integrating materials science into the mechanical...

2006 ASEE Southeast Section Conference

Integrating Materials Science into the Mechanical Engineering Curriculum

Judy Schneider1

Abstract – Much of the challenge in introducing Materials Science concepts into the engineering curriculum arises because this is the first time many students are tested on their conceptual understanding rather than just the ability to “plug in” numbers and solve equations. Several approaches can help to bridge this gap: tailoring the Materials Science course to a specific engineering curriculum, incorporating laboratory exercises to reinforce concepts, and including industrial "real world" examples. By building a Materials Science course around the requirements of a Mechanical Engineering (ME) Curriculum, students are introduced to the conceptual as well as computational understanding of the interaction between materials selection and suitable processing methods in the ultimate performance of an engineered device.

Keyword: Materials Science, Mechanical Engineering, Inquiry based exercises.

Background

At Mississippi State University (MSU), the ME department has taken ownership of the traditional service course entitled: "Introduction to Materials Science" by offering the renamed course within the ME Department. This allows the faculty member to tailor this "Materials for ME Design" course to ensure that students understand the materials science concepts as applied to the typical processes they may encounter in the design and fabrication of an engineered component.

Currently the "Materials for ME Design" incorporates a semester long project to compliment the lectures. The project, entitled: "Life of a Casting" is used to demonstrate the importance of materials science in manufacturing processes by providing the students with hands-on exposure. The students work in groups to collectively cast, heat treat, machine, and perform hardness and tensile tests on a given aluminum alloy and temper. To link the microstructural evolution with the mechanical properties, the students mount and polish specimens to view the microstructure before and after heat treatments. The various stages of the project correspond with lecture topics and serve as enforcement of basic concepts of diffusion, phase diagrams, strengthening mechanisms, and processing/property/microstructure relationships. Students correlate hardness tests results with their measured tensile test results. At the completion of the course, the students submit group reports documenting the correlation of processing, microstructure, and property relationships. When students are asked on the final student evaluation, "What did you like the most about the class?" One hundred percent of the students respond, "The project because I got to get out of the classroom and do something."

1 Associate Professor, P.O. Box ME, Mississippi State University, Mississippi State, MS 39762, [email protected].


Course Content

The Materials for ME Design Course is concerned with the selection of materials, how they are processed and how the selection of a material and a suitable manufacturing sequence enters into the practice of concurrent design. The specific course objectives include:

1) Demonstrate knowledge of the structure and behavior of the different classifications of materials (metals, ceramics, polymers, and glass).

2) Demonstrate knowledge of the applicability of various manufacturing processes toward the different classes of materials.

3) Combine principles and concepts covered in the class with the group project. The project will involve casting, heat treating, machining and testing of tensile bars of Al alloys. Data from the testing will be reduced to stress vs. strain plots. Groups of 3 students will work together on a specified aluminum alloy and temper. A final report, prepared by the group, will be due at the end of the semester.

Traditionally, an Introduction to Materials Science Course would have included a laboratory to demonstrate the concepts covered in class. An alternative approach is to allow the students to witness demonstrations in addition to hands on exercises, all structured around a particular project. By coordinating the class discussions with the various stages of the group project, the students are exposed to an inquiry based experience. Table I summarizes the learning objectives of the class with the associated stage of the student project. The student project is accomplished with 2 4-h demonstrations outside of class, 2 demonstrations during normal class periods, and 3 hands-on, independent activities for each student group. The students are expected to spend 4-h on each independent activity including the preparation time. Prior to gaining independent access, the students are required to complete and submit the necessary background information prior to receiving a key to the laboratory for a specified period of time. Failure to return the key after the specified period of time results in the student group failing the class. To date, no students have failed this class.

