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WAAM process optimization: Temperature control

Interpass/interlayer temperature control in Wire Arc Additive Manufacturing

This article discusses the role of heat input and interpass/interlayer temperature in the Wire and Arc Additive Manufacturing (WAAM) process. It further discusses the importance of controlling heat to produce quality defect-free parts and the consequences of not maintaining optimum thermal conditions during the production process. We finally introduce the temperature control module within MaxQ and how it can help manufacturers reliably automate temperature control during WAAM.

The role of temperature in materials science

A fundamental part of materials science is to understand how the microstructure of materials behave at different thermal conditions. For most alloys, thermal conditions have a direct impact on their mechanical properties and susceptibility to failure. To illustrate an example, if a low-carbon steel is allowed to naturally cool from 900°C to room temperature, it forms a low-strength high-toughness phase called ferrite; whereas quenching (rapid cooling) the hot steel forms a high-strength low-toughness phase called martensite [1]. This example illustrates another key facet of material behavior – the microstructure and properties of a material are not affected by just the temperature alone, but also by the heating/cooling rates it is subjected to. Thus, the thermal conditions have to be taken into account when designing manufacturing processes to yield products with desired properties and optimum performance. 

Heat input and interpass temperature

The Gas Metal Arc Welding (GMAW) based WAAM process commonly utilizes a parameter called ‘heat input’ that defines its thermal condition, which has the unit J/mm in most cases. The heat input is the amount of arc energy supplied by the welding power source to deposit a weld, and is a function of the process parameters used in welding – current, voltage and travel speed and process efficiency, which depends on the type of welding used. 

The wire feeding rate, which is a measure of the material usage, should match the heat input to reach a state of thermal and mass balance. This leads to a stable process where welds with desirable properties can be deposited. To maintain this process during cyclic heating and cooling, another important parameter called the interpass/interlayer temperature is defined, which is the temperature of the area between weld layers in a multi-layer weld [2].

These parameters influence the height, width and penetration depth of a weld. It affects the final geometry of the build part as they have an influence on the thermal distortion, thermomechanical stresses, resulting microstructure and susceptibility to form oxides and silicates on the surface and impurities within the deposited material.

Consequences of not maintaining interpass temperature during WAAM

Compromised Heat affected Zone

The heat affected zone (HAZ) is a region adjoining the unmelted area near the fusion zone of a weld. This region undergoes changes in material properties as a result of being exposed to high temperatures, as an example shown in Figure 1 between regions D and E. Maintaining a maximum interpass temperature helps prevent the formation of undesirable carbides and other intermetallics that affect the mechanical and corrosion properties of the welded component. Maintaining the interpass temperature also prevents grain growth that causes reduction in strength and grain boundary liquation, which propagates crack formation [3].

Overheating and surface anomalies

The surface finish is an important attribute to control in the manufacturing of metal parts. A non-homogenous surface finish can result in insufficient or excessive dilution, which could jeopardize the integrity of the deposited material. The interpass temperature needs to be controlled to avoid surface anomalies to form due to overheating. 

Figure 2 shows an example when metal is deposited at a high heat input without controlling the interpass temperature. There is a significant variation in surface topography that can have a multiplying effect when more layers are deposited. Although the heat source used in this study was a laser, the same effect is also observed for the WAAM process.

Figure 3 shows an example where proper interpass temperature was maintained, where a comparatively uniform surface finish can be achieved using a GMAW based process, the results of which are also comparable with the material deposited using a laser. This was possible due to low heat input techniques developed for GMAW, such as CMT from Fronius and SAWP from Panasonic, in which the energy delivered to the material is closely controlled. In this study, the SAWP welder mode was used to deposit Stellite 6 on various steel substrates.

Figure 1: HAZ in welding of 316L steel [source: RAMLAB] 

Figure 2: Continuous deposition of layer without temperature control [7]

Figure 3: Oxide and silicate impurities on deposited layer [8,source: RAMLAB]

Inhomogeneous thermomechanical response

Interpass/interlayer temperature is a form of preheating during multipass welding. For some materials, a minimum interpass temperature is defined to reduce the risk of cold weld cracking. As materials experience different thermal cycles during WAAM, stresses develop within the fresh deposits and its previous deposited material (weldments as progressively changed substrate). Compressive stresses are introduced during the heating cycle and tensile stresses are introduced during the cooling cycle in the substrate. This results in retained strain and residual stresses locked inside the material. The residual stresses in WAAM are mainly caused by sharp thermal gradients, thermal mismatch and phase transformations in the microstructures. Residual stresses can be categorized as macro and micro stresses. Macro stress acts over a range of a few grains to large distances in a material, while micro stresses acts within a grain over atomic scale distance. Understanding these stresses is necessary to prevent cracking initiation from phase transformation in microstructures [9].

