Title (Type Title of Paper Here)
Optimizing Mechanical Properties in Aluminum Alloys with Separately Cast Test Bars C.J. Chen, D. Schwam, D. Neff Case Western Reserve University, Cleveland Ohio
Copyright 2014 American Foundry Society ABSTRACT The influence of metal cleaning practices (degassing, filtration) on tensile properties of A356 alloy has been previously studied in phase I . The present work (phase II) focuses on optimizing tensile properties by grain refining, microstructure modifying, and heat treating. Four molds were employed. A Sand testbar mold and standard permanent mold test bar mold (Stahl mold ASTM B-108) are widely used in foundries, but the latter still suffers from micro-porosity and less than optimum mechanical properties. In order to improve the tensile properties, a modified mold involving knife ingate between the feeder and test-bar cavity, in-mold filter cavity and heater (CaseH mold) was evaluated. In addition, a Step mold is another useful tool to evaluate how porosity and SDAS affect the mechanical properties. A356 alloy was cast in CWRU Lab Foundry and commercial foundry A. 319 alloy was cast in commercial foundries A and B. The Case-H mold test-bar exhibits better tensile properties and quality index (Q) than Stahl mold test-bar due to better feeding. HIPping on specimen cut from step mold coupons showed improved tensile and fatigue poperties over non-hipped samples in the heat treated condition (T6 for A356, T7 for 319) for both alloys. Keywords: A356, 319, separately cast test bar molds INTRODUCTION
Aluminum casting alloys are widely used to replace steel especially in automobile industry. Aluminum alloys are around 1/3 density comparing to steel but are lower strength. Therefore, improving mechanical properties of aluminum alloys becomes an important task. A large number of papers have been reported which demonstrated to increase properties with chemical modification [2, 3], heat treatment [4-6], and reduced porosity [7]. Mechanical properties derived from separately cast test bars do not replicate properties in a full casting—except for perhaps those sections of a casting with similar SDAS. However, foundries can derive the potential for nearoptimum properties by pouring such test bars from their melt. The mechanical properties in the test bar will be a
reasonable measure of the melt quality, especially with regard to the presence or absence of impurities— inclusions, porosity, and micro-porosity--which will strongly influence resultant mechanical properties It is well known that the presence of micro-porosity has a key effect on mechanical properties of both A356 and 319 alloys. Degassing treatments for A356 and 319 alloy melt are generally required to reduce hydrogen content and thus minimize porosity in the casting. Improving the gating system of permanent mold separately cast test bars should provide better feeding and change the solidification path to prevent micro-porosity in the test bar. Hot isostatic pressing (HIP) is known to reduce microporosity and therefore can also increase mechanical properties. The Stahl Mold permanent test bar is widely used in most foundries to evaluate the melt quality. However, the test bar itself has porosity which will make the evaluation of the melt quality imprecise [8]. In the Stahl mold design, the sprue is narrow to control the flow into the mold and reduce turbulence. The feeder and test bar sections are designed to obtain good filling and reduce shrinkage porosity. However, foundry practice indicates the Stahl Mold still has difficulty in producing the best mechanical properties due to micro-porosity. In this work, a modification was proposed to improve the mechanical properties by applying a thin knife ingate between the feeder and the test-bar so as to improve the filling capability and reduce the micro-porosity due to shrinkage. This effect was predicted by simulation and corroborated by results from Phase I of this project. [1] The knife ingate is named after its shape which looks like a thin knife blade. The width plus narrow thickness provides better metal flow and feeding. Computer simulation [9] was used to predict the effect of the knife ingate on the solidification pattern and microstructures of test-bar castings consequently. A new mold was fabricated based on the modified design, and the performance was compared with the standard Stahl Mold by casting A356 aluminum test bars. Sigworth and Kuhn [8] compared the performance of the Aluminum Association step mold with a knife ingate and the standard Stahl Mold. It was found the Stahl Mold could not produce the high mechanical properties
compared to the AA step Mold. The reason was attributed to the micro-porosity due to shrinkage in the gage section of the Stahl mold. However, the AA step mold is also not perfectly porosity free in every section, especially in the 2” and 3” section. Guang Ran et al. [10] investigated the effect of hot isostatic pressing (HIP) on the microstructure and tensile properties of A356-T6 cast aluminum alloy. The A356 alloy was cast into a plate and was machined to test bar according to ASTM E8M. The solution treatment was 1000F (538C) for 5 hours and artificial aging was 320F (160C) for 4 hours. In Ran’s study, authors compared different SDAS with UTS and elongation. The results showed that if SDAS is lower than 80 um, the hipped sample has higher elongation then the non-HIP sample at the same UTS level.. In the present work—Phase II of this project, both A356 and 319 alloy casts were studied to determine further beneficial effects of microstructural enhancements—ie grain refining, modification, SDAS over and above good foundry practice with melt cleaning—fluxing, degassing. The several separately cast test bar molds were employed to evaluate resultant mechanical properties, particularly in the heat treated condition, on commercial foundry as well as laboratory melts.
EXPERIMENTAL PROCEDURES Melt Preparation A356 alloy was melted in an electric resistance melting furnace in the CWRU Foundry. Virgin and recycle (10% clean) alloy was used and the melt temperature was held at 1300,1350 and 1450F (704C, 732C and 788C) . In the CWRU foundry study, virgin versus recycled metal was evaluated. Degassing was performed by bubbling argon through a Foseco degassing unit for 30 minutes. The hydrogen level was controlled to 0.1 ml/100g Al by an ALSPEK continuous hydrogen measurement system, and mold pours were performed at this level. In addition, the reduced pressure test (RPT) was also employed to ascertain qualitative hydrogen, ie porosity levels in the melts solidified under reduced pressure (11). Generally, the RPT results were held at 2.60 S.G. at 1300 degF, and at 2.50 s.g. for the melt at 1450 degF. To evaluate the effect of micro-structural enhancement on the mechanical properties and build upon the results of Phase I, addition of Tibor and strontium for grain refinement and modification respectively was made to the melt(s). The chemical compositions of alloys used at CWRU and the two commercial foundries are listed in Tables 1 and 2. The alloys nominally called by Foundries A and B as 319 are special automotive grade compositions (similar to alloy 320) which suitable for their cast component requirements.