Student Project Description

Cast, quench, and relationship to tempers - In the first 2 weeks of the course, while the students are reviewing mechanical properties of materials, a casting demonstration is schedule for 2 evenings. The student groups were assigned one of two cast aluminum alloys and a temper. Although several groups are assigned the same alloy (A386 or A319), each group has a different temper. The students cast bars of an aluminum alloy, shown in Figure 1, using machine steel molds coated with ceramic slurry. The molds were designed with a 2-in. feeder reservoir at the top. Figure 2a shows the moisture being driven off of the mold and ceramic coating using a propane torch. This is done just prior to filling of the mold with molten metal to prevent any production of steam which may cause an explosion. After heating, the two halves of the mold are clamped together to ensure a proper seal.

In preparation for the class, the appropriate aluminum alloy was melted in a “tilt and pour” furnace at 820˚C. The molten aluminum was poured into a crucible, Figure 2b, and then transferred to the mold, Figure 2c. A continuous pour was maintained to ensure that uniformity of the cast metal. After the metal solidifies, the mold was separated, samples removed and quenched in water. The tilt-pour furnace holds 1 liter of molten metal which can fill each of the 2 molts, 4 times to produce


16 aluminum bars. This allows 4-5 groups to pour per session. At the completion of the casting demonstration, each group is given 3 tensile bars which they maintain for the duration of the semester.

Table I. Correlation of Student Project Outline with the Course Lecture Topics. Project Outline Class Concepts Demonstrated and Reinforced

Casting Demonstration (Evening exercise for class)

• Processing of crystalline materials. • Melting temperatures of metals versus ceramics. • Volume change versus temperature for a crystalline metal.

Introduction to Metallurgical Laboratory (During normal class time)

• Polycrystalline structure of metals. • Metal classifications. • Density of metal (constant for aluminum alloys).

Heat Treatment Experiments (Hands on time for students)

• Reference material for metals. • Phase diagrams. • Cooling curves • Solid state diffusion. • Quenching and tempering. • Strengthening mechanisms in metals.

Metallographic Preparation and Hardness Measurements (Hands on time for students)

• Microstructure – mechanical property relationships. • Non-destructive measurements to verify manufacturing

processing. Introduction to Machine Shop (During normal class time)

• Metal removal processes.

Machining Tensile Bars (Hands on time for students)

• Turning processes. • Engineering drawings – tolerances and finishes.

Tensile Test Demonstration (Evening exercise for class)

• Mechanical properties. • Elastic modulus of metal (constant for aluminum alloys).

Figure 1. Dimensions of the as-cast aluminum bar.

2.5 in.

Dia. = 0.5 in.

The tensile bar was cut from this section.

The metallurgical specimen disks were cut from this section.

7.25 in.

Dia. = 1.0 in.


(a) (b) (c)

Figure 2. Drying the molds (a), pouring the molten aluminum into a crucible (b), and transferring the molten aluminum into the molds (c).

Sectioning of samples and density measurements - A class period was scheduled for the students to tour the metallographic specimen preparation laboratory. Each group brought their cast aluminum bars and used the metallographic cutoff saw to remove the riser and cut samples for mounting and polishing. Density measurements were made of a section of the casting.

For this class period, the students are divided into 4 groups which alternate activities between using the cutoff saw, making density measurements, witnessing hardness measurements and obtaining an overview of metallographic preparation of samples. The abrasive cutoff saw, shown in Figure 3, was used to cut small ½-in. thick disks from each bar for microstructural viewing and hardness testing. Water was used as a coolant during cutting to prevent heat build-up from friction to avoid an undesired change in the microstructure of the metal.

Archimede’s Principle, illustrated in Figure 4, was used to determine the density of the unmounted sample. First the mass of the specimen is measured in air and recorded as “md” as illustrated in Figure 4a. Next the specimen is submersed in water and the mass measured and recorded as “mw” as illustrated in Figure 4b. The mass of water displaced is obtained by subtracting the two masses (m=md – mw). Using the density of water at 1 g/ cm3, and the mass of the water displaced, the volume, V, of the specimen can be obtained in cm3. Equation 1 was used to determine the density of the specimen. By doing this as part of the class, the students are able to confirm that all aluminum alloys have approximately the same density, ρ, of 2.7g/cm3.

