Surface and Coatings Technology 163–164(2003) 668–673

0257-8972/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž02.00693-X

Investigating ion nitriding for the reduction of dissolution and solderingin die-casting shot sleeves

Vivek Joshi , Amit Srivastava , Rajiv Shivpuri *, Edward Rolinskia a a, b

The Ohio State University, 1971 Neil Avenue, 210 Baker Systems, Columbus, OH 43210, USAa

Advanced Heat Treat Corporation, 1625 Rose St., Monroe, MI 48162, USAb

Abstract

As liquid aluminum at 7808C is pored into the horizontal shot sleeve of a die casting machine, the surface gets washed outby the molten stream exposing the die steel to chemical dissolution and soldering, which leads to failure. This paper investigatesthe efficacy of ion nitriding surface treatment for preventing surface dissolution and soldering. Cylindrical coupons with variousnitriding depths are dipped in hot liquid aluminum melt for a predetermined time. The amount of soldered metal and the weightloss due to dissolution was measured. Ion nitriding is shown to significantly reduce both of these wear modes at shorter diptimes, with larger reductions seen for deeper nitrided case depths. Also, ejection tests were done on a casting solidified on thenitrided pin and the force of ejection was measured. It is found that ion nitriding significantly reduces this force, which is asurrogate measure of wettability and adhesion.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Nitriding; Dissolution; Soldering; Shot sleeve; Wettability; Adhesion

1. Introduction

The die-casting process is defined as a net-shapedmanufacturing process in which the molten metal isinjected at high speeds and pressure into a metallic die.Pressure is maintained within this die until solidificationhas been completed. The die casting process is used tomake parts that have geometrically complicated shapeswhich cannot be made by other manufacturing processesor cannot be mass produced cost effectively with otherprocesses. Due to the use of metallic molds, this processis primarily used in casting of alloys of aluminum, zincand magnesiumw1x.

Typical operating sequence for the cold chamber diecasting process is shown in Fig. 1:

1. Die is closed and molten metal is ladled into the coldchamber cylinder.

2. Plunger pushes molten metal into die cavity. Then,the metal is held under high pressure until it solidifies.

3. Die opens and the plunger follows to push thesolidified slug from the cylinder. Cores, if any, retract.

*Corresponding author. Tel.:q1-614-292-7874; fax:q1-614-292-7852.

E-mail address: shivpuri.1@osu.edu(R. Shivpuri).

4. Ejector pins push casting off ejector die and plungerreturns to original position.

The shot sleeve, shown in Fig. 2, plays a criticalfunction in the die-casting operation. It receives themolten metal from the ladle, holds the hot metal untilthe pouring waves subside and provides guidance to theplunger during injection. Lewis et al.w2x consideredshot sleeve parameters and presented six independentoperating variables, which influence wave formation andcasting porosity. These independent variables are:(1)shot sleeve diameter;(2) filling percentage;(3) slowshot speed;(4) starting position of fast-shot speed;(5)pouring rate; and(6) shot delay time. These variablesand others such as molten metal temperature, moltenmetal impact, shot sleeve metal quality, shot sleevemetal type, and shot sleeve hardness also influence shotsleeve life.

The life of the shot sleeve used in aluminum diecasting has traditionally been measured by the numberof shots delivered without significant deterioration in itsoperation. H13 tool steel quenched and tempered to 46–50 HRC, has commonly been used for the fabricationof shot sleeves. This alloy generally displays toughness,

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Fig. 1. Typical operating sequence for the cold chamber die-castingprocess. Fig. 3. Photograph illustrating a cross-section of the washout area that

was immediately opposite the pouring hole on the shot sleeve with9600 shots.

Fig. 2. Picture of a shot sleeve.

thermal stability, and thermal fatigue properties suitablefor shot sleeve applicationw3x.

The types of shot sleeve failures include washout(dissolution), soldering, thermal fatigue, gross or large-scale cracking, and erosion or scoring of the internalsurface. Dissolution and soldering occur along the insidesurface where the plunger pushes the molten aluminum.Washout is located primarily below the pouring holewhere the molten metal contacts the inside of the sleeve.Fig. 3 illustrates the cross-section of the washout areajust below the pouring hole after around 10,000 shots.Thermal fatigue cracking occurs on the internal surfaceproducing an array of cracks at the location of contactwith molten aluminum. Gross checking occurs acrossthe wall thickness and produces catastrophic failure.Also, the inner diameter of the shot sleeve distorts dueto the thermal gradient and does not stay round or co-axial with the plunger, causing the molten metal toescape between the sleeve and the plunger. This escapedmetal can result in welding of the plunger to the shotsleeve.

