The present invention relates generally to the quantification of microstructural fineness of metal castings, and more particularly to the automated quantification of dendrite arm spacing (DAS) in dendritic microstructures of metal castings as a way to avoid having to take such measurements manually.
The resulting microstructure of all cast aluminum-based components (such as engine blocks, cylinder heads, transmission parts or the like) is generally determined by the alloy composition and more particularly by the solidification conditions. In hypoeutectic alloys (i.e., those that contain less of the other alloying constituents than corresponds to the eutectic composition, examples of which include but are not limited to A356 and 319), the materials tend to solidify dendritically. Other such aluminum alloy examples that exemplify dendritic solidification include 354, 355, 360, 380, 383 and others. A typical microstructure of this family of alloys consists of a primary dendritic phase and a second phase of particles such as silicon particles and iron-rich intermetallics. The relative amounts, sizes and morphology of these phases in the as-cast structure are highly dependent on the solidification condition as well as on the alloy composition. The dendrite cell size (DCS) and DAS, sometimes referred to as the secondary dendrite arm spacing (SDAS), have long been used to quantify the fineness of the casting, which in turn can be used to gain a better understanding of the material and its related properties where—as a general rule—cast components with smaller DAS tend to have better ductility and related mechanical properties. Discussions pertaining to aluminum alloy casting in general, as well as to DAS properties in particular, may be found in numerous other applications for patent that are owned by the Assignee of the present invention, including U.S. patent application Ser. No. 12/356,226 filed Jan. 20, 2009, U.S. patent application Ser. No. 12/402,538 filed Mar. 12, 2009, U.S. patent application Ser. No. 12/454,087 filed May 12, 2009 and U.S. patent application Ser. No. 12/932,858 filed Mar. 8, 2011, all of which are hereby incorporated by reference.
There have been numerous efforts at describing dendrite refinement and its relationship to the solidification condition, starting in 1950 with Alexander and Rhines, who first established a quantitative basis for the influence of composition and solidification rate on certain dendrite features. Table 1 below summarizes known literature to describe in quantitative terms the fineness of dendritic structure.
TABLE 1Microstructural parameters describing the dendritesParametersSymbolUnitDefinitionDendrite arm spacingDAS, λμmDistance between well(Levy et al., 1969;defined secondaryOswalt and Misra, 1980;dendrite arms (centerRadhakrishna et al., 1980;to center)Flemings et al., 1991)Dendrite cell sizeDCSliμmRandomly linear intercept(Spear and Gardner, 1963;among dendrite cellsJaquet and Hotz, 1992)Dendrite cell sizeDCSedμmArea equivalent circle(Cáceres et al. 1995)diameter of dendritecells including eutecticDendrite cell countCPUANumber of cells per field(McLellan, 1982)
Of these, Spear and Gardner (1963) quantitatively described the scale of dendritic structure using DCS obtained by a random linear intercept and is referred to as DCSli in their FIG. 3(a). Following Spear and Gardner, Jaquet and Hotz (1992) in their study also used DCSli to quantify the dendrites. Levy et al. (1969), Oswalt and Misra (1980), Radhakrishna et al. (1980) and Flemings et al. (1991) all discussed DAS to quantify the dendritic structure. In these approaches, DAS is obtained by a linear intercept method where the line is chosen to intersect a series of well-defined secondary dendrite arms.
McLellan (1982) used dendrite cell count (CPUA) to quantify the microstructure and claimed that it describes the deformation process more accurately than DAS. However, Levy et al. (1969) had critically analyzed the measurements of both DAS and CPUA to characterize the cast structure, and pointed out that the standard deviation for DAS measurement was less than for CPUA measurement and also the mean cell size calculated from CPUA is greater than the mean DAS. Measurement of CPUA involves primary, secondary, and tertiary arms of the dendrites, whereas DAS measurements usually refer only to the secondary arm spacing.
The methods associated with manual measurement of DAS have been frequently used by the Assignee of the present invention as a way to make DAS measurement of aluminum castings. Such a procedure generally first includes preparation of metallographic samples that are prepared in accordance with known standards, such as the American Society of Testing and Materials Standard Guide for Preparation of Metallographic Specimens (also known as ASTM E3), a portion of which is reproduced below in Table 2.
TABLE 2ASTM E3Abrasive Type/SizeForceAPlatenSurfaceLubricantANSI (FEPA)Time sec.N(lbf)RPMBRotationPlanar Grindingwater120-320 (P120-400)15-4520-30 (5-8) 200-300CCODpaper/stonegrit SiC/Al2O3Fine Grindingcompatible6-15 μm diamond180-30020-30 (5-8)100-150COheavy nylon clothlubricantRough Polishingcompatible3-6 μm diamond120-30020-30 (5-8)100-150COlow/no nap clothlubricantFinal Polishingcompatible1 μm diamond 80-12010-20 (3-5)100-150COmed/high nap clothlubricantsynthetic suedeEwater0.04 μm colloidal silica30-6010-20 (3-5)100-150CONTRAFor 0.05 μm aluminaAForce per 30 mm (1¼ in.) diameter mount.BPower heads generally rotate between 25 and 150 rpm.CHigh-speed stone grinders generally rotate at greater than 1000 rpm.DComplimentary rotation, surface and specimen rotate in same direction.EOptional step.FContra rotation, surface and specimen rotate in opposite directions.
The surface of the sample to be analyzed is expected to be of sufficient quality to reflect the truest possible size and shape of particles. In one form, the plane of the polish will include eutectic phases that will appear darker compared to the surrounding matrix. Thus, in one form, the metallographic samples are finally polished to obtain a flat, near mirror image surface finish. Chemical etching may be used to enhance the contrast of the dendrite structure, where in one form, the etching may be in accordance with ASTM E407. Preferably, the sample is clean and dry, while polishing artifacts (such as comet tailing, pitting, scratching, pullout and staining) should be kept to a minimum. Likewise, test conditions and deviations should be agreed upon beforehand. In a preferred form, each sample will be examined in numerous fields of view, each of which is subject to a high (for example, 100×) magnification, depending on the fineness of the material grain. After this, an image of the field of view to be measured should be captured. In one form, the linear intercept method may be used for measuring DAS, where three or more dendrites with visible dendrite trunks per field of view with at least three dendrite arms are selected. From this, a line is drawn from the outside edge of the first dendrite arm to the inside edge of the last dendrite arm; an example of this is depicted in FIG. 6B. The distance d for each dendrite may be recorded, while the number n1, n2, n3, etc. of dendrite arms counted for each measurement may also be recorded. These activities may be repeated for each field of view.
At present, both a volume percentage of eutectics and the DCS can be determined automatically via the use of an image analyzer. The local cooling rate affects not only the microstructural fineness but also porosity formation. Therefore, DAS tends to be used more frequently to quantify the microstructural fineness. The problem with the measurement of DAS is that it has to be performed manually by identifying well-defined dendrite arms in the image. Unfortunately, this is both very time-consuming and heavily dependent upon the skills of the user or individual doing the measuring.