Contact Temperature Measurements on Hybrid Aluminum–Steel Workpieces in a Cross-Wedge Rolling Process

Round 1

Reviewer 1 Report

Deformation heat treatment requires very precise temperature control. Especially if a heterophase material is subjected to it with phase transformations and the formation of funnel from aluminum and steel, with sharply differs of its recrystallization temperatures. Before deformation, rolling billets are fabricated by rotary friction welding. The temperature of individual areas of the deformed product was measured by precision sensors during the cross-wedge rolled (CWR) process. The need for precision temperature measurement is scientifically substantiated in the literature review. The investigations contained in the article were previously partially published on the project website (SFB 1153 - Sub-project B1 - Querkeilwalzen), funded by the Deutsche Forschungsgesellschaft (German research association) 07/01/2015 - 06/30/2027 and reported at the conference.

1. However, in no these cases not was shown the advantage of using thin film sensors compared to a traditional thermocouple pressed into a workpiece. A big request to the authors to indicate in more detail the advantage of a thin-film sensor compared to a thermocouple. This must be done, especially since a thin-film sensor is subject to destruction under deformation of complex geometry products.

2. It is necessary to specify the geometrical dimensions of the sensor, so that we can understand from what area we receive the average temperature data.

3. The phrase "high latency of the thermocouples cause only small spikes in the measurement curves" is not true. This remark should be removed or proved. It all depends on the sensitivity of the recording equipment.

4. Please recheck line 190 for an error. (55mm or 5.5mm). "The sensor contacts the workpiece at a Z-value of about 55 mm".

5. Please show the dimensions of the aluminum-steel billet and funnel in pictures 1 and 2.

6. On line 123 “direct electrothermal process”, the word “direct” must be deleted, since “direct electrothermal process” is contact electric heating.

 

7. It is necessary to formulate a Conclusions and indicate the advantages of a thin-film temperature sensor compared to a conventional thermocouple and measurements using an IR thermal imaging camera.

Author Response

Dear reviewer, 

thank you for your comments. I will address your concerns in the following paragraphs:

1. / 3. a thermocouple can't be in constant contact with the workpiece during cross-wedge rolling due to the rotation of the workpiece and due to the high degree of forming. Thermalcouples are already used as in Figure 2 to measure during the cross wedge rolling process. Only small temperature increases of 60 °C are measured by the thermocouple. Increasing the measuring frequency does not increase the maximum measured values. Due to the short contact time of 0.041 s between the workpiece and the sensor, only thin-film sensors are able to measure the temperature in direct contact. I have adapted the description of the tool concept to make this understandable.
2. The measurements of the sensor was added.
3. see above under 1. 
4. The workpiece has a length of 130 mm in the Z direction and was thermally measured along this axis using a thermographic measurement. Thin film sensor No. 1 contacts the workpiece at the beginning of the process before the workpiece is formed. The point at which the thin-film sensor contacts the workpiece can be described by Z = 55 mm. The thermographic measurement can therefore be compared to the result of the thin film sensor. I hope that this explanation provides clarity. The article has been adapted.
5. The measurements were added. 
6. The word direct has been deleted. 
7. The conclusion has been adapeted according to your comments. 

Reviewer 2 Report

The manuscript by P. Merkel et al. presents development and testing of temperature sensors for in situ temperature control during cross-wedge rolling. This study will be interesting for the readers of Journal of Manufacturing and Materials Processing. I have only minor remarks:

1) CWR is cross-wedge rolling, please, mention at the end of introduction.

2) Explain FEA on page 2.

3) Lines 116 and 119 on page 3, “were” should be “where”.

Author Response

Dear reviewer, 

thank you for your comments. The document has been adapted accordingly. 

