R.E. Showman, R.C. Aufderheide, N.P. Yeomans

Transcription

1 Advances in Thin-Wall Sand Casting R.E. Showman, R.C. Aufderheide, N.P. Yeomans Ashland Casting Solutions, Dublin, Ohio, USA Abstract Economic and environmental pressures continue to force metalcasters to search for ways to produce castings with thinner walls and lighter weight, but with equal or improved mechanical properties. Previous studies have shown that the use of low-density alumina-silicate ceramic (LDASC) as a sand additive/replacement can dramatically modify the thermal properties of sand molds and cores and can enable the production of thin (<3mm) sections without sacrificing metallurgical or mechanical properties. This paper reviews laboratory testing conducted with LDASC in the sand mold or core and summarizes effective thermal properties, resulting cooling rates and casting properties. Additional new data is provided detailing the use of LDASC to effectively enhance gating and feeding systems through the development of thermal channels within the mold cavity. Key words thin-wall, LDASC, thermal-channels, sand-castings 97/1

2 Introduction The demand for light-weight, thin-wall castings continues to grow. Section thickness reductions for automotive and other castings are driven by the need to reduce weight, tighten dimensional tolerances, enhance performance and reduce overall cost. This has resulted in a continuing shift toward more aluminum and magnesium castings and toward gravity and high-pressure diecasting. However, the strength-to-weight ratios for ductile and compacted graphite irons make them attractive if thin-wall (<3mm) parts can be produced with good dimensional accuracy and controlled physical and microstructural properties and precision sand casting offers a number of advantages over other casting methods[1]. Efforts by a number of researchers [2-5] have shown that thin-wall iron castings can be reliably produced in sand molds by using the proper metal chemistry, inoculation practice, gating design, and mold and core materials. These controls may tighten the processing window for iron foundries, but do not pose insurmountable problems. The procedures simply reflect an extension of existing practices rather than totally new manufacturing processes. However, foundries have been slow to adopt these practices and move toward thinner castings without more proven examples. Review of Previous Work with LDASC Earlier work by the authors [6] has shown that LDASC can be used as a sand additive/replacement to modify and control the thermal properties of molds and cores, and to make the production of thin-wall iron castings a reality. LDASC has a density about 1 / 4 of silica sand and a corresponding reduced specific heat and thermal conductivity. When used at levels of 20% 100% (volume %) with sand, the heat extraction characteristics are dramatically changed, making thin-wall iron castings with good metallurgical and physical properties possible. The change in thermal properties slows the cooling and solidification rates as shown with ductile iron. (Figure 1 [7]) The ability to produce thin-wall iron castings with LDASC has been demonstrated in several ways. First, standard fluidity spiral castings were produced with various blends of LDASC and silica sand. These blends reduced the cooling and solidification rates in these castings, enabling much increased metal flow. Where a standard sand mold produced a flow distance of approximately 71 cm, a 40% sand, 60% LDASC (volume %) blend resulted in flow all the way to the end of the spiral, or about 147 cm. (Figure 2) Thin-wall plate castings were also produced to show the effectiveness of LDASC additions. The test casting was approximately 200mm x 200mm x 1.5mm with two ingates. The mold was produced in phenolic urethane nobake, first with sand and then faced with 100% LDASC on both the cope and drag surfaces. The sand mold produced a misrun when poured with 97/2