Table.1 Chemical Composition of A356 Alloy
Foundry
Si%
Fe%
Cu%
Mn%
Mg%
Ti%
Sr%
A356
CWRU
6.491
0.084
0.000
0.012
0.307
0.093
0.024
A356
Foundry A
6.727
0.102
0.011
0.003
0.341
0.119
0.020
Table.2 Chemical Composition of 319 Alloy
Foundry
Si%
Fe%
Cu%
Mn%
Mg%
Ti%
Sr%
319
Foundry A
7.081
0.537
3.358
0.089
0.321
0.135
0.017
319
Foundry B
8.790
0.520
2.990
0.357
0.357
0.146
0.007
Commercial Foundry Practices Commercial melt A356 alloy was evaluated at Foundry A. Commercial melts of 319 alloy were evaluated at Foundry A and Foundry B. At Foundry A, the metal was continuously rotor degassed with nitrogen and filtered inline with a bonded particle filter prior to casting. A rod grain refiner was employed. For the A356 alloy, casting temperature was 1350F (732C); for 319 casting
temperature was 1320F (716C). Specific gravity was held to 2.55 for A356; to 2.67-2.70 for 319 alloy. In both instances the established practices were commensurate with customer requirements for those castings and yielded acceptable results. At foundry B, the 319 melt was continuously rotordegassed with argon, treated with strontium rod, and
filtered in-line with a bonded particle filter prior to casting. Specific gravity was held to 2.71. The Stahl and Case molds were poured at each foundry at a mold temperaturer at or near 625F (329C) or slightly above in most instances. The step mold was poured at 400 degF (204C) mold temperature.
Mold Preparation Four types of molds were used - the Stahl test-bar permanent mold (Fig.1a), Case-H mold (Fig.1b) a sand mold (Fig.1c), and a step mold (Fig.1d). The Case-H mold contains the knife-ingate into the gage section and embedded heating elements for very consistent thermal
Fig.1 (a) Stahl Mold and the cast bars
Fig.1 (c) Sand cast bars
Heat Treatment The A356 test bars which were poured at CWRU and Foundry A were heat-treated with T6 condition – solution treatment at 1000˚F for 12 hours, quench, artificial aging
control. The Stahl and Case-H mold were coated with Dycote 34ESS (FOSECO) on the sprue and runner sections and graphite on the gage section surface, the latter to create best solidification conditions. The Stahl mold and Case-H mold were pre-heated to 400F (204C) for mold coating application, then poured at 625F (329C) mold temperature in all foundries; the step mold was preheated and poured at 400F (204C) mold temperature. The step mold produces sections of different thickness, forming a step like shape. The variation in thickness allows one to examine the mechanical properties of castings solidified under different cooling rates in one pour. However, since the mold does not produce a test bar shaped casting, significant machining labor is required to prepare test bars. Tensile test-bars were machined from the 2” section and fatigue samples were machined from the 1” section (Fig.1d).
Fig.1 (b) Case-H Mold and the cast bars
Fig.1 (d) Step casting at 320F (160C) for 6 hours. The 319 test bars poured at Foundry A were heat-treated with T6 and T7 conditions, T6 - solution treatment at 950F (510C) for 8 hours, quench, artificial aging at 320˚F for 6 hours. The T7 solution treatment at 925F (496C) for 8 hours, quench, artificial aging at 462F (239C) for 4.75 hours was applied
to the 319 alloy test bars poured both at Foundry A and Foundry B. Tensile Testing The diameter of gauge section of Stahl mold and Case-H mold is 0.5 inch. The test bars in the as-cast, T6 and T7 condition were pulled to fracture with a tensile test machine at room temperature at a strain rate of 10-3s-1. In addition to the UTS and elongation, the Quality Index [12] was used to evaluate the overall mechanical properties of the A356 test bars. The Quality Index (QI) is defined by the equation QI = UTS+150log10 (elongation).
(1)
Fig.2 Tensile test machine The UTS and elongation values were determined from an average of 6 bars for both Stahl mold and Case-H mold and at least 3 bars for step mold and sand mold. In the latter stages of this work, the yield stress was also determined. Fatigue Test Fatigue test bars were cut and machined from step mold 1” section as shown below (Figure 3) .Testing was conducted at C-T-C at 125 Mpa in fully reversed sinusoidal loading at 60 Hz. φ 0.250+/-0.001 1.050 (Ref.)
1.050 (Ref.)
φ 0.500+/-0.001 R 2.000 3.50
Fig.3 Dimensions of fatigue test bar
RESULTS AND DISCUSSION The mechanical properties of heat-treated separately-cast test bars using the four molds were evaluated to discern the effects of microstructural enhancement, i.e. modification and grain refinement and the beneficial effects of reduced micro-porosity
Tensile Properties of A356 Alloy The tensile properties in the separately cast test bars poured at CWRU and at Foundry A are shown in Figure 4. At both foundries, the metal quality is principally virgin with just 10 % recycle, and all melts are fluxed, degassed, grain refined and modified, with in-line furnace filtration.. Fig.4 shows the Case-H mold both improved UTS about 2ksi in Foundry A and CWRU respectively. However the Case-H mold exhibited improved elongation almost twice that of the Stahl mold at CWRU, but less in Foundry A. This may relate to the iron content of A356 in CWRU being lower than in Foundry A. The figure also shows that Case-H mold test bar result at CWRU has a much higher quality index than any other test bars principally because of the high elongation. In fact, despite the very high strontium content (0.024) and possible overmodification (with visible microporosity), the achieved mechanical properties obtained in the testbars easily exceed the 40-30-10 (T.S., Y.S. El) results most casting designers call for. This also suggests further confirmation that the enhanced feeding afforded by the knife ingate in the Case-H mold overcomes the inherent loss of properties that such microporosity would suggest. Naturally, the permanent mold testbar results show improvement over the sand mold test bar results due to faster solidification and therefore smaller SDAS.
