Engineering Failure Analysis 17 (2010) 1285–1289

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Engineering Failure Analysis

journal homepage: www.elsevier .com/locate /engfai lanal

Cold shut formation analysis on a free lead yellow brass tap

V. Vazquez a, A. Juarez-Hernandez a,*, A. Mascarenas b, P. Zambrano a,M.A.L. Hernandez-Rodriguez a

a Universidad Autónoma de Nuevo León, Facultad de Ingenieria Mecanica y Electrica, Ciudad Universitaria, San Nicolas de los Garza, Nuevo Leon, C.P. 66450, Mexicob American Standard, Carretera Mexico Laredo Km 1011, Cienega de Flores, Nuevo Leon, C.P. 65550, Mexico

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 October 2009Received in revised form 5 March 2010Accepted 13 March 2010Available online 16 March 2010

Keywords:SolidificationLead freeYellow brassTilt angleProCast

1350-6307/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.engfailanal.2010.03.002

* Corresponding author. Tel.: +52 81 14920365; fE-mail address: artjua@yahoo.com (A. Juarez-He

In order to minimize the risk of the lead in the health, the chemical composition of the plumb-ing components have suffered changes. However, lead improves machinability and castabil-ity in copper alloys. In this work a failure created during solidification process on a lead freetap of yellow brass alloy was investigated. Visual inspection, SEM, EDS and a ProCast V.2008simulation were carried out in order to determine the cause of this failure. The results exhib-ited a close correlation between the failure formation and a convergent metal flow and solid-ification involved during the tilt process and determines that failure was caused by a coldshut, also inclusions carrying from the core and metal oxide films were also found.

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1. Introduction

The use of lead free brass castings in the fitting industry has been increasing due to the new world regulations and torestrictions on the leachable lead in water [1]. Bismuth has been found to be the best replacement for lead as a free machin-ing additive although Bi is found to segregate toward the grain boundaries because of its lower melting point and solubilityin copper [2–4]. As it is well known that a change in composition brings a change in alloy castability and as a consequencecommon casting defects such as: porosity cracks, hot tearing and shrinkage can appear. Defects play an important role in thecasting mechanical properties [5]. For a casting process it is challenging to be controlled due to combined phenomenaresponsible for its manufacture including: restricted interdendritic fluid flow in solidifying metal, soluble gas partitioningbetween liquid and solid, pore nucleation and growth, and interaction of pores with developing microstructure [6–8].

The purpose of the present study was to determine the root cause of a failure presented in a Bismuth yellow brass castingand to know the effect of tilt angle and casting conditions on the failure formation for one side of the tap. During the castingprocess the defect tap percentage was around 40%, after visual inspection, the investigation analyzed the damage zone usinga stereoscope, and microanalysis with an scanning electron microscope, simulations with a commercial software ProCastV.2008 were made at different process conditions, finding the tilt angle as the most relevant parameter to control this defect.

2. Experimental procedure

2.1. Casting process

For doing the casting process a semiautomatic casting machine, a permanent mould consisted of two lateral parts and onesand core were used, experimental time conditions were set with the help of a PLC in seconds, Table 1, for each cycle. Melt

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ax: +52 81 10523321.rnandez).

Table 1Cycling process times in seconds.

Filling

Tilt Process

Solidification

Cast ejection

Water cooling

Mould closing

Sequence Operation

0-5

2-5

5-14

14-15

15-18

18-19

Time operation

68οi

ii

45.3 ο

27.2 ο0 ο

a b c

d eFig. 1. Temperature distribution at different times during the casting process before tilt at: (a) 1, (b) 2 s, during tilt at (c) 3, (d) 3.86 s and after tilt at (e) 5 s;which consists of (i) two mold metal parts and (ii) one sandcore.

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was pour into the mould cavity during 5 s with an inlet of 0.490 kg/s at a temperature of 1050 �C, during the pouring be-tween 2 and 5 s, mould was tilt from the angle selected to the vertical position at a constant angular velocity, Fig. 1, solid-ification process took 9 s and cast was ejected in 1 s, afterwards mould was deep 4 s into a graphite solution in order to cool itand finally it was closed in 1 s to start the cycle again, having a total cycle time of 19 s, Table 1.

In order to characterize the failure on the tap component, Fig. 2a, visual inspections were performed, subsequently thefailure was studied by scanning electronic microscope (SEM) and energy disperse spectroscopy (EDS) in order to explorein high magnifications the morphology and identify elements or contaminants involved in the failure formation.