3 ductile iron at 1400 o C. The mold faced with LDASC filled completely, with no indications of misruns or coldshuts. (Figure 3) Somewhat more complex thin-wall castings were produced to show that the LDASC material could be used either on the mold or core surfaces. A manifold test casting was used that was approximately 380mm long by 55mm in diameter with a 2mm wall (Figure 4). By gating into all three heavy sections, the casting could be 90% 95% filled with ductile iron. However, by using at least 25% LDASC in either the core or mold facing, the casting filled completely using a single gate, nearest the sprue. Even in thin-wall castings, the use of LDASC controlled the solidification rates and resulting microstructures of iron castings. This was first demonstrated with gray iron chill wedges. These castings are typically used to show the chilling tendencies of gray iron with different chemistries or inoculation. However, they can also be used to show the effects of slower cooling with LDASC additions on casting microstructure, particularly on carbide formation. (Figure 5) The effects of LDASC additions on ductile iron microstructure were most dramatically shown in the thin-wall manifold casting in the 2mm section. When both a 100% sand core and mold were used, the 2mm section contained massive carbides in fine pearlite. When a 50/50 (by volume) mixture of sand and LDASC was used in both the core and mold, the 2mm section showed small graphite nodules in ferrite and pearlite, with some small retained carbides. However, when 100% LDASC was used in both the core and mold, the 2mm section contained large graphite nodules in a typical bulls-eye structure that might be expected in section of 5mm and above. (Figure 6) The physical properties of thin-wall casting are dependent on the cooling rates and resulting microstructures. Hardness is often used as an indicator of tensile strength in castings where it is impractical to cut and test actual sections from the casting. For the 2mm manifold test casting, hardness values were recorded from the 2 mm section and plotted against the volume percentage of LDASC in contact with the casting. The resulting graph shows a dramatic reduction in hardness, which would indicate a corresponding increase in ductility and yield. (Figure 7) Labrecque, Gagne & Javaid [8] also performed tensile tests on samples cut from thin-wall ductile iron castings produced with varying amounts of LDASC. They found a significant decrease in ultimate tensile strength (UTS) and yield strength (YS) and a corresponding increase in percent elongation (%E) with increasing amounts of LDASC. (Figure 8) Thermal Channels When tests were conducted on the thin-wall manifold casting, there was an improvement between castings made with LDASC in the core versus the castings with LDASC in the mold facing, also covering the gating 97/3

4 system. The LDASC appeared to improve the performance of the gating system. The use of LDASC in the mold or core reduced heat extraction from the metal and thus reduced cooling and solidification rates. In effect, the area with LDASC acted like a heavier section. This concept generated the idea to use LDASC inserts as thermal channels. The inserts could be placed on the mold surface leading from the ingates, across thin sections of the casting. The inserts would cause the area to act like a heavier section, thus creating a thermal channel for the metal from the gate to flow through. Thermally, the insert would create an effective heavier section, but the actual casting section would remain thin and uniform. An extreme test casting was chosen for pouring trials. The 200mm x 200mm x 1.5mm casting could not be poured with a sand mold in either gray or ductile iron at normal pouring temperatures without misruns. It was an impossible casting without the use of special techniques. Pouring trials in normal no-bake sand molds produced castings that showed the expected flow pattern spreading from the ingates, but the metal cooled and solidified before the entire mold could fill. (Figure 9) Thermal channels were created by placing 25mm x 12mm bars of 100% LDASC as inserts on the casting surface. Four configurations were developed: extension from one gate, extensions from both gates, one cross piece between extensions, and two cross pieces between extensions. Initially molds were made with the extensions only in the drag, but a second set of molds were made with the extension in both cope and drag. (Figure 10) The molds were poured in Class 30 gray iron at approximately 1370 o C. The castings with the inserts in the drag only showed improved metal flow and fill distance and the castings with the inserts in both the cope and drag showed even greater improvement. It was apparent that gate extensions had acted as thicker sections and helped fill the impossible casting. (Figure 11) Thermal channels can also be used to control feeding in thin-wall castings. Typical thin-wall castings are prone to centerline shrinkage [9] because of the very low modulus (volume to surface area ratio) and the inability to maintain feeding pathways through the thin sections. The use of LDASC in the mold or core can change the effective modulus of the section and thus improve feeding through the section. This is similar to the traditional use of padding to improve feeding characteristics. By selectively using LDASC inserts to create thermal channels through the thin sections, feed pathways can be maintained and directional solidification will occur toward the appropriate gates, risers or heavy sections. (Figure 12) Modeling Thermal Channels The thin plate casting was modeled using Novaflow to show the flow and cooling patterns. The initial model of the plain no-bake mold showed flow 97/4