at 1450F (788C) pouring temperature would be slower than at1300F (704C), and therefore more and larger porosity is expected. Despite these points, only a minimum deterioration of mechanical properties was observed due to higher temperature exposure. UTS(ksi)
Fig.4 Mechanical properties of A356 alloy cast in Foundry A and CWRU With the addition of grain refining and modification, the Quality Index result is further improved versus that achieved with clean metal practice alone, as previously reported (1). This is shown in Figure 5 with the ‘star’ data point.
50 45 40 35 30 25 20 15 10 5 0
Stahl 1300˚F T6 CWRU UTS(ksi) 41.54 El(%) 11.55
El(%)
Stahl 1450˚F T6 CWRU 39.59 9.9
Case-H 1300˚F T6 CWRU 43.1 18.5
Case-H 1450˚F T6 CWRU 42.59 17.63
Fig.6 The effect of pouring temperature on mechanical properties of A356 alloy Tensile Properties of 319 Alloy In Foundry A, 319 alloy was cast from melts that were cleaned, degassed, grain refined and modified per that foundry’s normal commercial practice. The results of test bars poured at Foundry A were well within agreement of their own test results. . Fig.7 shows the mechanical properties of 319 alloy cast in Foundry A comparing both T6 and T7 heat treatments. . T7 heat treatment is known to increase elongation by sacrificing a small amount of UTS. In each case here there is no great difference in result between the Stahl and Case molds.
Figure 5 Quality Index Plot for A356 Alloy (Sigworth and Kuhn) Effect of Pouring Temperature To evaluate the influence of high melting and/or holding temperatures on the possible deterioration of mechanical properties, the melt at CWRU was evaluated at 1300F (704C) and 1450F (788C) Fig. 6 shows mechanical properties of A356 alloy for test bars poured with the Stahl and Case-H molds. The result reveals that higher pouring temperature reduces both UTS and elongation of the two molds. The higher melt and hence pouring temperature also resulted in a much higher hydrogen content as measured by Alspek readings and evaluated by RPT. In addition the higher pouring temperature created a larger difference between the mold temperature and pouring temperature, and therefore the solidification rate
Fig.7 Mechanical properties of 319 alloy cast in Foundry A
The 319 alloy was also cast in Foundry B. Fig.8 shows that here the Case-H mold showed improvement in UTS and elongation in both T6 and T7 conditions versus the Stahl mold. UTS(ksi) 60 55 50 45 40 35 30 25 20 15 10 5 0
T6
Stahl Case-H Foundry B Foundry B UTS(ksi) 44.42 51.5 YS(ksi) El(%) 0.95 1.23
YS(ksi)
El(%)
T7
Effect of Hot Isostatic Pressing The Step mold has four steps – 0.5”, 1”, 2” and 3” thickness sections. In this study, 2” section is chosen to compare with Stahl and Case-H mold test bars. It is easy to imagine that the 2” part of step mold sample has large SDAS and more porosity than Stahl and Case-H mold test bars. Fig.9 shows the porosity of Stahl/Case-H mold test bars and step mold 2” section. Fig.10 presents the SDAS of Stahl/Case-H mold test bars and step mold 2” section sample where the SDAS was determined to be 18μm and 40μm,respectively.
Stahl Case-H Foundry B Foundry B 38.09 42.98 32.69 32.59 1.18 2.32
Figure 8: Mechanical Properties of 319 Cast in Foundry B
500
Fig.9 The porosity of Stahl/Case-H mold (a) and step mold 2” section test bars(b)
500
Fig.10 The SDAS of Stahl/Case-H mold (a) and step mold (b) 2” section test bars Hot isostatic pressing (HIP) should be able to remove most of porosity in the step mold 2” section sample. Fig.11 shows the results of A356 alloy with and without HIP at T6 condition and 319 alloy with and without HIP at T7 condition respectively. HIPping improves the mechanical properties of A356 alloy better than 319 alloy. Although the HIP improves much UTS and elongation for 319 alloy, the tensile properties of the step mold samples are much lower than the separately cast Stahl and Case-H mold samples. The reason is that the step mold has a higher SDAS. X. Zhu et al. [13] investigated effects of microstructure and temperature on fatigue behavior of 319-T7 alloy. The result showed that the UTS of 319 alloy is 42ksi and 26ksi with respect to 30μm and 70μm SDAS. Therefore, in this study, the step mold 2” section 319-T7 sample with about 40μm SDAS has 32ksi UTS is reasonable. Another study by Guang Ran et al. [14] revealed that the UTS of A356-T6 alloy decreases from 35.4ksi to 34.5ksi when the SDAS increases from 82μm to 96μm. Therefore, in this study, the step mold 2” section A356-T6 sample with about 40μm SDAS has 37ksi UTS is also reasonable.
Fig.11 Mechanical properties of A356 and 319 Alloy Step Mold 2” tensile samples with and without HIP Fatigue Properties Fig.12 shows the fatigue properties of step mold 1” sample with and without HIP which compares to the reference data with 125Mpa fully reversed sinusoidal loading at 60Hz. The reference curve of A356 alloy is from the Casting Technologies Company. The blue triangles represent A356 alloy without HIP at T6 condition and the red circles represent it with HIP. The no HIP samples have worse results than the reference curve and the HIP samples have better results than reference curve. HIP also improve 319 alloy fatigue properties. The blue stars represent 319 alloy without HIP at T7 condition and the red circles represent it with HIP. The 319 reference curve is from X. Zhu’s study [13]. Therefore, reducing micro-porosity by HIP improves the fatigue properties
dramatically. While certainly not an original result, the improvement with Hipping does confirm that microporosity in a separately cast test bar, or tensile specimens
harvested from a test mold, can be counter-acted with this technique to improve desired mechanical properties.
