There are various mechanical tests such as hardness, tensile properties, stress relaxation, and strain rate sensitivity mea- surements to evaluate precipitation reactions in Al. Among these, the hardness test is the simplest and quickest method that can provide a precise overview of the macroscopic/mi- croscopic effects of the precipitation reaction process. The technique is in-situ wherein the measurement time is compa- rable to the rate of formation of the precipitates in the prima- ry Al matrix immediately after quenching and subsequent to solution treatment. The macro hardness measurement tech- nique has been used by researchers for studying precipita- tion reaction during aging of Al-Si-Mg alloys,23,31,33,37,41
how-
ever, the results do not identify details and do not quantify the kinetics of the formation of initial individual-clusters, co-clusters and GP-Zones of Mg and Si atoms.
Micro-hardness measurements carried out continuously and immediately after quenching the Al-Si-Mg alloy samples from the solution heat treatment temperature is a viable technique to analyze the kinetics of the precipitation reactions during incubation. The size of the indentation in micro-hardness is in micrometers and is generally smaller than the size of an aver- age grain in the samples solidified at nominal cooling rates, akin to those observed in conventional casting processes. The micro-hardness measurements are very sensitive to subtle changes in the distribution of solute atoms in the primary Al phase and hence, could detect the formation of self-clusters and GP zones during the precipitation reaction.
Objectives and Research Plan
The objectives of this study are: · Examine the sequence of precipitation reaction of the Mg and/or Si atoms in the primary Al phase of A356.2 alloy using micro-hardness measurements and tensile property assessment coupled with the information from the prior-art in the field. Spe- cifically, hypothesize the specific stages occur- ring during incubation at room temperature and subsequently, the artificial aging process at high temperature.
·
Identify the effect of various incubation times on the precipitation reactions during artificial aging at high temperature; subsequently, evaluate the resultant tensile properties of the cast A356.2 component.
Experimental Plan A set of experiments was carried out to evaluate the effect of the rate of quenching on the kinetics of the precipitation re- action during incubation. In these experiments, samples cast with A356.2 alloy were solution treated at 540C (1004F) for 12 hours at temperature and subsequently quenched in three different quenching media: · A mixture of anti-freeze solution with dry ice maintained at a constant -40C (-40F) temperature. Water at room temperature of about 23C (73F). Water maintained at 80C (176F) temperature.
· ·
20
Subsequent to quenching, each sample was maintained at room temperature for incubation. During incubation, mi- cro-hardness measurements were carried out on the matrix from 0 to 150 hours aging time. Measurements were taken at very small intervals during the first few hours and sub- sequently, the measurement intervals were increased me- thodically.
A second set of experiments was carried out to understand the sequence of precipitation in the primary phase during incuba- tion treatment and subsequently, the artificial aging treatment. In addition, the effect of various incubation times coupled with artificial aging treatment on the mechanical properties were evaluated through tensile property measurements.
In the second set of experiments, two sets of sample geom- etries, namely, cast cylindrical samples of about 12.70 mm in diameter and 12 mm in length, and as-cast tensile test bars (ASTM B557-06 standard) permanent mold cast specified by ASTM B108 M-08 Standard were used. Solution heat treatment was carried out on all the samples at 540C (1004F) for 12 hours at temperature in a conventional electric resis- tance furnace. The samples were quenched in water main- tained at 80C (176F). Incubation was carried out at room temperature for various times ranging from 0 to 20 hours for the cylindrical samples and tensile test bar samples. The sample from each incubation time was subjected to artificial aging treatment at 155C (311F) for various times ranging from 1.5 to 10 hours for the cylindrical samples and tensile test bar samples. Micro-hardness measurements on the ma- trix of the primary Al phase in the cylindrical samples and uni-axial tensile test on the tensile test bars were carried out. There were a total of 104 cylindrical samples and 160 tensile test bar samples (5 samples repetition for each condition) in the second set of experiments.
Materials and Procedure Details of materials and procedures are presented below.
Alloy Alloy A356.2 was obtained from a commercial vendor and the composition of the alloy as measured by Glow Dis- charge Optical Emission Spectroscopy (GDOES) Induc- tively Coupled Plasma/Mass Spectrometry (ICP/MS) is presented in Table 2.
Casting A356.2 alloy was cast in a direct pour gravity casting pro- cess in a metal mold. Cast component design and the cast- ing process specified in ASTM B108M-08 standard was followed. Test bars for tensile test evaluation were cast and ASTM B557M-07 testing procedure was followed (a strain rate of 1 mm/min was used in the uni-axial tension mode on a 51 mm gauge sample fitted with a digital on-line ex- tensometer). Tensile samples were of standard specimen dimension described by ASTM B108M-08. The alloy was
International Journal of Metalcasting/Fall 2011
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