The objective of this study was to investigate theefficacy of ion nitriding treatment in preventing thefollowing:

● soldering in the inside surface of the shot sleeve thatcan affect the normal operation of the plunger, and

cause sticking of the plunger during the slow shotsequence; and

● dissolution of the shot sleeve surface below the pourhole that results in the formation of deep grooves inthe shot sleeves and its operational failure.

The tools used in this investigation include numericalmodeling of the pouring and injection operations todetermine the thermo-mechanical conditions experi-enced by the shot sleeve inner surface, hot dip(immer-sion) tests to determine the soldering and dissolutionrates, and ejection tests to determine the adhesivetendency of the treated surface.

2. Numerical modeling

The objective of numerical modeling was to simulatethe physical phenomena during the pouring and shotfilling stages of the injection process. Commercial soft-ware FLOW-3D was used for simulating pouring of thehot liquid into the shot sleeve, its filling and injectioninto the biscuit. It is assumed that once the slow shotvelocity is completed, the liquid metal is no longer incontact with the surface near the pouring hole.

FLOW 3D is a finite difference code that uses volumeof flow (VOF) approach to model the fluid flow andsolidification in internal cavities. Details of this codeare available in Kannanw4x. While it can adequatelymodel fluid flow and solidification inside the shotsleeve, it is not strong in heat transfer. Therefore, thethermal boundary conditions needed to be simplified toget reasonable results.

Fig. 4 shows the 3-D model of the shot sleeve andthe sprue(pipe) that transfers the liquid metal from thedosing furnace to the shot sleeve. To simplify the modelit is assumed that the dosing furnace tube, the shotsleeve, and the runner system are surrounded by H13metal. The heat flux into the shot sleeve surface isprimarily controlled by heat transfer coefficients and the

670 V. Joshi et al. / Surface and Coatings Technology 163 –164 (2003) 668–673

Fig. 4. Three-dimensional model of the shot sleeve and the sprue(pipe) that transfers the liquid melt from the dosing furnace to theshot sleeve.

Fig. 5. Wall temperature vs. timew4x.

temperature difference between the cast metal and theshot sleeve surface. This assumption is justified as thefocus of this study is on thermo-mechanical conditionsat the shot sleeve surface and not on the distortion ofthe shot sleeve.

Since the soldering and dissolution cannot be simu-lated directly, the objective of the simulation is todetermine the velocity, temperature, contact time, andsolid fraction for the shot sleeve during a typical die-casting cycle. The simulations were carried out with thefollowing factors: A 380 aluminum–silicon alloy cast atpouring temperature 6808C; H13 die steel with shotsleeve of 147 mm in diameter, and an active strokelength of 920 mm at 2508C; pouring time of 4.5–8.5s; shot delay time 4 s; pouring temperature of 7008C;and shot weight of 26.78 kg.

The results of the simulations showed that the velocityat the shot sleeve inner surface, below the pour-hole,varied from 0.8 to 1.13 mys, and the temperature variedfrom 5008C (steady state temperature) to a peak of 5508C. The shot sleeve surface never reaches the liquidmelt surface temperature unless the lubricant(and oxidecoatings) breaks down and the soldered metal is directlyin contact with the die steel. Fig. 5 is the plot of walltemperatures at three different locations as a function oftime. The temperatures are measured at a distance of 5mm from the inner surface of the shot sleeve. The walltemperature is calculated for location right underneaththe pour hole. The temperature falls as the distance fromthe pour-hole increases.