Fig.12 Fatigue properties of Step Mold 1” sample with and without HIP
Investigation of Micro-porosity Analysis of qualitative and quantitative micro-porosity is very difficult. In the present work the application of computed tomography was investigated on fractured tensile samples after consultation and potential successful application by J. T. Garant and Associates, a Metrology firm. Initial results using this technology appear
Conclusions 1. Clean metal practices (fluxing, degassing, flux injection, filtration) and appropriate application of grain refining and modification result in acceptable separately-cast test bar results in both lab and commercial foundry melts in A356 and 319 alloys in their respective T6 and T7 heat treat conditions. 2. The Case test-bar mold with knife ingate consistently produces measurable mechanical property increases over the standard Stahl ASTM
promising as it is possible to ascertain pore size and size distribution in a cast sample, with sensitivity down to just a few microns. At this time, however, it has not been possible to incorporate specific results and comparisons between tensile samples of different conditions in this report.
3.
B108 testbar mold and should be given further consideration for use as a standard methodology. Despite perhaps over-modification with high strontium and apparent porosity, separately-cast Case test bar mold tensile properties yielded a very high Quality Index number aided by high elongation results (13-15%). This is due to low iron content (less than 0.10%) and the effect of the knife-ingate configuration on the Case testbar mold which provides better feeding in spite some
4.
5.
micro- porosity experienced with the high strontium. HIPping improves the tensile and fatigue properties of both alloys by closing off any micro-porosity. Computed Tomography (CT Scan) techniques may be useful in ascertaining microposity distribution and quantification as influenced by testing or processing variables.
References 1.C.J. Chen, D.V. Neff, D.S. Schwam, Y. Wang, “Optimization of As cast Mechanical Properties in A356 via Simulation and Permanent Mold Test-bars”: AFS Transactions, 2011, vol. 115, pp. 53-59. 2. H.R. Lashgari, M. Emamy, A. Razaghian, A.A. Najimi, “The Effect of Strontium on the Microstructure, Porosity and Tensile Properties of A356-10%B4C Case Composite”: Materials Science and Engineering A, 2009. 3. F.J. Tavitas-Medrano, J.E. Gruzleski, F.H. Samuel, S. Valtierra, H.W. Doty, “Effect of Mg and Sr-modification on the Mechanical Properties of 319-type Aluminum Cast alloys Subjected to Artificial Aging”: Materials Science and Engineering A, 2008, pp. 356-364. 4. D. Apelian, S. Shivkumar and G. Sigworth, “Fundamental Aspects of Heat Treatment of Cast Al-SiMg Alloys”: AFS Transactions, 1989, vol. 97, pp. 727. 5. S. Shivkumar, S. Ricci, C. Keller and D. Apelian, “Effect of Solution Treatment Parameters on Tensile Properties of Case Aluminum Alloys”: Journal of Heat Treating, 1990, vol.8, pp. 63-70. 6. S. Shivkumar, R. Ricci, Jr.B. Steenhof, D. Apelian and G.K. Sigworth, “An Experimental Study to Optimize the Heat Treatment of A356 Alloy”: AFS Transactions, 1989, vol. 97, pp. 791-810. 7. A. Knuutinen, k. Nogita, S.D. McDonald, A.K. Dahle, “Porosity Formation in Aluminum Alloy A356 Modified with Ba, Ca, Y and Yb”: Journal of Light Metals I, 2001, pp. 241-249. 8. G.K. Sigworth and T.A. Kuhn, “Use of Standard Molds to Evaluate Metal Quality and Alloy Properties”: AFS Transactions, 2009, vol.117, pp. 55-62. 9. Y. Wang, D. Schwam, D.V. Neff, C.J. Chen, X. Zhu, “Improvement in Mechanical Properties of A356 Tensile Test Bars Cast in a Permanent Mold by Application of a Knife Ingate”: Metallurgical and Materials Transactions A, 2011. 10. G. Ran, J. Zhou, Q.G.Wang, “The Effect of Hot Isostatic Pressing on the Microstructure and Tensile Properties of an Unmodified A356-T6 Cast Aluminum Alloy”: Journal of Alloys and Compounds, 2006, pp.8086. 11. S. Dasgupta, L. Parmenter, D. Apelian, ”Relationship Between the Reduced Pressure Test and Hydrogen Content of the Melt”: AFS International Conference on Molten Aluminum Processing, 1998, pp. 801-816 12. M. Drouzy, S. Jacob and M. Richard, “Interpretation
of Tensile Results by means of Quality Index and Probable Yield Strength”: AFS International Cast Metals Journal, 1980, vol.5, pp. 43-50. 13. X. Zhu, A. Shyam, J.W. Jones, H. Mayer, J.V. Lasecki, J.E. Allison, “Effects of Microstructure and Temperature on Fatigue Behavior of E319-T7 Cast Aluminum Alloy in Very Long Life Cycles”: International Journal of Fatigue 28 (2006) 1566-1571 14. Guang Ran, Jingen Zhou, Q.G. Wang, “The Effect of Hot Isostatic Pressing on the Microstructure and Tensile Properties of an Unmodified A356-T6 Cast Aluminum Alloy”: Journal of Alloys and Compounds 421 (2006) 8086 Acknowledgements: The authors wish to express their gratitude to American Foundry Society and the Aluminum Division for their financial and advisory support of this work; to the management of Foundry A and Foundry B for their direct participation in conducting a portion of this work, and to C-T-C for their in-kind contribution to fatigue test specimen preparation and testing.
