Metalcasting's Future Through New Ideas and Innovation: A sampling of presentations from the AFS Metalcasting Congress in Fort Worth, Texas, focuses on ways to move the industry forward into a new period of prosperity.
One of the biggest draws of the annual AFS Metalcasting Congress is the presentation of completed research projects. They are shared and discussed and hopefully help metalcasters across North America improve their operations, better service customers, and stay ahead of the game in a very competitive market.
Three of the presentations are summarized below. These papers from the leading minds in the industry represent studies and ideas that could have wide-ranging positive impacts on the future of metalcasting.
Research into the Quantitative Evaluation of Casting Surfaces Using 3-D Laser Scanning
Nathaniel Bryant, University of Northern Iowa (Cedar Falls, Iowa)
Surface roughness is an integral part of casting quality specifications. The surface finish of fluid handling castings, piping components, and related geometries can be a critical factor in their service life and overall efficiency. This is especially important in high velocity liquid and gas applications, because rough surfaces affect the allowable flow. Rough surfaces are also subject to increased wear since there is increased frictional interference during the material transfer.
In production facilities, quality engineers are still bound to qualitative forms of surface roughness measurement using cast comparators and visual inspection. The University of Northern Iowa is researching a method to quantitatively describe cast surfaces using a laser scanning device. The results can be represented by industry accepted scales such as root means square (RMS) and roughness average (Ra). Defect analysis and part variation measurement capabilities are also possible with this new method.
From an aesthetic standpoint, smooth and regular parts appeal to casting customers. It is more appealing for the casting customer if parts are smooth and regular. The surface quality of castings is the first impression for the customer, so it is advantageous to have smooth casting surfaces.
Additionally, the evaluation of process changes that can affect the surface roughness of castings can benefit from quantitative measurement.
Measurement of surface roughness on cast surfaces is difficult and relies on qualitative methods with significant variability. These methods give an estimate for surface finish, but lack a numerical value for the measured surface. Methodology based on laser metrology offers the ability to quantitatively measure casting roughness on non-reflective surfaces without part destruction. It also helps quality inspectors identify casting defects with a way to measure their severity. According to the gage R&R results, the methodology shows promise in terms of repeatability and reproducibility. Without a true calibration standard for cast roughness, it proves difficult to propose a methodology to measure surface finish.
Influence of Mn and S on the Microstructure of Cast Iron
Richard B. Gundlach, B. Gundlach, Element Materials Technology (Wixom, Michigan)
Sulfur is generally considered a tramp element in cast iron, and its level must be controlled. Manganese will react with sulfur to form MnS inclusions and, thereby, tie up sulfur before solidification occurs. When manganese is not present at sufficient concentrations, sulfur reacts with iron to produce a low-melting phase that can produce hot-shortness and embrittlement in iron castings. Consequently, the industry has always added manganese to control sulfur in cast iron.
Various approaches for balancing manganese and sulfur have been promulgated in the industry. To avoid FeS formation, many look to the stoichiometric ratio of MnS (1.7 to 1), and rely on adding excess manganese to control sulfur. Some employ a manganese to sulfur ratio (such as in the range 5-7). Others advocate that the sulfur content must simply be at, or above 0.04%S to obtain adequate inoculation response. With the exception of a few investigators, none has considered the solubility of MnS (from thermodynamic principles) as a guide to balance manganese and sulfur.
While the concept of "excess manganese" suggests that manganese can completely tie up sulfur prior to the solidification of the melt, thermodynamic calculations show that the reaction manganese + sulfur--MnS is weak and does not go to completion before the metal reaches the eutectic temperature (typically around 2,150F), see Figure 1. Based on thermodynamic calculations, there can be significant amounts of sulfur in solution when eutectic solidification begins.
At the eutectic temperature for typical commercial cast iron chemistries, the equilibrium constant, Keq, for the reaction Mn + S--> MnS is around 0.03. That is, the following relation is true:
Keq %Mn x %S = 0.03 (at 2,150F) When the value %Mn x %S exceeds 0.03, the solubility of MnS is exceeded and some MnS will precipitate in the liquid metal before the metal reaches the eutectic temperature. The precipitation of MnS will reduce the level of sulfur in the melt until the solubility limit is attained (Figure 2). Consequently, some free sulfur is always present when eutectic solidification begins, and the "free sulfur" content in the liquid metal is primarily controlled by the manganese concentration.
Some "free" sulfur is beneficial. As a surface active element, sulfur promotes undercooling which, in turn, promotes a good response to heterogeneous nucleation of graphite, i.e., a good response to inoculation. The level of free sulfur is also expected to influence the growth of the graphite flakes, promote branching, and limit the growth of the eutectic cells, all of which positively influence the mechanical properties of cast iron.
Microstructure evaluations and additional mechanical testing were conducted on the materials from AFS Research Project 12-13 #04. The test results from this investigation provided more insight on the cause for reduced strength with increasing sulfur contents.
* Cell count was found to decrease with increasing sulfur.