Eqn. 1


(a) (b)

Figure 3. Metallographic abrasive cutoff saw (a) and abrasive cutoff wheel (b).

(a) (b)

Figure 4. Archimede’s Principle.

Heat Treatment of Alloys - Each student group is required to research the heat treat schedule for their particular alloy temper. This introduces them to the ASM materials handbooks typically found in the reference section of the library, facilitates discussion of phase diagrams [1], heat treatment schedules [2], quenching mediums [2, 3], and “H” values [3]. The “H” values, listed in Table II, are metallurgical terms which describe quench severity for various cooling mediums. Figure 5 shows the appropriate binary phase diagrams for a 3xx series of cast aluminum alloy which is alloyed with Si and Cu. After approval of their proposed heat treat schedule the students may check out a laboratory key and signup to conduct their approved heat treatment schedule. Often the possibility of incorporating the heat treat schedule into the casting process is suggested, which expands the discussion into how the heat treatment process could be combined with the casting process if the cooling rate could be better controlled through various mold designs. The students complete the heat treatment of their cast bars and at least one metallographic section. The furnace used is shown in Figure 6 with the student showing the appropriate safety protection. Trade-off between time and temperature are discussed in conjunction with their schedules and availability to monitor the furnace.


Table II. H coefficient and severity of quench for several media [3].

Medium H Coefficient Cooling Rate of a 1 inch bar

(°C/s)

Oil (no agitation) 0.25 18

Oil (agitation) 1 45

Water (no agitation) 1 45

Water (agitation) 4 190

Brine (no agitation) 2 90

Brine (agitation) 5 230

(a) (b)

Figure 5. Appropriate Al-Cu & Al-Si Phase diagrams for 3xx cast aluminum alloys [1]. Cu is added for precipitation strengthening (a) while Si is added to form a eutectic (b).

5.5-6.5% Si 3-4 % Cu

Al-Cu Phase Diagram Al-Si Phase Diagram

Al Al Wt% Si Wt% Cu


Figure 6. Heat treatment furnace with air atmosphere.

Metallographic Specimen Preparation and Hardness Measurements - The next stage for the student project requires the students to consult the ASM Handbook for example micrographs [4] and expected hardness values [5, 6]. The use of material databases such as MatWeb [6] are beneficial because they reinforce the concept of material classifications, which are needed to find an alloy of interest. Upon showing expected microstructures and hardness values, students may check out a laboratory key and schedule time to prepare their specimens for metallographic inspection and hardness measurements. A month is allocated for the students to complete this stage of the project. The hardness measurements are made soon after the heat treatment so that the students can witness the difference in hardness. This establishes an interest to examine the microstructure to understand why heating a metal would make it stronger and provides evidence of solid state diffusion.

The metallurgical examination begins by mounting an as-cast and a heat treated disc in an epoxy mount (Epo-Kwick Fast Cure Epoxy) which sets up in 24 hours, followed by mechanical grinding and polishing. Inquisitive groups may decide to investigate intermediate stages of the heat treatment process. A Buehler hand grinder, shown in Figure 7a, was used to mechanically grind the specimen surface. Water was used to cool the samples and remove grinding debris. SiC sandpaper was used in progression from 240, 320, 400, to 600 grit. The sample was rotated 90 deg between each grit sandpaper and ground until the previous grit markings were no longer observed.

(a) (b)

Figure 7. Mechanical grinding of mounted specimens (a) and final polishing (b).


The next step in the disc preparation was polishing using the polishing turntable shown in Figure 7b. The first polishing stage used 1 micron gamma alumina polishing compound on a Buehler Microcloth with water lubricant. The second polishing stage used 0.05 micron gamma alumina polishing compound on a new Buehler Microcloth with water lubricant. After polishing to a mirror finish, the sample was taken to an inverted light microscope shown in Figure 8 and photomicrographs were taken of the microstructure as illustrated in Figure 9. Because cast alloys are used, no use of acids for etching is required to view the various eutectic microstructures.