Copyright 2014 American Foundry Society ABSTRACT The influence of metal cleaning practices (degassing, filtration) on tensile properties of A356 alloy has been previously studied in phase I . The present work (phase II) focuses on optimizing tensile properties by grain refining, microstructure modifying, and heat treating. Four molds were employed. A Sand testbar mold and standard permanent mold test bar mold (Stahl mold ASTM B-108) are widely used in foundries, but the latter still suffers from micro-porosity and less than optimum mechanical properties. In order to improve the tensile properties, a modified mold involving knife ingate between the feeder and test-bar cavity, in-mold filter cavity and heater (CaseH mold) was evaluated. In addition, a Step mold is another useful tool to evaluate how porosity and SDAS affect the mechanical properties. A356 alloy was cast in CWRU Lab Foundry and commercial foundry A. 319 alloy was cast in commercial foundries A and B. The Case-H mold test-bar exhibits better tensile properties and quality index (Q) than Stahl mold test-bar due to better feeding. HIPping on specimen cut from step mold coupons showed improved tensile and fatigue poperties over non-hipped samples in the heat treated condition (T6 for A356, T7 for 319) for both alloys. Keywords: A356, 319, separately cast test bar molds INTRODUCTION
Aluminum casting alloys are widely used to replace steel especially in automobile industry. Aluminum alloys are around 1/3 density comparing to steel but are lower strength. Therefore, improving mechanical properties of aluminum alloys becomes an important task. A large number of papers have been reported which demonstrated to increase properties with chemical modification [2, 3], heat treatment [4-6], and reduced porosity [7]. Mechanical properties derived from separately cast test bars do not replicate properties in a full casting—except for perhaps those sections of a casting with similar SDAS. However, foundries can derive the potential for nearoptimum properties by pouring such test bars from their melt. The mechanical properties in the test bar will be a
reasonable measure of the melt quality, especially with regard to the presence or absence of impurities— inclusions, porosity, and micro-porosity--which will strongly influence resultant mechanical properties It is well known that the presence of micro-porosity has a key effect on mechanical properties of both A356 and 319 alloys. Degassing treatments for A356 and 319 alloy melt are generally required to reduce hydrogen content and thus minimize porosity in the casting. Improving the gating system of permanent mold separately cast test bars should provide better feeding and change the solidification path to prevent micro-porosity in the test bar. Hot isostatic pressing (HIP) is known to reduce microporosity and therefore can also increase mechanical properties. The Stahl Mold permanent test bar is widely used in most foundries to evaluate the melt quality. However, the test bar itself has porosity which will make the evaluation of the melt quality imprecise [8]. In the Stahl mold design, the sprue is narrow to control the flow into the mold and reduce turbulence. The feeder and test bar sections are designed to obtain good filling and reduce shrinkage porosity. However, foundry practice indicates the Stahl Mold still has difficulty in producing the best mechanical properties due to micro-porosity. In this work, a modification was proposed to improve the mechanical properties by applying a thin knife ingate between the feeder and the test-bar so as to improve the filling capability and reduce the micro-porosity due to shrinkage. This effect was predicted by simulation and corroborated by results from Phase I of this project. [1] The knife ingate is named after its shape which looks like a thin knife blade. The width plus narrow thickness provides better metal flow and feeding. Computer simulation [9] was used to predict the effect of the knife ingate on the solidification pattern and microstructures of test-bar castings consequently. A new mold was fabricated based on the modified design, and the performance was compared with the standard Stahl Mold by casting A356 aluminum test bars. Sigworth and Kuhn [8] compared the performance of the Aluminum Association step mold with a knife ingate and the standard Stahl Mold. It was found the Stahl Mold could not produce the high mechanical properties
compared to the AA step Mold. The reason was attributed to the micro-porosity due to shrinkage in the gage section of the Stahl mold. However, the AA step mold is also not perfectly porosity free in every section, especially in the 2” and 3” section. Guang Ran et al. [10] investigated the effect of hot isostatic pressing (HIP) on the microstructure and tensile properties of A356-T6 cast aluminum alloy. The A356 alloy was cast into a plate and was machined to test bar according to ASTM E8M. The solution treatment was 1000F (538C) for 5 hours and artificial aging was 320F (160C) for 4 hours. In Ran’s study, authors compared different SDAS with UTS and elongation. The results showed that if SDAS is lower than 80 um, the hipped sample has higher elongation then the non-HIP sample at the same UTS level.. In the present work—Phase II of this project, both A356 and 319 alloy casts were studied to determine further beneficial effects of microstructural enhancements—ie grain refining, modification, SDAS over and above good foundry practice with melt cleaning—fluxing, degassing. The several separately cast test bar molds were employed to evaluate resultant mechanical properties, particularly in the heat treated condition, on commercial foundry as well as laboratory melts.
EXPERIMENTAL PROCEDURES Melt Preparation A356 alloy was melted in an electric resistance melting furnace in the CWRU Foundry. Virgin and recycle (10% clean) alloy was used and the melt temperature was held at 1300,1350 and 1450F (704C, 732C and 788C) . In the CWRU foundry study, virgin versus recycled metal was evaluated. Degassing was performed by bubbling argon through a Foseco degassing unit for 30 minutes. The hydrogen level was controlled to 0.1 ml/100g Al by an ALSPEK continuous hydrogen measurement system, and mold pours were performed at this level. In addition, the reduced pressure test (RPT) was also employed to ascertain qualitative hydrogen, ie porosity levels in the melts solidified under reduced pressure (11). Generally, the RPT results were held at 2.60 S.G. at 1300 degF, and at 2.50 s.g. for the melt at 1450 degF. To evaluate the effect of micro-structural enhancement on the mechanical properties and build upon the results of Phase I, addition of Tibor and strontium for grain refinement and modification respectively was made to the melt(s). The chemical compositions of alloys used at CWRU and the two commercial foundries are listed in Tables 1 and 2. The alloys nominally called by Foundries A and B as 319 are special automotive grade compositions (similar to alloy 320) which suitable for their cast component requirements.