* There is a tendency for type D graphite to form in the cell boundaries with increasing sulfur.
* The formation of spiky graphite occurred in irons of higher sulfur concentrations in all cast sections and in all three manganese series.
* Intercellular carbides and sulfides were observed in high-sulfur alloys, but only in the 0.28% manganese series.
* Tensile testing revealed that the reduction in strength with increasing sulfur was caused by a reduction in the elongation at fracture, without a significant change in the yield point.
* The premature fracture in the tensile test was accompanied by a change in fracture mode in the pearlite matrix from ductile tearing to transgranular cleavage.
An Aluminum Casting Alloy for High Performance Engines
David Weiss, Eck Industries (Manitowoc, Wisconsin)
Auto manufacturers are under commercial and regulatory pressure to improve engine efficiency. This requires increased specific power output (horsepower per liter of engine displacement), accomplished by use of direct fuel injection, higher compression ratios and turbochargers. However, these improvements result in higher operating temperatures and pressures. This causes increased thermo-mechanical fatigue, particularly in the valve bridge between inlet and exhaust ports on the combustion face of cylinder heads. Engine designers have realized that further improvements will require better materials. This has been the main impetus for the development of new materials.
Briefly, the alloys currently in use or proposed are:
* 319 alloy.
* Primary A356 or 357.
* C355 alloy.
* Optimized 319/320.
* 224 alloy.
* 351 alloy.
It will also be useful to look at some other properties. The relative costs have been calculated using A356 alloy as a base point. These calculations used typical costs and recoveries for alloying materials. (In these calculations copper was $3 per pound; and 351 alloy was assumed to contain 0.15% each of V and Zr.) Other factors enter into the cost of production, of course, but the values tabulated here are a good starting point for comparison. The costs tabulated for 319 and 319+MnVZr are for primary versions of these alloys. Secondary versions would normally be less costly, but they would have significantly lower fatigue strength.
How these values may be used are illustrated is an example, comparing the two strongest alloys: RTA's version of 224 alloy and Alcoa's 351 alloy. They have nearly the same strength, so the design (shape and size) of a cylinder head should be nearly the same for both materials. However, the density of 224 is 2.81/2.70=1.041 times greater. The cost per pound is also slightly higher. If the base price of A356 is $1 per pound, then the material cost of a 20-lb. cylinder head in A356 alloy will be $20. Using 351 alloy, the cost is:
20 lb. x 2.70/2.67 x $ 1.126/lb. or $22.77
The same cylinder head in 224 alloy will cost:
20 lb. x 2.81/2.67 x $ 1.128/lb or $23.74
Obviously, there is no cost incentive to use the 224 alloy. In addition, the thermal conductivity of 224 is less; as expected, because of its higher copper content. Similar comparisons can be made with other alloys.
Castability is not presented in Table 1, but this is an important consideration. Typical experience with alloys similar to 224 (especially A206) suggests that typical production costs will be 20 to 30% greater than for A356. With experience casting 354 alloy, and considering its similarity, 351 alloy was expected to exhibit similar castability.
For these reasons 351 alloy was chosen when customers requested material in cylinder heads and blocks for racing engines.
The production of automotive blocks and heads puts significant demands on the foundry to produce sound, pressure tight castings in a wide range of shapes that have not necessarily been optimized for the casting process, from strong alloys. Many high strength alloys, particularly those that contain copper, sometimes suffer from castability concerns and may require more sophisticated gating or the use of extensive chilling to produce. 351 alloy generally has better mechanical properties than A356 or C355, with castability very similar to aluminum-silicon alloys without copper.
Caption: Figure 1. Depiction of the solubility of MnS in cast iron at temperatures near and well above the eutectic temperature.
Caption: Figure 2. Estimation of amounts of MnS precipitation and soluble sulfur in cast iron containing 0.78%Mn and 0.15%S.
Caption: Figure 3, This is a high-performance block produced from 351 alloy.
Caption: The annual technical sessions were an important part of the Metalcasting Congress.
Caption: Many of the sessions at Metalcasting Congress were interactive and challenged the people in attendance.
Table 1. Other Properties of selected aluminum casting alloys Alloy Density (gm/cc) Thermal Conductivity Temper (25 C, W/m-K) 319 2.79 112 F A356 2.67 150 T6 A356-Cu (2.68) (<150) T6 C355 2.71 150 T6 A356-CuZrMn (2.68) (<150) T6 319+MnVZr (2.79) (<112) F 224 2.81 117 T62 224(RTA) (2.81) ([less than T6 or equal to] 117) 354 2.71 125 T61 351 2.70 140-160 T7 Alloy Cost Differential (cents/lb) 319 6.9 A356 -0- A356-Cu 1.1 C355 2.2 A356-CuZrMn 2.7 319+MnVZr 13.1 224 7.5 224(RTA) 12.8 354 4.9 351 12.6
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|Title Annotation:||METALCASTING CONGRESS|
|Date:||May 1, 2018|
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