Commercial software: ProCast V.2008 was used to simulate the filling and solidification behaviour of the yellow brass alloy(0.7 Bi, 0.04 Sn, 0.6 Al, 0.01 Fe, 0.01 Si, 36.7 Zn and Cu balance in wt.%). The parameters used in simulation were as shown inTable 1, using a 2000 W m2/K as a heat transfer condition between metallic mould and casting and 1000 W m2/K as a heattransfer condition between sand core and casting, with these parameters different tilt angles were studied.

3.5 cma b

dc

e fFig. 2. Picture shows: (a) as cast yellow brass tap (zone defect white circle), (b) without defect and SEM micrographs and EDX analysis defects for, (c) of thecrack zone and (d), and particle (e) and (f) respectively, for a tilt of 68�.

1288 V. Vazquez et al. / Engineering Failure Analysis 17 (2010) 1285–1289

3. Results and discussion

3.1. Visual inspection

Fig. 2a exhibits an extended failure located on the surface of one side of the casting, a zone of this failure was analyzed(white circle), Fig. 2c and e shows a SEM micrograph at higher magnifications from this failure zone, the line of the failureruns on very important section of the casting as seen in Fig. 2a. It is feasible to view different particles as contaminants of thisalloy on different sections of the failure and cavities. The failure appears on the surface of the casting and it is believed todepend on gate and runner designs and on the casting processes such as pouring temperature, tilt angle and mould temper-ature, the failure occurred regularly and equally in the castings.

3.2. Microscopy analysis and simulations

The EDS spectrums shown in Fig. 2d and f correspond to particles marked (white circles) on the SEM images. Fig. 2c showsa higher magnification of the failure zone (white circle) whose morphology and metal oxide composition is related to a con-vergent metal flow. A higher magnification Fig. 2e shows composition of some inclusions finding Si and Al, Si is present in thesand core while Al is presented in the alloy and has a high oxidation potential, inclusions could be trapped due to a conver-gent fluid flow and a high solidification velocity.

In order to understand better this phenomenon ProCast V.2008 simulations were made, temperature distributions in �Cand metal fluid flow during the fillings are shown in Fig. 3. It was found at 68� tilt angle and 1050 �C of pouring temperaturethat the molten metal flowed quite uniformly thorough the gate, however at 2.32 s, Fig. 3a, liquid metal starts moving up-wards as a result of the tilt, tilt process starts in second 2 and it is possible to observe a convergent metal flow at 2.48 s,Fig. 3b, coinciding with the failure location on the yellow brass tap. Furthermore, the front liquid flow losses temperatureduring the transport permitting a rapid solidification, as consequence traps film oxides formed on the liquid metal surface,and in this case some inclusions were carried to the final trajectory marked in Figs. 2e and 3c at 2.55. So regarding to metal-

Fig. 3. ProCast solidification simulation temperature profiles in Celsius: (a) 2.32, (b) 2.48 and (c) 2.55 s for 68� tilt; and (d) 2.66, (e) 2.83 and (f) 2.9 s for 78�tilt, filling times.

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lographic characterization and simulation results it is believed that this failure is caused by a cold shut due to the castingprocess parameters and it is expected that this problem is controlled by the tilt angle.

Fig. 3d–f show results increasing the tilt angle from 68� to 78� at the same pouring temperature of 1050 �C and processconditions. The casts manufactured with this new tilt angle show that the convergent fluid flow moves further up at 2.83 s,Fig. 3e. On the other hand at collation time 2.89 s, Fig. 3f, temperature profile is slightly higher than in the case of 68� Fig. 3c,so that it could explain the reason of cold shut control, being this defect agreed with casting of simulation results on thefailure formation and experimental results and so removing this failure formation, Fig. 2b.

4. Conclusions

The effect of process parameters for permanent mould casting conditions and the actual casting for a yellow brass wasinvestigated:

1. A failure was presented in a yellow brass tap being characterized as cold shut caused by casting process parameters.2. With the increase in the tilt angle from 68� to 78� convergent metal flow moves, castability and fluidity increased and the

cold shut failure defect despairs as observed experimentally and in ProCast simulations.3. The proposed sequence of failure is: due to the lower tilt angle there is a convergent metal flow which carries out film

oxides and inclusions at low temperatures, resulting in a formation of a big cold shut due to the rapid solidification.4. In order to prevent wear failure, process conditions must be controlled being in this case a precise control of the tilt

angles.

Acknowledgement

The authors would like to acknowledge to CONACYT Mexico for its support to this research.

References

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