Table.1 Chemical Composition of A356 Alloy
Foundry
Si%
Fe%
Cu%
Mn%
Mg%
Ti%
Sr%
A356
CWRU
6.491
0.084
0.000
0.012
0.307
0.093
0.024
A356
Foundry A
6.727
0.102
0.011
0.003
0.341
0.119
0.020
Table.2 Chemical Composition of 319 Alloy
Foundry
Si%
Fe%
Cu%
Mn%
Mg%
Ti%
Sr%
319
Foundry A
7.081
0.537
3.358
0.089
0.321
0.135
0.017
319
Foundry B
8.790
0.520
2.990
0.357
0.357
0.146
0.007
Commercial Foundry Practices Commercial melt A356 alloy was evaluated at Foundry A. Commercial melts of 319 alloy were evaluated at Foundry A and Foundry B. At Foundry A, the metal was continuously rotor degassed with nitrogen and filtered inline with a bonded particle filter prior to casting. A rod grain refiner was employed. For the A356 alloy, casting temperature was 1350F (732C); for 319 casting
temperature was 1320F (716C). Specific gravity was held to 2.55 for A356; to 2.67-2.70 for 319 alloy. In both instances the established practices were commensurate with customer requirements for those castings and yielded acceptable results. At foundry B, the 319 melt was continuously rotordegassed with argon, treated with strontium rod, and
filtered in-line with a bonded particle filter prior to casting. Specific gravity was held to 2.71. The Stahl and Case molds were poured at each foundry at a mold temperaturer at or near 625F (329C) or slightly above in most instances. The step mold was poured at 400 degF (204C) mold temperature.
Mold Preparation Four types of molds were used - the Stahl test-bar permanent mold (Fig.1a), Case-H mold (Fig.1b) a sand mold (Fig.1c), and a step mold (Fig.1d). The Case-H mold contains the knife-ingate into the gage section and embedded heating elements for very consistent thermal
Fig.1 (a) Stahl Mold and the cast bars
Fig.1 (c) Sand cast bars
Heat Treatment The A356 test bars which were poured at CWRU and Foundry A were heat-treated with T6 condition – solution treatment at 1000˚F for 12 hours, quench, artificial aging
control. The Stahl and Case-H mold were coated with Dycote 34ESS (FOSECO) on the sprue and runner sections and graphite on the gage section surface, the latter to create best solidification conditions. The Stahl mold and Case-H mold were pre-heated to 400F (204C) for mold coating application, then poured at 625F (329C) mold temperature in all foundries; the step mold was preheated and poured at 400F (204C) mold temperature. The step mold produces sections of different thickness, forming a step like shape. The variation in thickness allows one to examine the mechanical properties of castings solidified under different cooling rates in one pour. However, since the mold does not produce a test bar shaped casting, significant machining labor is required to prepare test bars. Tensile test-bars were machined from the 2” section and fatigue samples were machined from the 1” section (Fig.1d).
Fig.1 (b) Case-H Mold and the cast bars
Fig.1 (d) Step casting at 320F (160C) for 6 hours. The 319 test bars poured at Foundry A were heat-treated with T6 and T7 conditions, T6 - solution treatment at 950F (510C) for 8 hours, quench, artificial aging at 320˚F for 6 hours. The T7 solution treatment at 925F (496C) for 8 hours, quench, artificial aging at 462F (239C) for 4.75 hours was applied
to the 319 alloy test bars poured both at Foundry A and Foundry B. Tensile Testing The diameter of gauge section of Stahl mold and Case-H mold is 0.5 inch. The test bars in the as-cast, T6 and T7 condition were pulled to fracture with a tensile test machine at room temperature at a strain rate of 10-3s-1. In addition to the UTS and elongation, the Quality Index [12] was used to evaluate the overall mechanical properties of the A356 test bars. The Quality Index (QI) is defined by the equation QI = UTS+150log10 (elongation).
(1)
Fig.2 Tensile test machine The UTS and elongation values were determined from an average of 6 bars for both Stahl mold and Case-H mold and at least 3 bars for step mold and sand mold. In the latter stages of this work, the yield stress was also determined. Fatigue Test Fatigue test bars were cut and machined from step mold 1” section as shown below (Figure 3) .Testing was conducted at C-T-C at 125 Mpa in fully reversed sinusoidal loading at 60 Hz. φ 0.250+/-0.001 1.050 (Ref.)
1.050 (Ref.)
φ 0.500+/-0.001 R 2.000 3.50
Fig.3 Dimensions of fatigue test bar
RESULTS AND DISCUSSION The mechanical properties of heat-treated separately-cast test bars using the four molds were evaluated to discern the effects of microstructural enhancement, i.e. modification and grain refinement and the beneficial effects of reduced micro-porosity
Tensile Properties of A356 Alloy The tensile properties in the separately cast test bars poured at CWRU and at Foundry A are shown in Figure 4. At both foundries, the metal quality is principally virgin with just 10 % recycle, and all melts are fluxed, degassed, grain refined and modified, with in-line furnace filtration.. Fig.4 shows the Case-H mold both improved UTS about 2ksi in Foundry A and CWRU respectively. However the Case-H mold exhibited improved elongation almost twice that of the Stahl mold at CWRU, but less in Foundry A. This may relate to the iron content of A356 in CWRU being lower than in Foundry A. The figure also shows that Case-H mold test bar result at CWRU has a much higher quality index than any other test bars principally because of the high elongation. In fact, despite the very high strontium content (0.024) and possible overmodification (with visible microporosity), the achieved mechanical properties obtained in the testbars easily exceed the 40-30-10 (T.S., Y.S. El) results most casting designers call for. This also suggests further confirmation that the enhanced feeding afforded by the knife ingate in the Case-H mold overcomes the inherent loss of properties that such microporosity would suggest. Naturally, the permanent mold testbar results show improvement over the sand mold test bar results due to faster solidification and therefore smaller SDAS.
