THE USE OF THE WEIBULL STATISTICAL METHOD TO ASSESS THE RELIABILITY OF A DEVELOPMENT ENGINEERED CASTING COMPONENT


R. MacKay


Nemak of Canada Corp., Windsor, Ontario, Canada D. Szablewski


Transportation Technology Center, Inc., Pueblo, CO, USA Copyright © 2010 American Foundry Society Abstract


The work contained herein examines the effect of liquid metal quality, solidification rate, tensile test sample extraction and tensile test preparation, on tensile result reliability as measured using Weibull Statistics. The prototype test casting used for this study was made from sand with two possible sized chills. The level of dissolved hydrogen was examined; as were tensile test sample placement within a cast structure having a gradient of structural fineness imposed by the chill, the effect of increase chill mass, and the effect of sample preparation,


Introduction


The tensile strength of many cast aluminum alloys is limited by the size and distribution of the primary phase constitu- ents such as the secondary dendrite arm spacing (λ2


tion by the secondary phase constituents such as β-Al5 α-Al8


in the interdendritic regions.3-13 Fe2 Si, α-Al12 Fe3 Si2 , and α-Al15 The size and distribution of (Fe,Cr,Mn)3 Si2


FeSi and Si platelet crystals) due to the low so- lution temperature used for Cu containing alloys.7-9


loys such as 356 and 357, which have solution temperatures around 530°C (986°F), spheroidization of β-Al5


platelet crystals can occur.11-13


The tensile properties for castings can have more scatter than what is typically found for wrought aluminum. It is this scatter in tensile properties that can limit aluminum casting


International Journal of Metalcasting/Winter 10


the porosity within the same structure containing the afore- mentioned phases is also a factor. Typically the heat treat- ment T6 (peak age) and T7 (over age), solution, quench and then artificial age used in most 300 series alloys modify the properties within the FCC aluminum structure via precipita- tion, which elevates the matrix microhardness, elongation, and yield point. Effects of the T7 treatment contained in this work do not modify the morphology of the phases that grow via the Twin Plane Re-entrant Edge (TPRE) growth method (i.e. β-Al5


For al- FeSi and Si ), and the


Si particle morphology of the Al-Si eutectic. Al-Si-Cu sand castings will range in elongation between 1-8%, rarely below 1%, and seldom exceeding 8%.1,2


There is also a contribu- FeSi,


located


In practice, most literature reports a large number of ten- sile tests due to the scatter mentioned, where using averag- ing methods and a standard deviation are used for a final result. Boileau5


reported that a minimum of 16 test samples is needed for 98% confidence interval for a true mean in coarse structures encounter for sand castings. The param- eters that tend to scatter the most are elongation and UTS. For a similar HT condition and cooling rate the YS should have a much lower level of scatter.14-16


For cast component development however, there will be a desired mechanical property requirement determined from stress analysis modeling. The casting developer must then develop a process which will constantly achieve that mechanical test result in a production environment. The problem that may exist with an average or mean of test- ing results with a standard deviation (σ) is that tensile results being presented in this fashion are implied as a Gaussian (also known as Normal) distribution, or ‘bell-


31


on the Weibull Modulus. The test sample population for each iteration was thirty samples, which is reasonable for a Weibull modulus to be determined. A detailed metallographic analysis of the tensile bar fracture surfaces was determined using Light Optical Microscopy (LOM) and Scanning Electron Microscopy in Secondary Electron (SEM/SE) mode.


Keywords: tensile testing, Weibell statistics, metal casting, aluminum, engineered component


usage in several dynamic applications. Wrought aluminum products can achieve nearly 20% elongation. This is due to the fact that cast alloys will have a larger volume of sec- ondary phases, higher volume fraction of porosity, and the absence of surface texture. An additional reason as reported by Campbell et al.,14


Byczynski15 and Green et al.,16 the pres-


ence of bifilms within the cast structure can lower elongation and UTS. Bifilms, even in the Hot Isostatic Pressing (HIP) condition will not improve scatter in tensile results.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85