5 extending from the ingates and cooling to create a likely misrun. This was very similar to the fill patterns seen in the actual test castings. (Figure 13) LDASC inserts were then added to the model in the same configurations as the four-test casting. The density and thermal properties of the inserts were adjusted for the LDASC material to show its effects on the fill pattern. (Figure 14) The resulting casting models showed fill patterns very similar to the actual castings (Figure 15). This confirmed the ability of the model to show the effects of the gate extensions and indicates that the modeling of gate extensions can be applied to complex castings. Conclusions Previous work has shown the effectiveness of LDASC to modify the thermal properties of sand molds and to reliably produce thin-wall iron castings with good metallurgical and physical properties. LDASC can be used to create thermal channels that improve the flow and feeding through thin-wall sections. This provides an additional design and process option for thin-wall castings including a reduction in runner size, reduction in the number of ingates and improved feeding through thin sections. Models can be used to show the effects of the thermal channels and to predict flow and feeding for design purposes. References 1. Busby, A.D., Archibald, J.J., Expanded Opportunities for Precision Sand Casting Utilizing Coldbox Binders, AFS Transactions, , Charoenvilaisiri, S., Stefanescu, D., Rusanda, R., Piwonka, T., Thin-wall Compacted Graphite Castings, ASF Transactions, , Javaid, A., Thomson,J., Davis, K., Critical Conditions for Obtaining Carbide-Free Microstructures in Thin-Wall Ductile Irons, AFS Transactions, , Katz, S., Thin-wall Iron Castings Planning the Future, Foundry Management & Technology, June 1997, pp Ruxanda, R., Stefanescu, D., Piwonka, T., Microstructure Characterization of Ductile Thin-wall IronCastings, AFS Transactions, , Showman, R.E., Aufderheide, R.C., A Process for Thin-Wall Castings, AFS Transactions , Aufderheide, R.C., Showman, R.E., Controlling the Skin Effect on Thin-Wall Ductile Iron Castings, AFS Transactions , Labrecque, C., Gagne, M., Javaid, A., Optimizing the Mechanical Properties of Thin-Wall Ductile Iron Castings, AFS Transactions , /5

6 9. Woolley, J.W., Stefanescu, D.M., Microshrinkage Propensity in Thin Wall Iron Castings, AFS Transactions , 2005 Figures Sand vs. LDASC Molds - 5mm & 3mm Plate Cooling Curves Degrees C Time (seconds) 3 mm Section in LDASC Mold 5 mm Section in LDASC Mold 3 mm Section in Sand Mold 5 mm Section in Sand Mold Figure 1. Cooling curves for 3 & 5 mm ductile iron plates 1 Sand Mold 40% sand, 60% LDASC mold Figure 2. Fluidity spiral castings, gray iron Figure 3. Thin-wall plate castings: LDASC mold left, sand mold right 97/6

7 Manifold pattern Manifold casting Casting section Figure 4. Thin-wall manifold test casting Figure 5. Gray iron chill wedge castings from 100% sand to 100% 2mm section, sand core & mold, 100x 2mm section, 50/50 sand/ldasc core & mold, 100x 2mm section, 100% LDASC core & mold, 100x LDASC in 20% increments Figure 6. Ductile iron microstructures from 2mm manifold test casting 97/7

8 Figure 7. Graph of casting hardness vs. LDASC % Thin-Wall DI Strength Thin-Wall DI % Elongation Strength, MPa % LDASC UTS Yield % Elongation % LDASC % Elongation Figure 8. Thin-Wall DI casting properties as a function of % LDASC in the mold sand Figure 9. Thin plate casting poured in no-bake sand mold 97/8

9 Sand Mold Test 1 Test 2 Test 3 Test 4 Cope & Drag inserts Figure 10. Test molds with different gate extension configurations Figure 11. Thin plate castings poured with gate extensions; top row - drag only, bottom row cope & drag Figure 12. Sketch of thermal channel to promote feeding through thin section 97/9

10 Figure 13. Modeled fill pattern in thin plate casting with sand mold Figure 14. Examples of the model with gate extension in the drag and both cope and drag TC from1 gate (Test1) TC from both gates (Test 2) Plus 1 cross piece (Test 3) Plus 2 cross pieces (Test 4) Figure 15. Models of fill patterns with thermal channels in the drag (top row) and both cope and drag (bottom row) 97/10