at 1450F (788C) pouring temperature would be slower than at1300F (704C), and therefore more and larger porosity is expected. Despite these points, only a minimum deterioration of mechanical properties was observed due to higher temperature exposure. UTS(ksi)
Fig.4 Mechanical properties of A356 alloy cast in Foundry A and CWRU With the addition of grain refining and modification, the Quality Index result is further improved versus that achieved with clean metal practice alone, as previously reported (1). This is shown in Figure 5 with the ‘star’ data point.
50 45 40 35 30 25 20 15 10 5 0
Stahl 1300˚F T6 CWRU UTS(ksi) 41.54 El(%) 11.55
El(%)
Stahl 1450˚F T6 CWRU 39.59 9.9
Case-H 1300˚F T6 CWRU 43.1 18.5
Case-H 1450˚F T6 CWRU 42.59 17.63
Fig.6 The effect of pouring temperature on mechanical properties of A356 alloy Tensile Properties of 319 Alloy In Foundry A, 319 alloy was cast from melts that were cleaned, degassed, grain refined and modified per that foundry’s normal commercial practice. The results of test bars poured at Foundry A were well within agreement of their own test results. . Fig.7 shows the mechanical properties of 319 alloy cast in Foundry A comparing both T6 and T7 heat treatments. . T7 heat treatment is known to increase elongation by sacrificing a small amount of UTS. In each case here there is no great difference in result between the Stahl and Case molds.
Figure 5 Quality Index Plot for A356 Alloy (Sigworth and Kuhn) Effect of Pouring Temperature To evaluate the influence of high melting and/or holding temperatures on the possible deterioration of mechanical properties, the melt at CWRU was evaluated at 1300F (704C) and 1450F (788C) Fig. 6 shows mechanical properties of A356 alloy for test bars poured with the Stahl and Case-H molds. The result reveals that higher pouring temperature reduces both UTS and elongation of the two molds. The higher melt and hence pouring temperature also resulted in a much higher hydrogen content as measured by Alspek readings and evaluated by RPT. In addition the higher pouring temperature created a larger difference between the mold temperature and pouring temperature, and therefore the solidification rate
Fig.7 Mechanical properties of 319 alloy cast in Foundry A
The 319 alloy was also cast in Foundry B. Fig.8 shows that here the Case-H mold showed improvement in UTS and elongation in both T6 and T7 conditions versus the Stahl mold. UTS(ksi) 60 55 50 45 40 35 30 25 20 15 10 5 0
T6
Stahl Case-H Foundry B Foundry B UTS(ksi) 44.42 51.5 YS(ksi) El(%) 0.95 1.23
YS(ksi)
El(%)
T7
Effect of Hot Isostatic Pressing The Step mold has four steps – 0.5”, 1”, 2” and 3” thickness sections. In this study, 2” section is chosen to compare with Stahl and Case-H mold test bars. It is easy to imagine that the 2” part of step mold sample has large SDAS and more porosity than Stahl and Case-H mold test bars. Fig.9 shows the porosity of Stahl/Case-H mold test bars and step mold 2” section. Fig.10 presents the SDAS of Stahl/Case-H mold test bars and step mold 2” section sample where the SDAS was determined to be 18μm and 40μm,respectively.
Stahl Case-H Foundry B Foundry B 38.09 42.98 32.69 32.59 1.18 2.32
Figure 8: Mechanical Properties of 319 Cast in Foundry B
500
Fig.9 The porosity of Stahl/Case-H mold (a) and step mold 2” section test bars(b)
500
Fig.10 The SDAS of Stahl/Case-H mold (a) and step mold (b) 2” section test bars Hot isostatic pressing (HIP) should be able to remove most of porosity in the step mold 2” section sample. Fig.11 shows the results of A356 alloy with and without HIP at T6 condition and 319 alloy with and without HIP at T7 condition respectively. HIPping improves the mechanical properties of A356 alloy better than 319 alloy. Although the HIP improves much UTS and elongation for 319 alloy, the tensile properties of the step mold samples are much lower than the separately cast Stahl and Case-H mold samples. The reason is that the step mold has a higher SDAS. X. Zhu et al. [13] investigated effects of microstructure and temperature on fatigue behavior of 319-T7 alloy. The result showed that the UTS of 319 alloy is 42ksi and 26ksi with respect to 30μm and 70μm SDAS. Therefore, in this study, the step mold 2” section 319-T7 sample with about 40μm SDAS has 32ksi UTS is reasonable. Another study by Guang Ran et al. [14] revealed that the UTS of A356-T6 alloy decreases from 35.4ksi to 34.5ksi when the SDAS increases from 82μm to 96μm. Therefore, in this study, the step mold 2” section A356-T6 sample with about 40μm SDAS has 37ksi UTS is also reasonable.
Fig.11 Mechanical properties of A356 and 319 Alloy Step Mold 2” tensile samples with and without HIP Fatigue Properties Fig.12 shows the fatigue properties of step mold 1” sample with and without HIP which compares to the reference data with 125Mpa fully reversed sinusoidal loading at 60Hz. The reference curve of A356 alloy is from the Casting Technologies Company. The blue triangles represent A356 alloy without HIP at T6 condition and the red circles represent it with HIP. The no HIP samples have worse results than the reference curve and the HIP samples have better results than reference curve. HIP also improve 319 alloy fatigue properties. The blue stars represent 319 alloy without HIP at T7 condition and the red circles represent it with HIP. The 319 reference curve is from X. Zhu’s study [13]. Therefore, reducing micro-porosity by HIP improves the fatigue properties
dramatically. While certainly not an original result, the improvement with Hipping does confirm that microporosity in a separately cast test bar, or tensile specimens
harvested from a test mold, can be counter-acted with this technique to improve desired mechanical properties.
