MATS23502: Deformation Mechanisms: Mechanical Testing Lab

The deformation mechanisms of “light” alloys

Location: MECD teaching laboratories (meet at Core 3 meeting point, level 3)

Staff Responsible for Lab: Prof. João Fonseca, Mr Stuart Morse, Mr Ken Gyves

Demonstrators:

Aims and objectives

The aim of this lab is to explore the strengthening mechanisms of “light” alloys. It will involve bend testing 2 aluminium alloys with different thermal histories, and comparing the deformed microstructures of an aluminium and a magnesium alloy. The objectives are:

-    to test a series of samples in bending with different heat treatments

-    to analyse the bend test data to extract material properties

-    to observe the changes in the microstructure caused by deformation

-    to explain the experimental behaviour in terms of the deformation  and strengthening mechanisms operating in the alloys

Learning outcomes

Knowledge and understanding

-    Review and reinforce the mechanics of the bending of beams

-    Understand the effects of thermal history on the strength of aluminium alloys

-    To understand the link between the deformation behaviour and the associated changes to the microstructure

-    Learn about alloy designations

Intellectual skills

-    Develop improved logical reasoning, critical thinking, problem solving and ability in applied mathematics.

Practical Skills

-    Carrying out bend testing using a universal testing machine

-    Obtain data produced by the mechanical testing machine and use  Python to analyse it

-    Use optical microscopy to characterise deformed microstructures

Transferable skills

- General experimental skills including sample handling, machine operation and note taking

- Independent learning and literature research skills

- Working effectively in a group to carry out experiments

- Using computing to analyse experimental data

- Analysing results critically and reporting them in an appropriate and effective manner

Introduction

In this lab, you will use bend testing to explore the deformation and strengthening mechanism of 2 aluminium alloys: Al5754 and Al6111. The are both wrought alloys, which means they are deformed during production to produce the final product.

The product you will be testing is automotive sheet, which is used to   make lightweight cars. Car manufacturers like Ford and Tesla in the USA, Jaguar Land Rover  in  the  UK  (see  Fig.  1),  and  Audi  in  Germany  all  exploit  the  high  specific properties of these alloys to reduce the weight of their vehicles, improving efficiency and reducing emissions.  By replacing steel with aluminium in the new Range Rover and Range Rover Sport, Jaguar Land Rover saved 400 kg in weight!

Fig.1: Aluminium bodies in the Jaguar Land Rover assembly line.

“Light” alloys get their name because of their high specific properties, i.e. the ratio of the property (Young’s modulus, or yield strength) and their density. The density of Al is 2.7 Mg m-3 , whereas it is 8.0 Mg m-3  for steel. Light alloys like Mg and Al also have low  melting  points,  which  makes  recycling  them  much  more  energy  efficient. However, steel is very strong, has very high stiffness and is easy to form, all of which make it very competitive with light alloys despite its higher density. It is also usually half the price.

In order to compete with steel, light alloys have to use every trick in the book to increase their strength. In this lab we will explore these strengthening mechanisms using a combination of mechanical testing and optical microscopy. We will use the bend test as it is easier to perform. than tensile testing, and it is a better mimic of the forming operations during car making.

Experiencing  the  actual  testing  and  microscopy  of  materials  is  one  of  the  main outcomes of this lab. Therefore, you must try to run the machines and microscopes yourself for at least one sample.

You  should  use  last  semester’s  notes  on  plastic  bending  of  beams  to  help  you analyse your data. Some of this is also covered in the Jupyter notebook provided in Blackboard.

General instructions and safety considerations

You should start by assigning a role to each member of your team. A list of possible roles is:

1.  Measuring samples

2.  Transferring and loading samples

3.  Testing machine operator

4.  Note taker  (to  record  sample  dimensions  and  any  observations  during the tests)

5.  Sample labeller and test tracker (using the test matrix provided)

The demonstrators will show you how to load and test the first samples and will assist with the straightening operations. Otherwise, you should run the tests yourselves, as a team, with the advice and support of the demonstrator.