Reviewer 3 Report

The paper entitled "Contact Temperature Measurements on Hybrid Aluminum-Steel Workpieces in a Cross-Wedge Rolling Process" shows the use of a different type of temperature sensor for controlling the Cross-Wedge Rolling process. The results are interesting, but some adjustments should be made in the article to improve the readers' understanding. Suggestions for improvement are in the attached document. I hope I have contributed to improve the article.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer, 

thank you for your comments. I will address some of them in the following paragraphs: 

1. The abstract has been adapted according to your suggestions.
2. The citation was done according to the guidelines of the IPH - Institut für Integrierte Produktion Hannover. For the moment, I will leave it as it is, since the editor will still read and correct the article. I am open to changes in this regard and will let the journal decide on that. 
3. Chapter 1.1 was rewritten to include more details. I hope you find it to be more informative.
4. The notes to the table have been checked against the original source and modified accordingly. The position of the table legend is done by Latex automatically through the MDPI template. 
5. Regarding the measurement accuracy that would be required: This is an interesting question, that is hard to answer. A accuracy of at least 3% is desirable to control the process. Both presented method (pyrometry, thermocouples) don't deliver any information at all that could be extracted due to the described conditions during cross-wedge rolling. The article has been adapted to include more information. 
6. Regarding fig. 2: I am very sorry, that it wasn't clear from the description in the article. The thin film sensors can be modularly added to the tool not the workpiece. I have adapted this in the description. 
7. I have also adapted the Article to include a chapter "Materials and Methods" and one "Results and Discussion".
8. The conclusion has been adapted to include more information according to your comment. 
9. Regarding the last sentence in "Further research": The minimization of the growth has been done due to the optimization of temperature profiles, but we still don't completly understand the mechanism of growth during CWR since this is a unique research field. We have four years of research ahead of us and plan to learn a lot more about the aluminum-steel combination by automating the CWR-process and systematically study the influence of different temperature profiles on the intermetallic phase. 

I have found your suggestions to be very helpful. I thank you for your time. I hope that the article now meets your requirements.

Reviewer 4 Report

Dear Authors! I reviewed your submission "Contact Temperature Measurements on Hybrid Aluminum-Steel Workpieces in a Cross-Wedge Rolling Process" and found it ready for publication. 

I have some doubts about uniformity of weld seam as a result of rotary friction welding and some ideas about stricture studies.

1. Rotary friction welding of cylindrical parts intrinsically generates different welding conditions along the radius of cross section. This even may cause residual cavity or at least different thickness of welding seam and intermetallic layer. It would be useful to discuss how fast temperature front penetrates cross section and influences the quality of ready workpieces.
   
   
2. Temperature gradients during a Cross-Wedge Rolling Process, of course, cause the gradient of structure and properties. It would be useful to discuss how to correlate measured surface temperatures with the uniformity of properties and quality.    

Author Response

Dear reviewer, 

thank you very much for your interesting comments. I will address them in the following paragraphs: 

1. We have found that rotary friction welding produces different properties in the joined part. Other authors from different subprojects of the CRC 1153 had the opportunity to further investigate the properties of the joining zone. We found out that there are different joining conditions in the joining zones and that they influence the resulting temperature profiles. This is one reason why the thin film sensor is such an interesting possibility for our process monitoring. For the first time, we can examine from the outside without destroying the workpiece whether a large cavity has formed in the joining zone. The influence of the void is very large for our process, because only the steel part is heated and the heat conduction through the rest of the material leads to the heating of the aluminum. In CWR of aluminum-steel hybrid parts, it has happened that a workpiece has fractured in the joining zone without any visual difference from the other workpieces. Often, it is assumed, these were the workpieces with the largest blowholes. In the future, we hope to develop a monitoring system that will detect these process deviations earlier to prevent breakage during CWR. 

2. For future research, I will take this comment and do metallographic studies of the part, as I find it very interesting. It should be noted that the statements about properties after cross wedge rolling are unfortunately of little value, since machining and heat treatment follow and have a large influence on the properties of the material. 

We adapted the article according to your comments. 

Round 2

Reviewer 3 Report

Dear;

The paper has been revised according to my suggestions for improvement. Thus, I believe the article is fit to be published.

 

Reviewer 4 Report

Thank you for taking into account my comments.