Fig.12 Fatigue properties of Step Mold 1” sample with and without HIP
Investigation of Micro-porosity Analysis of qualitative and quantitative micro-porosity is very difficult. In the present work the application of computed tomography was investigated on fractured tensile samples after consultation and potential successful application by J. T. Garant and Associates, a Metrology firm. Initial results using this technology appear
Conclusions 1. Clean metal practices (fluxing, degassing, flux injection, filtration) and appropriate application of grain refining and modification result in acceptable separately-cast test bar results in both lab and commercial foundry melts in A356 and 319 alloys in their respective T6 and T7 heat treat conditions. 2. The Case test-bar mold with knife ingate consistently produces measurable mechanical property increases over the standard Stahl ASTM
promising as it is possible to ascertain pore size and size distribution in a cast sample, with sensitivity down to just a few microns. At this time, however, it has not been possible to incorporate specific results and comparisons between tensile samples of different conditions in this report.
3.
B108 testbar mold and should be given further consideration for use as a standard methodology. Despite perhaps over-modification with high strontium and apparent porosity, separately-cast Case test bar mold tensile properties yielded a very high Quality Index number aided by high elongation results (13-15%). This is due to low iron content (less than 0.10%) and the effect of the knife-ingate configuration on the Case testbar mold which provides better feeding in spite some
4.
5.
micro- porosity experienced with the high strontium. HIPping improves the tensile and fatigue properties of both alloys by closing off any micro-porosity. Computed Tomography (CT Scan) techniques may be useful in ascertaining microposity distribution and quantification as influenced by testing or processing variables.
References 1.C.J. Chen, D.V. Neff, D.S. Schwam, Y. Wang, “Optimization of As cast Mechanical Properties in A356 via Simulation and Permanent Mold Test-bars”: AFS Transactions, 2011, vol. 115, pp. 53-59. 2. H.R. Lashgari, M. Emamy, A. Razaghian, A.A. Najimi, “The Effect of Strontium on the Microstructure, Porosity and Tensile Properties of A356-10%B4C Case Composite”: Materials Science and Engineering A, 2009. 3. F.J. Tavitas-Medrano, J.E. Gruzleski, F.H. Samuel, S. Valtierra, H.W. Doty, “Effect of Mg and Sr-modification on the Mechanical Properties of 319-type Aluminum Cast alloys Subjected to Artificial Aging”: Materials Science and Engineering A, 2008, pp. 356-364. 4. D. Apelian, S. Shivkumar and G. Sigworth, “Fundamental Aspects of Heat Treatment of Cast Al-SiMg Alloys”: AFS Transactions, 1989, vol. 97, pp. 727. 5. S. Shivkumar, S. Ricci, C. Keller and D. Apelian, “Effect of Solution Treatment Parameters on Tensile Properties of Case Aluminum Alloys”: Journal of Heat Treating, 1990, vol.8, pp. 63-70. 6. S. Shivkumar, R. Ricci, Jr.B. Steenhof, D. Apelian and G.K. Sigworth, “An Experimental Study to Optimize the Heat Treatment of A356 Alloy”: AFS Transactions, 1989, vol. 97, pp. 791-810. 7. A. Knuutinen, k. Nogita, S.D. McDonald, A.K. Dahle, “Porosity Formation in Aluminum Alloy A356 Modified with Ba, Ca, Y and Yb”: Journal of Light Metals I, 2001, pp. 241-249. 8. G.K. Sigworth and T.A. Kuhn, “Use of Standard Molds to Evaluate Metal Quality and Alloy Properties”: AFS Transactions, 2009, vol.117, pp. 55-62. 9. Y. Wang, D. Schwam, D.V. Neff, C.J. Chen, X. Zhu, “Improvement in Mechanical Properties of A356 Tensile Test Bars Cast in a Permanent Mold by Application of a Knife Ingate”: Metallurgical and Materials Transactions A, 2011. 10. G. Ran, J. Zhou, Q.G.Wang, “The Effect of Hot Isostatic Pressing on the Microstructure and Tensile Properties of an Unmodified A356-T6 Cast Aluminum Alloy”: Journal of Alloys and Compounds, 2006, pp.8086. 11. S. Dasgupta, L. Parmenter, D. Apelian, ”Relationship Between the Reduced Pressure Test and Hydrogen Content of the Melt”: AFS International Conference on Molten Aluminum Processing, 1998, pp. 801-816 12. M. Drouzy, S. Jacob and M. Richard, “Interpretation
of Tensile Results by means of Quality Index and Probable Yield Strength”: AFS International Cast Metals Journal, 1980, vol.5, pp. 43-50. 13. X. Zhu, A. Shyam, J.W. Jones, H. Mayer, J.V. Lasecki, J.E. Allison, “Effects of Microstructure and Temperature on Fatigue Behavior of E319-T7 Cast Aluminum Alloy in Very Long Life Cycles”: International Journal of Fatigue 28 (2006) 1566-1571 14. Guang Ran, Jingen Zhou, Q.G. Wang, “The Effect of Hot Isostatic Pressing on the Microstructure and Tensile Properties of an Unmodified A356-T6 Cast Aluminum Alloy”: Journal of Alloys and Compounds 421 (2006) 8086 Acknowledgements: The authors wish to express their gratitude to American Foundry Society and the Aluminum Division for their financial and advisory support of this work; to the management of Foundry A and Foundry B for their direct participation in conducting a portion of this work, and to C-T-C for their in-kind contribution to fatigue test specimen preparation and testing.
