten A380 alloy for 5 s with ultrasonic vibration
300
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9 Soldering area fraction of different coatings as function of dipping time (t) when coated pin is subject to high intensity ultrasonic vibration in molten A380 alloy
The similarity of the coating failure mechanism identified in the sample tested using this new method with that observed using the conventional methods indicates that soldering tendency of a core pin can also be determined using this new method. If each cycle of ultrasonic vibration represents one cycle of die casting operation or dip test, soldering occurs in a short time frame because the core pin vibrates at a frequency of 20 kHz. This makes ultrasonic vibration ideal for the accelerated testing of die soldering.
Conclusions
8 Image (SEM) and EDS results of intermetallic layer and remaining coating (AlCrN) in soldering area on H-13 pin with Alcrona coating: pin was subject to high intensity ultrasonic vibration
Figures 7 and 8 show the SEM observation that soldering on the coated pins initiates at the site of localised coating failure(s) or pre-existing coating defects. When molten metal comes in contact with the steel substrate where a coating discontinuity exists, reactions with the molten aluminium begin which preferentially and continuously corrode or erode the steel matrix. The fragile, suspended section of the coating layer can also be easily broken by the flowing molten metal or by the varying pressures in molten metal near the end of the pin. The initiation of soldering on a coated pin (Figs. 7 and 8) has observed by other researchers using the conventional soldering testing methods.10 As soon as such a soldering initiation starts on a coated pin, soldering of a whole coated pin will occur soon. Thus, it is important to have a coating with strong bond to the H-13 steel matrix in order to prevent premature coating failure during die casting operations.
An accelerated method for testing die soldering tendency of core pins has been proposed and tested. High intensity ultrasonic vibration is used to generate high relative velocities between a coated core pin and the molten metal in which it is dipped. The high instantaneous pressures on the pin simulate the conditions experienced during die casting operations. The results indicate that tests under high intensity ultrasonic vibration accelerate die soldering and have potential for use as a rapid test to indicate performance in practice. Under simple dipping, it took more than 1 min for soldering to initiate on uncoated H-13 steel pins, but only 1–2 s under high intensity ultrasonic vibration. Coated H-13 pins experienced soldering initiation times between 15 s and 3 min, compared with many hours or several thousand cycles using conventional soldering tests. The experimental results are in qualitative agreement with industry experience of the coatings examined.
Table 3 Soldering area fraction of commercial coatings after given dipping times Dipping time, s Coating
1
5
10
15
30
60
120 240
Uncoated H-13 Steel Alcrona D Futura nano Lumena
1
…
…
…
…
…
…
…
… … … …
0.14 0.32 0.14 …
0.41 0.38 0.22 …
0.56 0.40 0.58 …
0.97 1 0.92 0.003
1 1 0.97 0.03
… … … 0.15
… … … 0.74
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5. J. L. Lin, S. Myer, O. Salas, S. Carrera, P. Ried, J. Brennan, B. Mishra and J. Moore: in ‘Surface engineering in material science’, 163–175; 2005, Warrendale, PA, TMS. 6. Z. W. Chen, and M. Z. Jahedi: Mater. Design, 1999, 20, 303–309. 7. R. Shivpuri, M. Yu, K. Venkatesan and Y.-L. Chu: J. Mater. Eng. Perform., 1995, 4, (2), 145–153. 8. J. Wallace, D. Schwam, Y. Zhu and S. Birceanu: in ‘Transactions of the North American Die Casting Association’, T03–041; 2002, Rosemont, IL, North American Die Casting Association. 9. G. Engleman, S. Viswanathan, C. A. Blue, Q. Han and N. B. Dahotre: ‘Transactions of the North American Die Casting Association’, T02–43; 2002, Rosemont, IL, North American Die Casting Association. 10. J. Moore, S. Carrera, J. Lin, O. Salas, B. Mishra, G. Mustoe and P. Ried: in ‘Transactions of the North American Die Casting Association’, T03–025; 2003, Rosemont, IL, North American Die Casting Association. 11. D. M. Stefanescu (ed.): ‘Metals handbook’, 9th edn, 291; 1988, Metals Park, OH, ASM International. 12. O. Abramov: ‘High-intensity ultrasonics theory and industrial applications’, 122; 1998, New York, Gorden and Breach Science Publishers. 13. H. Xu, X. Jian, T. T. Meek and Q. Han: Mater. Lett., 2004, 58, 3668–3672. 14. Q. Han and S. Viswanath: Metall. Mater. Trans. A, 2003, 34A, 139–146. 15. X. Wan, Q. Han and J. D. Hunt: Metall. Mater. Trans. A, 1998, 29A, 751–755. 16. S. Shankar and D. Apelian: Metall. Mater. Trans. B, 2002, 33B, 465–476. 17. M. Yu, R. Shivpuri and R. A. Rapp: ASM Int., 1995, 4, 175–181.
Acknowledgements Research was partly sponsored by the North American Die Casting Association and partly sponsored by the US Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCar and Vehicle Technologies, Automotive Lightweighting Materials Transportation Technology Program, the ORNL SHARE user facility, under contract no. DE-AC05-00OR22725 with UT-Battelle, LLC.
References 1. S. Chellapilla, R. Shivpuri and S. Balasubramaniam: in ‘Transactions of the North American Die Casting Association’, 295–305; 1997, Rosemont, IL, North American Die Casting Association. 2. Y. Tsuchiya, H. Kawaura, K. Hashimoto, H. Inagaki and T. Arai: in ‘Transactions of the North American Die Casting Association’, 315–323; 1997, Rosemont, IL, North American Die Casting Association. 3. M. Sundqvist, J. Bergstrom, T. Bjork and R. Westergard: in ‘Transaction of the North American Die Casting Association’, 325–328; 1997, Rosemont, IL, North American Die Casting Association. 4. W. Jiang and P. Molian: Surf. Coat. Technol., 2001, 135, 139– 149.
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