Residual Stresses Control in Additive Manufacturing

Round 1

Reviewer 1 Report

The manuscript proposes a simulation approach to estimate the residual stresses in metal additive manufacturing. The approach is based on a three-bar model and combines with an experimental calibration. To uncover the key mechanisms controlling the residual stresses, the proposed approach is implemented to study the effect of the scan pattern, the influence of preheating, deposit height and energy density, and the effect of the substrate stiffness and the part geometry.

 

Overall speaking, the topic of the study is interesting and the conclusions may provide some guidance for metal additive manufacturing. The reviewer can recommend the manuscript for publication. However, the following comments should be considered before.

 

1. More information on the simulation should be provided, for example, the temperature boundary condition. How is the three-bar model implemented in the simulation processes?

 

2. In Fig. 1, what is 40-layer rectangular block? Can the authors give more explanation on the FE model? Especially, how are the different materials combined with each other.

 

3. The curves in Fig. 2 possess considerable fluctuations. For the experimental results, the fluctuations can be expected. But the reviewer cannot understand the fluctuations for the simulation. Can the author explain it?

 

4. It is necessary to explain how to achieve the different scan patterns (Fig. 4a) in the simulation. The maps shown in Fig. 4b-e are the final state? It is better to provide directly the values of the mechanical and thermal properties, since Fig. 4 shows the simulation results with absolute values.

 

5. The simulation adopts the elastic-perfectly plastic model for the materials. But in fact, the mechanical behaviors of the materials used in additive manufacturing are more complex. Can the authors give some comments on it. For example, how does the plastic harden has effects on the residual stresses?

 

6. It is better to give some comments on the potential comparison between simulation and experiments. Are there some methods that can verify the proposed simulation approach, as well as the conclusions.

Author Response

Response:

The authors thank the reviewer for his/her positive appraisal of our work.

1. All information on the simulation has been added in Section 3.

In this work, the three-bar model is used as a conceptual aid to understanding the formation of residual stresses and the related key factors in AM, but it is not implemented in the FE software used to predict the mechanical response of the built. The thermomechanical model explained in Section 3.1 is used to this end.

2. The figure has been changed for clarity and more explanations on the FE model have been added in Section 3.

In the FE modelling of AM, the birth-and-death element technique is used to activate the elements belonging to the new deposit layer according to the depositing path. The elements are classified into: activated, active and inactive elements. At each time step, the elements corresponding to the melting pool (active) and to the deposit layer (activated) are sought and included in the current computational domain, but the inactive elements are neither assembled nor computed.

3. Similarly to the experimental results, the fluctuations in the simulated plots are due to the use of the alternating longitudinal-transverse scan path in the DED process (see Figure 1b). Once again, the simulated curves agree well with the measured results.

4. The scan paths are defined in an input file (CLI format), which can be parsed by the software used to simulate the AM process. This is explained in Section 3.2.

Fig. 5b-c shows the temperature and von Mises stresses when depositing the 2^nd layer, while the maps shown in Fig. 5d-g are the final state. This has been clarified in the description of the figure.

Figure 5 provides the values of the thermal and mechanical results. In detail, the values in Figure 5b are the simulated temperatures, the values in Figure 5c,d,f represent the predicted von Mises stresses, the values in Figure 5e and 5g stand for the final distortion (displacement norm) and the vertical displacement, respectively.

5. In order to accurately capture the mechanical response during the printing process, a J2-thermo-elasto-visco-plastic model including the strain-hardening was used in this work. The description of this model has been added in Section 3.

For Ti-6Al-4V, plastic hardening barely affects the residual stresses.

6. The remarkable agreement between the simulated and experimental results illustrates that the FE software is a powerful tool to predict the AM process. Hence, the software can be employed to study the effect of different AM variables in order to optimize the fabrication process. More comments regarding this have been added in Section 3.3 and the Conclusions.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript reports interesting results regarding residual stress in additive manufactured (AMed) parts. The residual stress is a problem with AMed components and until the present it wasn’t solved. As it was presented in the manuscript, a few methods are currently applied to reduce (not eliminate) the residual stresses. The state of the art on residual stress in AM field is mainly focused on numerical simulations and very little on actual experimental analysis.

Even if it is an interesting manuscript, the authors should clarify or revise a few aspects, as follows:

1. The acronyms should be defined on their first appearance (e.g. DED appears in the Introduction section – line 43 but is explained in Experimental setup section – line 77);
2. The AM equipment used should be mentioned in the Experimental setup section;
3. At least an image of the experimental set-up used to realize the in situ measurements for model calibration should be provided;
4. All symbols from Figure 1 should be defined on the picture or in the figure’s description;
5. Figures 4 and 6 descriptions should be written using the same font;
6. In the 3.1 section the authors mention that the Base-bar temperature was set at 0°C, isn’t it to low? Please explain.
7. In the same section the authors mention the maximum heating temperature, Tmax, what value was considered for Tmax?
8. Please explain the reason for selecting the 500°C for the preheating temperature of the building plate from section 3.3. Moreover, it should be mentioned in the section 2 if the building plate was heated or not for the calibration set-up;
9. Please explain how can be experimentally applied the cases described in section 3.4 and Figure 6.

Author Response

Response:

The authors thank the reviewer for his/her positive appraisal of our work.

1. The modification has been done.

2. More experimental details have been added in Section 2.

3. The figure of the in-situ measurement used in this work has been added in Section 2.

4. More explanations about the symbols have been included in the figure’s description.

5. The modification has been done.

6. As thermal straining depends on incremental temperature, the Base-bar temperature is set at 0°C for simplicity. This is used only in Section 4.1 as a conceptual aid. In the real FE simulations, the actual baseplate temperature plays the role of the Base-bar temperature.

7. In AM, T_max is up to 2000-4000°C, corresponding to the peak temperature of the melt pool.

8. Usually, a temperature of 500°C is regarded as the lowest temperature used in stress annealing.

The baseplate is not preheated in the calibration setup and this has been clarified in Section 2.

9. The steps are as follows:

(a) determining the deposited zone on the substrate surface and designing the cut setting for several grooves;

(b) machining the grooves by wire-cut electrical discharge machining (WEDM);

(c) performing the building process on the optimized substrate, as shown in Figure 7.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I would like to thank the authors for their answers and for improving the manuscript.