Please be careful. The sand is at the temperature of a hot oven at home. Use the tongues provided to handle hot samples.

Equipment

-    Instron machine

-    Callipers

-    Hot plate with a sand “bath”

-    Thermocouple and reader

-    Tongues

-    Gloves

-    Permanent marker

Materials

- Al 6111 annealed at 550˚C for 1 hour and water quenched

- Al 5754 annealed at 550˚C for 1 hour and water quenched

This heat treatment is called a “solution heat treatment”. It’s carried out just below the melting point of the alloy to dissolve all second phases and make sure all alloying elements are in solid solution.

Experiments: Mechanical testing

-Part 1: Bending of Al alloys

In this experiment, you will measure the load and displacement during the bending of 2 different aluminium alloys and then during its reversal.

You will need:

-    2 samples of Al 5754 as provided

-    2 samples of Al 6111 as provided

Before you testing, measure  your  samples  and  label  them using  the  marker provided.  Before each of the tests, you should always reset the displacement value on the testing machine. However, you should never only reset the load value at the start of the test. Resetting it during testing could invalidate your results.

-    Insert two  undeformed samples  (one of each alloy) in the sand and leave them there for 30 minutes.

-    Use  the  mechanical  testing  machine  to  test  2  samples  of  each  alloy     (4 samples in total) in bending. Remember to add a sample name to the test in the computer. Deform all samples to a maximum deflection of about 5mm.  At the end of each test, remember to save your data.

If you finish your testing before the 30 minutes are up, use this time to complete your notes and discuss the results so far.

-    What are the dimensions of the bending jig (measure carefully)?

-    How different are the results from the repeat tests in step 1?

-    What might cause these differences?

This critical analysis of your results will be important during your write up.

- Part 2: The effects of annealing on the behaviour of Al alloys

In this experiment you will explore the effect of deformation and annealing on the properties of the two aluminium alloys. You will use the samples that have been annealing in the sand bath.

You will need:

- Two undeformed samples (one of each Al alloy), heat-treated in the sand

Remember to name the samples in the testing software before each test. Reset the displacement NOT the load.

- take the 2 undeformed samples from the sand (one for each alloy) and cool them  down  to   room  temperature.  Test  them   in   bending  to   a  maximum deflection of 5 mm.

Don't forget to save your data!

Experiments: Optical microscopy of deformed samples

In this experiment, you will use an optical microscope to examine cross sections of deformed samples. The demonstrators have prepared metallographic specimens for you. Please be mindful and do not to touch the polished surface as that will damage it and make it hard to image the microstructure. There should be 2 samples:

-    Mg alloy (AZ31) deformed in bending

-    Al alloy tested in bending

Use the microscopes provided to record images of the deformed microstructures of these samples.

-    Step 1: calibrate your microscope at the different magnifications

-    Step 2: study the microstructure of the different samples noting how it changes with distance from the neutral axis

-    Step  3:  Save  images  to  the  computer  provided  for  using  in the  report to illustrate your findings

Data analysis and visualization

You can analyse and plot the data using Python. A example notebook is provided to help  you  with  this  task.  Remember  to  label  all  axes,  including  units  where appropriate.

Assessment and feedback:

You should report your findings using the report template provided on Blackboard in PDF  format.  The  deadline  for  submission  of  your  lab  reports  is available on Blackboard. You should start working on your report as soon as possible, so you have time to analyse the data, do the required reading and ask questions. The usual late penalties apply.

Important: You must attend the lab. If you don’t attend the lab*, you can write up the report but your mark will be deducted 50% of the marks.

The assessment criteria are as follows:

Content (75%)

Each section of the report will be assessed as noted on the template. The template contains several suggestions for what to include and discuss in each section. Note that these are just suggestions; you should write the best report you can. Excellent reports are expected to go beyond what is already outlined.

Presentation (25%)

The  presentation  score  includes  structure,  style,  clarity,  spelling,  and  quality  of figures and equations.

Feedback

You will receive feedback in the form of a marked lab report 15 working days after the submission date

* If you miss the lab due to unavoidable circumstances, like illness, you must apply for mitigation.