3. Nitriding process

Ion nitriding is an extension of the conventionalnitriding process using the plasma-discharge physics. Invacuum, high-voltage electrical energy is used to formplasma through which nitrogen ions are accelerated toimpinge on the workpiece. This ion bombardment notonly cleans the workpiece surface but also heats up thesurface and provides active nitrogen for nitriding. The

parts to be nitrided are cleaned, loaded into the vacuumchamber and secured. The chamber is evacuated bymeans of a roughing pump so that the pressure isreduced to a level of 7–14 Pa. The initial air and anycontaminants are removed from the chamber. Resistanceheaters or cathode shields are used to bring the load tonitriding temperatures(375–6508C) before the glowdischarge. While heating, the pressure is increased sothat the glow stream does not get too thick and causelocalized overheating. After the workload is heated tothe desired temperature, process gas is introduced intothe chamber at a flow rate determined by the loadsurface area. Pressure is maintained between 140 and1400 Pa. The process gas is normally a mixture ofnitrogen, hydrogen and sometimes, small quantities ofmethane. In the presence of this process gas, thisworkload is maintained at a high negative DC potential(500–1000 V) with respect to the vessel. Under theinfluence of this high voltage, the nitrogen gas isdissociated, ionized and accelerated towards the work-load (cathode). Upon impact with the workpiece, thekinetic energy of the nitrogen ion is converted to heat,which can bring the load to the nitriding temperature.The glow discharge surrounding the negatively chargedworkpiece forms at voltages of 200–1000 V with gaspressures of 140–1400 Paw5x.

During the glow discharge process, different speciesand iron atoms from the workpiece combine with thenitrogen as it diffuses into the material, forming ahardened surface and case. A uniform glow discharge isnecessary for uniform case depth. After the nitridingcycle, cooling is achieved by back filling the chamberwith nitrogen or other inert gas and re-circulating thegas through a cooling device.

The advantages of the process are the following:pollution is totally absent, efficient use of gas andelectrical energy is possible, total process automation ispossible, selective nitriding can be accomplished bysimple masking techniques, process span that encom-passes all subcritical nitriding, and reduced nitridingtime and the lower process temperatures can be used.

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Fig. 6. Hardness depth profiles for nitriding depths after 4, 20 and 60h.

Fig. 7. Soldering after a:(a) 5; and(b) 2 h dip in Plain H13 and 390mm nitrided pin.

The common disadvantages are: high capital cost of thesystem; the need for precision fixturing with electricalconnections; and longer times compared to other nitro-carburising techniquesw6x.

Fig. 6 shows the depth hardness profiles from thepins nitrided in three batches. The first batch was 60 h,the second 4 h, and the third was 20 h. The finaltemperature was 525–5358C. Gas composition, 20%nitrogenq80% hydrogen. The case was defined as 50HK above the core hardness. The total case depth0.1gms

for 4 h nitriding was 0.099 mm, for 20 h nitriding was0.256 h, and for 60 h nitriding was 0.391 mm.

The microstructure of H13 tool steel nitrided for 4,20 and 60 h, respectively, at 5108C shows a singlelayer of Fe N and a diffusion zone of nitride containing4

tempered martensite. The thickness of compound zonefor 4 h nitriding was 2–4.5mm, for 20 h nitriding was3.4–5.6mm and for 60 h nitriding was 5.6–6.7mm.

4. Test results and discussions

The coupons are standard DME core pins that wereheat treated as follows: stress relieved for 0.5 h at 5378C; vacuum hardened for 90 m at 10248C; quenched,tempered for 3 h at 5378C; tempered again for 3 h at600 8C; and tempered again for 2 h at 5408C. The finalhardness was measured as 46–48 HRC.

The aluminum–silicon alloy A380 is heated to themelt temperature 6808C in an alumna crucible. The testcoupons are then immersed in the molten aluminum.The temperature of the aluminum bath is automaticallycontrolled by a temperature controller. After dipping forthe predetermined time, the pins are extracted andcooled. The coupons are subjected to further analysis.The detailed test procedure and coupon geometries areexplained elsewherew8x.

As explained earlier, in the present study, four differ-ent types of coupons are used. These are plain H13,H13 nitrided at 100, 250 and 390mm. These couponswere subjected to five different dip times 5 s, 30 s, 300s, 30 min and 2 h. The test procedure was as follows:

1. Dipping in A380 at 6708C for the predeterminedtime (as shown above) and allowing the coupons tocool.

2. Macroscopic analysis.3. Measuring the height and length of the soldered layer.4. Microstructural analysis

Fig. 7a,b shows the appearance of pins after dippingin molten aluminum for 5 s and 2 h, respectively. Fig.8 shows the results of the height of the soldering profilefor 5, 30 and 300 s dip time. Fig. 9 shows themicrostructural analysis of the soldered layer for 5 and

672 V. Joshi et al. / Surface and Coatings Technology 163 –164 (2003) 668–673

Fig. 8. Soldering profile peak heights.