Figure 8. Inverted microscope with digital image capture.

After examining the microstructure, the hardness of the as-cast and the heat treated samples are made using a Rockwell B hardness test. A 1/16-in. steel sphere was used as the indenter with a 100 kg mass as shown in Figure 10. Hardness testing is a simple and inexpensive, nondestructive test for evaluating the strengthening of a material by comparison of results to that of the initial material. The indent is made and the corresponding hardness value read off the gauge. Four or five indentions were made on each disk ensuring that each indention was away from the edge of the material, other indentions, or flaws in the surface. Some students will intentionally expand their study to investigate intermediate steps in the heat treatment process as shown by Table III.


(a) (b) (c)

Figure 9. Comparison of as-cast, 6-hour, and 12-hour heat-treated samples. Al-Si eutectic phase and other secondary precipitates found between dendrites (a). After 6-hour heat treat, dendrites are eliminated and elements are well dispersed (b). Only variance between 6 and 12-hour heat-treated samples is slightly larger grouping of silicon (c).

Table III. AA 319-T6 Rockwell B (RB) Hardness Measurements.

Hardness Results As Cast

Solution Heat-Treated Sample

(Not Aged)

T6 Heat Treated

(Artificially Aged)

1 42 52 72

2 43 52 73

3 47 53 72.5

(a) (b)

Figure 10. Rockwell Hardness Measurement device (a) with close-up of 1/16-in. ball indenter tip indenting the cast aluminum alloy bar (b).

Distance between outside bars on scale is 50 microns

Eutectic phase Silicon


Machining, drawings and surface finishes - A class period is devoted to touring the machine shop where students receive an engineering drawing (Figure 11) for the tensile bars in addition to the machining sequence, summarized in Table IV. After the introduction, the students have 1 month to sign up for a 2-4 hour block of time to machine their tensile bars. The students begin machining the cast bars, with the riser removed, by marking the cutting surface with a Sharpe pen. This enhances contrast so the students can visually following the cutting path. A facing bit is used to cut the bar to the proper length. With one end of the bar clamped in the rotating spindle, a shallow hole was drilled on the other end in line with the axis of the bar. A center was then moved into the hole so that the bar was supported on both ends. A round nose bit (used for lighter turning and finer finishes) was used for the turning operation. The bit was run back and forth along the length of the rotating bar, while progressively moving inward in small increments. This was continued until the desired diameter was reached. Figure 12 shows the specimen in the lathe during the turning operation. The final surface finish on the gage section was obtained by use of a fine emery cloth.

Figure 11. Engineering Drawing of Tensile Bar.

Figure 12. Turning operation on tensile bar in lathe.

Tensile testing - The final laboratory exercise was devoted to tensile testing of the machined tensile bars [3]. With this exercise, a full circle was made of the materials science course, by returning to the topic of the beginning lecture of engineering properties. For this exercise, an evening was setup for all students to come to the laboratory and use an Instron model 5689 electrical-mechanical load frame. Students recorded measurements and predicted failure load based on their hardness measurements. The competitive nature came through for most students as they focused on accurate predications of failure strength. With the entire class in attendance, they could witness different strength values for the different alloys and tempers. An additional reinforcement

Cast aluminum bar in lathe


is made between physical and mechanical properties as the students witness the same elastic modulus for all aluminum alloys, but with very different strength and elongation values.

Table IV. Turning Operation on Lathe.

1) install appropriate tool for turning operation 2) chuck cut end of bar in lathe

a. separation cut to length b. load center drill into tailstock c. center drill end (Drilling Operation)

3) replace turning tool a. turn OD final dimension

4) define the axial dimensions a. mark radius location b. change tool c. contour gauge section to 0.25-in. diameter with radius d. final process for required surface finish

Three tensile bars are tested by each group to obtain an average value for modulus, 0.2% offset yield, and ultimate tensile strength. Each tensile bar is clamped by wedge grips on the load fame, shown in Figure 13, and the test run under constant crosshead velocity. An extensometer was used to accurately measure the amount of elongation. A computer was used to record the force versus displacement data. From this data the engineering stress-strain values were calculated and summarized in Table III for the AA 319-T6 cast bars.