Cracks initiate once the thermal stresses, shrinkage stresses or residual stress exceed the Ultimate Tensile Strength (UTS) of the material locally, which can lead to failure and eventually the rejection of a part.
In addition to a minimum interpass temperature, a maximum interpass temperature is specified for certain materials such as austenitic stainless steel or Nickel alloys to ensure that satisfactory material properties can be obtained. In such cases, the weld layer must be below the maximum interpass temperature and above the minimum interpass temperature before welding continues. The minimum temperature in most of the cases is the same as the preheat temperature.

For simple welds or geometry, this can be handled by an experienced welder. However, there are challenges to maintain quality without human errors for larger parts with complex geometry, or interpass temperature above 400 °C. These challenges are more common when manufacturing larger critical industrial components using WAAM. It is therefore important to ensure that the preheating conditions are maintained by monitoring and controlling the minimum and maximum interpass temperature. Such a monitoring and control system can help to achieve process automation using robots to manufacture quality parts through improved work efficiency with lower costs.

Interpass/interlayer temperature can be controlled using MaxQ

The previous sections discuss the importance of maintaining the interpass temperature during additive manufacturing. Depending on the material thermal properties, size of the part and the desired mechanical properties, an optimum interpass temperature can be defined. The homogeneity of the part can be maintained through a production process with monitoring and control of key process variables. 

RAMLAB’s MaxQ monitoring and control system offers the capability of not only maintaining interpass conditions but also offers other features that enable the WAAM printing jobs to be done in an automated production environment. Temperature control as one of the features in the MaxQ system, which is currently equipped with an Optris XI 80 (Figure 5) infrared thermal imaging camera. It is integrated with the welding system and ensures that the component reaches the set temperature boundaries during GMAW based Wire Arc Additive Manufacturing.

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References

[1] Wang, H., Cao, L., Li, Y. et al. Effect of cooling rate on the microstructure and mechanical properties of a low-carbon low-alloyed steel. J Mater Sci 56, 11098–11113 (2021). https://doi.org/10.1007/s10853-021-05974-3

[2] Peng, Y., Wang, A. H., Xiao, H. J., & Tian, Z. L. (2012). Effect of Interpass Temperature on Microstructure and Mechanical Properties of Weld Metal of 690 MPa HSLA Steel. Materials Science Forum, 706–709, 2246–2252. https://doi.org/10.4028/www.scientific.net/msf.706-709.2246

[3]  Gáspár, M. (2019). Effect of Welding Heat Input on Simulated HAZ Areas in S960QL High Strength Steel. Metals, 9(11), 1226. https://doi.org/10.3390/met9111226

[4] de Gouveia, R. R., Pukasiewicz, A. G. M., Capra, A. R., Henke, S. L., & Okimoto, P. C. (2014). Effect of interpass temperature on microstructure, impact toughness and fatigue crack propagation in joints welded using the GTAW process on steel ASTM A743-CA6NM. Welding International, 29(6), 433–440. https://doi.org/10.1080/09507116.2014.932983

[5] Mandal, N., & Sundar, C.V. (1997). Analysis of welding shrinkage. Welding Journal, 76.

[6] The Welding Institute global website. Retrieved on August 4th, 2021, from URL: https://www.twi-global.com/technical-knowledge/faqs/faq-why-is-preheat-used-when-arc-welding-steel-and-how-is-it-applied

[7] Ya, W. (2015). Laser materials interactions during cladding: analyses on clad formation, thermal cycles, residual stress and defects.

[8] Lin, Z., Ya, W., Subramanian, V. V., Goulas, C., di Castri, B., Hermans, M. J., & Pathiraj, B. (2020). Deposition of Stellite 6 alloy on steel substrates using wire and arc additive manufacturing. The International Journal of Advanced Manufacturing Technology, 111(1), 411-426.

[9] Ya, W., & Pathiraj, B. (2018). Residual stresses in Stellite 6 layers cladded on AISI 420 steel plates with a Nd: YAG laser. Journal of laser applications, 30(3), 032007.