Fig. 9. Appearance of soldered layer at magnification 100=, etched in 2% Nital for(a) 5, and(b) 300 s dipping.

300 s dipping time for both plain H13 and nitridedcoupons. The following is observed in these figures:

● For smaller dip times(5 s), nitriding compared toplain H13 significantly decreases the amount ofsoldered material on the pin(Fig. 7a) with thereduction in soldering is directly proportional to thedepth of the nitrided case(Fig. 8).

● For larger dip times, dissolution of the pin increasesand the pin diameter decreasesw7x. This counteractsthe increase in diameter due to soldering. This effectis evident at 30 s and even more evident at 5 m dips.At 2 h dip, the diameter of pin after soldering and

dissolution is smaller than the starting diameter(Fig.7b).

● At smaller dip times(5 s), while there is solderingon the pin, there is negligible corrosive interactionbetween the pin surface and the soldered material(Fig. 9a). This interaction is even less with heaviernitrided case. This lack of wetting is important fortribology of the interface.

● At higher dip times intermetallic layer forms on theplain H13 surface. While the heavy nitrided casesuccessfully protects the substrate from the chemicalinteraction(Fig. 9b). This was investigated further inthe ejection test.

Molten aluminum has an affinity for the steel surface,bare or coated H13 die steel. This affinity results inchemisorption and adhesion(welding) of aluminum onthe pin surface. This adhered surface substantiallyincreases the ejection force required to separate thecasting from the die surface. A tribologically soundsurface(well lubricated with no adhesion) will permitcleanylow force ejection of the casting. Adhesivestrength between a die casting alloy and die steel isrelated to the soldering tendency of the two materials.This adhesive strength was measured by a speciallydeveloped ejection test.

The test consisted of solidifying a casting aroundcoated pin, and then pulling the casting from the pin

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Fig. 10. Ejection force test results for a 390mm nitrided coupon vs.plain H13.

using a fixture mounted on an MTS machine. Thedetailed test set-up and testing procedure is explainedelsewherew8x. This force of separation is a measure ofthe adhesive tendency of the casting to the pin surface.A small crucible was used as a mold for the castingprocess. Measured amount of A380 was melted in thiscrucible and coated pin was dipped to a constant depth.After a predetermined time, the melt solidified onto thedipped pin. This solidified cylinder(casting) was thenejected from the pin using a specially designed someclamping mechanism. And the force of ejection wasmeasured. Fig. 10 shows the plots for the ejection testsfor nitrided pins and H-13 pins. It can be seen thatnitrided pins show minimum ejection forces and thusthe minimum soldering tendency.

5. Conclusions

In this study, H-13 hot working die steel pins wereion nitrided with varying case depths. The nitrided pins

were then dipped in liquid aluminum alloy for predeter-mined time and removed. Soldering tendency of thesurface was measured from the amount of solderedmetal and the dissolution tendency by the dissolution inpin surface. It is seen that ion nitriding significantlyreduced both the soldering and dissolution interactionof the surface with higher reductions seen at higher casedepths. This improvement in tribology(lower wettabilityand adhesion) is confirmed by measuring forces ofejection between the nitrided surface and the solidifiedcast metal.

Acknowledgments

The authors acknowledge the support received fromthe Office of Industrial Technologies, Department ofEnergy for the SBIR grant(DE-FG02-98ER82702), andto UES, Inc. which made this research possible.

References

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w2x R.L. Lewis, Experimental investigation into the investigationof shot delay time on air entrapment and metal flow in coldchamber die casting, NADCA Transactions, 15th Die CastingCongress and Exposition, October 16–19, St. Louis, MO,1989, G-T89-061.

w3x G.M. Goodrich, Aluminum die-casting shot sleeves: a metal-lurgical study comparing long life and short life, NADCATransactions, Die Casting Congress and Exposition, Detroit,MI, 1991, T91-073.

w4x S. Kannan, Masters Thesis, The Ohio State University, 1999.w5x K. Kulkarni, Masters Thesis, The Ohio State University, 2000.w6x ASM Handbook of Heat Treating, vol. 4, August 1991, pp.

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