Analysis of Sheet Metal Forming (Warm Stamping Process): A Study of the Variable Friction Coefficient on 6111 Aluminum Alloy

Round 1

Reviewer 1 Report

Authors have studied the friction behaviour of a 6xxx aluminium alloy at warm temperatures. The alloy selection and the topic relevance is high as application of aluminium sheets in automotive applications is increasing significantly.

Authors have developed a new tribotester and have performed friction tests at different contact pressures, sliding velocities and forming temperatures.

The experimental results have been fitted to a force dependant and velocity dependant laws. Finally, these friction laws have been used in simulation to verify the results by drawing a U shape component.

Although the results are interesting, some aspects must be improved for approval:

General minor changes

• Please review all the comas after an and – some examples:

Sentence 15 – delete coma in “of parts, and”

Sentence 19 – delete coma in “microscope, and”

Sentence 21 – delete coma in “of temperature, and decreases”

Sentence 27 – delete coma in “measurement, and further”

Sentence 54 – delete coma in “the die, and the temperature”

• Sentence 57 – “forming cold die queuing” – quenching
• Sentence 79 – “fairing function arctangent function, and introduced the relative slip velocity” = fairing an arctangent function and introduced the relative slip velocity
• Sentence 16 – “widely used” – I wouldn’t say that
• Sentence 20 – “which is determined as the temperature” means optimum temperature?
• Sentence 175 – “in Figure 2(a)” is figure 3(a)
• In figure 9 use the same legend values for all the images to make the comparison easier

Compulsory major changes

• Please better explain the friction testing procedure. The heating method, how the samples are fixed during the test and how the sheet and H13 block are heated is not well explained. Modify figure 2 for this. Authors state that boundary lubrication conditions are imposed? How is this verified? Authors analyse the first 5 seconds of the test. Why? Please provide variability in all figures (for example in graph 3b)
• As observed in figure 3, the curve is not stabilized. It seems 5 mm stroke is not enough to obtain steady conditions and galling like effects are continuously occurring during measurements. Even the curve is not constant at room temperature. Please explain this effect as it is an important aspect of the study. Do you observe galling or sticking of aluminium on H13 blocks?
• Why only the surface of the sheet at different temperature conditions was analysed at a speed of 30 mm/s? were the other speeds analysed? Same applies for different pressures. Probably you could show the most relevant pictures and summarize the results with roughness measurements?
• Paragraph in sentence 194 has very small scientific background. This is possibly true for the friction behaviour but in a real process a lot of different aspects need to be analysed (formability, galling, final properties, etc.)
• Could the authors provide 3d measurements instead of 2d roughness values? Sa, Sq etc. instead of Ra for example. The use of a profilemeter to measure the morphology of the aluminium sheets after the test would be appreciated. Have the H14 blocks been analysed? This could explain some effects of the friction behaviour
• The friction law is fitted for the testing force. This is not usefull and needs to be in function of the contact pressure for any application in sheet metal forming simulation. Please change this in all the paper and the numerical analysis. Authors have the data of friction in function of temperature, pressure and sliding velocity. A more general friction law which is dependant of these three variables should be used for the numerical analysis instead of two different models which are in function of load (no sense in simulation, change it by force) and of sliding velocity. For example a modified Filzek friction model would be used for the analysis. Is the simulation isothermal? Please provide more details of the numerical model.
• Authors need to better explain the experimental test of the U-Drawing tests. How were the steel blocks heated? Which is the machine used and its kinematiks? Is the 2000 mm/s speed real? How is the load controlled?

Author Response

Please see the attachment. Thank you!

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear authors of the manuscript,

I believe you did a good job with your measurements of variable friction coefficient on 6111 Al alloy, and the manuscript is well thought out and understandable. However, I do have some questions and remarks that will hopefully help you improve your submitted manuscript.

Introduction:

While it is generally well written and gives a good introduction, it lacks specificity regarding your own measurements. In fact, you give several models to describe/explain friction coefficient and relevant parameters, but in the rest of the manuscript you do not use either one of them or compare their results. To improve the introduction, rewritting of this part with more specific data/literature/explanation would be beneficial.

Experimental:

What type of optical microscope did you use? In further work, you refer to laser scanning microscope vk-x100. Be more specific.

Results:

Figure 3: in text you say that only first 5 s of measurement are used to evaluate the average friction coefficient. Can you improve the figure by marking the first 5 seconds, so that this is clear also just by looking at the figure? And in figure 3b, the determined friction coefficient lacks estimated error bars (and due to the variability of mu in the first 5 seconds, also an explanation of how you estimated error bars would be useful)

Figure 4: scalebars are poorly visible. Can you also indicate with an arrow the direction of wear test? Additional image of the “new” sample (before any test was performed) would also be quite useful to compare.

Figure 5: typesetting errors in fig5b for values of a b and c.

Regarding function, validation and fitting: I dont really understand why did you choose to present your work this way. In fact, I believe it would be much better (and scientifically sounder) if you plotted all measurements on a curve and fit all measurements. This way the fit would be of 10 datapoints with 3 parameters, so better to see if the model chosen really fits. Also, which of the models in the intoduction would be comparable to this one?

Figure 6: the figures should be of same size (not necesarily magnification, but at least inage size) as the other similar images.

Eq. 8 and Fig7: similar remark as above. Why did you prefer “validation” as opposed to actually using the datapoints in the curve fitting? And which of the models in intro is relevant here? Also, validation points seem to fall way out of the plotted curve. Also, missing error bars.

Table 5: what is the meaning of error rate? 

Conclusions:

realtively long, but a good summary of the results.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

in the paper te authors try to identify the best conditions to form in the work we try to identify the best conditions to form 6111 aluminum plates. The work starts from a well-established scientific basis and uses an appropriate methodology. The results are well presented and correct. I thinck the paper can be published in the present form.

page 5 row 150 Fig. 3 a not Fig. 2 a

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Some aspects have been optimized by the authors.

However, for its publication a deeper chnage is needed:

1) The friction test need to be improved to have a higher sliding distance/stroke

2) Lubrication application procedure needs to be explained and lubrication ammount needs to be accuratelly controlled

3) 3D measurements of blocks and sheet is desirable prior and after the test to understand the friction mechanisms

4) Friction tests need to be cpompleted for various temperatures and pressures

5) Real stamping needs to be better documented (heating time, tranfer time, tool temperatures, blank holder presure measurement, punch force measurement, etc.)

6) Numerical simulation nedds to be considerably improved (non isothermal, friction law, press kinematiks, boundary conditions explained)

Please consider these aspects to improve the paper and submit a better study.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear authors, 

I think the manuscript is now better and can be considered for publication. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

Although the experimental testing can be completeed for different combinations of temperature, pressure and sliding velocity the authors included critical aspects that guarantee the scientific quality of the paper.