## 2 Methodology and Bibliometrics
### Bibliometric Analysis  
The collected papers in this article amount to 202 which fall within the time frame of 2015-2022. The year-wise distribution of the papers included in this review is indicated in Figure 4. As can be gleaned from the figure, the research output on the present theme has remained virtually constant pointing to the continued interest in the topic. Nonetheless, there has been an evolution in the themes that have been taken up in research which are increasingly gravitating towards automation with simulation techniques such as Augmented Reality (AR), Virtual Reality (VR), Digital Twins (DT), and hybrid simulations [22]. Figure 4: Year-wise distribution of covered research topics. Apart from the papers spanning the time frame 2015–2021, which form the mainstay of the research, some of the papers which provide supportive evidence to the research results have not been cataloged above. Figure 3: The documents as returned by the SCOPUS database with the above-described parameters and search strings. The number of studies shows a consistent increase as illustrated by the trend line. The research results collected for this review spans the globe and have been published in various journals. The topical relationship between the countries, keywords, and the publishing journals is depicted as three-field plot in Figure 5. The three-field plot in large part agrees with the country citation index such that India, USA, Italy, and Germany have 30, 31, 16, and 14 articles, respectively. Besides this, the sources which have been published most on the present topic include the journal of sustainability (Switzerland), Procedia CIRP, and Journal of cleaner production with 15, 12, and 14 articles respectively included in this review. The keywords that mediate between the two in the plot are typical of the research theme and include Supply chain 4.0, energy efficiency, and simulation, etc. The abstracts of the collected papers were analyzed, using the Bibliometrix application, for the recurrent themes and words. Excluding the trivial words and focusing on two-worded and three-worded themes, a thematic tree map was generated which is shown in Figure 6. The theme of energy simulation and modeling dominates as it is also reflected in the amount of research dedicated to this topic. A total of 40 papers in the collected research area directly or indirectly deal with this topic. Some of the most-cited of those papers include themes such as simulating energy conservation in the milling process [23; 24] and selective laser melting [25], etc. Besides this, lifecycle assessment shows a frequent presence in the abstracts. Some of the research includes topics such as social LCA [13], system dynamics and LCA [26], lifecycle cost assessment, and LCA [27], energy assessment and LCA in the Industry 4.0 [28], sustainable machining process [29], and lifecycle inventory gate-to-gate process energy use [30], among others. An alternate theme of the product lifecycle is also present in the abstracts which is similar to that of the LCA. A contemporaneously important topic of additive manufacturing also finds use in LCA [31; 32; 33]. The collected literature was subjected to the Bibliometrix online tool wherein, using the co-occurrence analysis on the 492 keywords, a keyword network was generated. The keyword network, showing the outline of published research, is depicted in Figure 7. The four clusters are named according to characteristic keywords. They are the lifecycle assessment cluster, decision support system, manufacturing methods, and numerical model. The cluster of product lifecycle includes themes such as environmental impact, benchmark, energy efficiency, and costs [10; 34; 35; 36; 37]. The cluster of decision support system includes  
Figure 5: The three-field plot or the Sankey diagram showing the relationship between the publishing countries, author keywords, and the journals. The figure shows the most significant studies in the present context which appear in peer-reviewed journals.