Course Assessment

Through the introduction of this semester long project, the instructor evaluation rating in this course improved from a 3.0 to a 4+ (scale of 1 to 4.5). On the evaluation, the following comments were made by the students with regards to the question “What did you like the most about this course?”

“This is one of the more fun engineering courses I have had, and, at the risk of being presumptuous, it seems like we as students have a degree of ownership over the projects. When all is said and done, it is satisfying to look back and think – I helped do this.”

“The labwork and hands-on experiments were a good way to learn the material being covered in class.”

“I liked relating theoretical properties of metals to practical applications.”

“I like the way she gets the class involved.”

“This is cool stuff! I want to be a metallurgist when I graduate.”


(a) (b)

Figure 13. Instron load frame with extensometer (a), and broken tensile bars (b).

Table III. AA 319-T6 Property Data

Elastic

Modulus Yield

Strength

Ultimate Tensile

Strength %Elongation

to failure

Hardness

Test Results 9,995 ksi 30.6 ksi 39.2 ksi 2.30% 72 RB

Published Values [6] 10,700 ksi 26.8 ksi 34.0 ksi 2% 56 RB

Conclusion

Incorporation of a semester long group project entitled “Life of a Casting” has proven to be a great motivator for students to understand the importance of materials science in the selection and manufacturing of engineered components. In contrast to an additional 1 credit hour of a standard laboratory, the student project can be incorporated into a 3 credit hour course. All aspects of the standard Introduction to Materials Science laboratory exercises are covered, but by following one cast aluminum alloy through its processing and testing, the exercises provide a continuity reflecting actual engineering processes. The use of standard reference materials to guide their projects


exposes student to the typical types of tradeoffs engineers must make in the manufacturing processes coupled with lectures on the underlying theory to help guide the decision. On this basis, students can more readily transfer their experience to processing of other material classifications with an appreciation of possible concerns and questions to ask. It has been interesting to note the biggest evidence of student pride, by the presence of broken tensile bars showing up on students’ key chains and as zipper pulls on backpacks.

Acknowledgments

All material presented was taken from the final reports of the undergraduate students in enrolled in ME 3403 at MSU. A special thanks to Mr. Victor Latham for his patience in allowing my students access to the machine shop to machine their tensile bars.

REFERENCES

[1] ASM Materials Handbook, Vol. 3, Alloy Phase Diagram, Publication 44073-0002, H. Baker, ed., Materials Park, OH. ASM International, 1992.

[2] ASM Materials Handbook, Vol. 4, Heat Treatment, J.R. Davis, ed., Publication 90-115, Materials Park, OH. ASM International, 1991.

[3] Smith, William F., Foundations of Material Science and Engineering, McGraw Hill Publishers, New York, 2004.

[4] ASM Materials Handbook, Vol. 9, Metallography and Microstructures, G.F. VanderWoort, ed., Publication 204-7586. Metals Park, OH. ASM International, 2004.

[5] ASM Materials Handbook, Vol. 2, Properties and Selection-nonferrous alloys and Special Purpose Materials, J.R. Davis, ed., Publication 90-115.Materials Park, OH. ASM International, 1990.

[6] Matweb: Material Property Data, http://www.matweb.com

Judy Schneider

Judy Schneider is an Associate Professor in the Mechanical Engineering Department at Mississippi State University. Her primary area of research is the engineering of the microstructure by control of the processing parameters to obtain the desired mechanical performance of structural materials. Much of her research centers on characterization of the microstructual evolution during either the processing or service life of the material. Dr. Schneider received her B.S. degree in Mechanical Engineering from the University of Nebraska – Lincoln, and her M.S. and Ph.D. degrees in Materials Science Engineering from the University of California – Davis.

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