A Review of Muli-Material Additive Manufacturing (AM)


Heat Sheilding Application to Hypersonic and Re-entry Vehicle


Aside from the capability of Rapid manufacturing methods in fabricating components with complex geometries, two crucial potentials of this manufacturing process that are worth mentioning are its flexibility in being combined with other production methods as well as the use of a variety of materials in a single production platform to make multi-material and composite products. Implementation of multiple materials in integrated structures has been shown to improve the functionality, weight reduction, and, by merging the assembly and production into one stage, modify the manufacturing processes. Different approaches towards the modification of additive manufacturing processes aimed to reach multi-material or composite parts are being reviewed in this paper. Also, an application of suitability of usage of additively manufactured Multi-material objects in extreme environments has been discussed. A mufti-material Honeycomb-shaped structure is designed and simulated under flight conditions corresponding to the conditions when a hypersonic vehicle re-enters the earth's atmosphere. The materials are chosen based on 3D printability and references from previous studies on re-entry vehicle heat-shielding materials. Steady state thermal and structural stress analysis have been done using ANSYS simulation software. The analysis shows a reduction of 66% temperature is obtained and the sustainability of the structural configuration at high pressure. The results obtained are compelling and explain the effectiveness of multi-material additively manufactured components in extreme conditions and can be substituted in the place of ordinary heat shields.


  1. 1 Rapid Manufacturing

The term “additive manufacturing” references technologies that grow three-dimensional objects one superfine layer at a time. Each successive layer bonds to the preceding layer of melted or partially melted material. It is possible to use different substances for layering material, including metal powder, thermoplastics, ceramics, composites, glass and even edibles like chocolate.

Objects are digitally defined by computer-aided-design (CAD) software that is used to create .stl files that essentially "slice" the object into ultra-thin layers. This information guides the path of a nozzle or print head as it precisely deposits material upon the preceding layer. Or, a laser or electron beam selectively melts or partially melts in a bed of powdered material. As materials cool or are cured, they fuse together to form a three-dimensional object [2].

1.2 Other Important Topics

1.2.1 Multi-Material Additive Manufacturing

Aside from the capability of additive manufacturing (AM) methods in fabricating components with complex geometries, two crucial potentials of this manufacturing process that are worth mentioning are its flexibility in being combined with other production methods as well as use of a variety of materials in a single production platform to make multi-material and composite products. Implementation of multiple materials in integrated structures has been shown to improve the functionality, weight reduction and, by merging the assembly and production into one stage, modify the manufacturing processes. Different approaches towards modification of AM processes aimed to reach multi-material or composite parts are being reviewed in this paper.

1.2.2 Heat shielding for Hypersonic and Re-entry vehicles

Objects entering an atmosphere from space at high velocities relative to the atmosphere will cause very high levels of heating. Reentry heating comes principally from two sources:[4]


Convective heating, of two types:

Hot gas flow past the surface of the body and catalytic chemical recombination reactions between the object surface and the atmospheric gases radiative heating, from the energetic shock layer that forms in front and to the sides of the object

As velocity increases, both convective and radiative heating increase. At very high speeds, radiative heating will come to quickly dominate the convective heat fluxes, as convective heating is proportional to the velocity cubed, while radiative heating is proportional to the velocity exponentiated to the eighth power. Radiative heating—which is highly wavelength dependent—thus predominates very early in atmospheric entry while convection predominates in the later phases.[4]

1.3 Objectives of the work:

  • Designing of wafer type Honey-comb structure

  • Validating suitability of choosing Rapid Manufacturing Technology for manufacturing Multi-material based Homey comb structure

  • Steady State Numerical Simulation and analysis of heat shielding effectiveness of the manufactured structure.

  • Numerical simulation and analysis of strength of the manufactured structure.

2. Methodology and Experimental Work:

The major setps for Additive manufacturing process[2] are as follows:

  • CAD : Producing a digital model is the first step in the additive manufacturing process. The most common method for producing a digital model is computer-aided design (CAD). There are a large range of free and professional CAD programs that are compatible with additive manufacture. Reverse engineering can also be used to generate a digital model via 3D scanning. There are several design considerations that must be evaluated when designing for additive manufacturing. These generally focus on feature geometry limitations and support or escape hole requirements and vary by technology.

  • STL conversion and file manipulation: A critical stage in the additive manufacturing process that varies from traditional manufacturing methodology is the requirement to convert a CAD model into an STL (stereo lithography) file. STL uses triangles (polygons) to describe the surfaces of an object. A guide on how to convert a CAD model to an STL file can be found here. There are several model limitations that should be considered before converting a model to an STL file including physical size, water tightness and polygon count. Once a STL file has been generated the file is imported into a slicer program. This program takes the STL file and converts it into G-code. G-code is a numerical control (NC) programming language. It is used in computer-aided manufacturing (CAM) to control automated machine tools (including CNC machines and 3D printers). The slicer program also allows the designer to customize the build parameters including support, layer height, and part orientation.

  • Printing: 3D printing machines often comprise of many small and intricate parts so correct maintenance and calibration is critical to producing accurate prints. At this stage, the print material is also loaded into the printer. The raw materials used in additive manufacturing often have a limited shelf life and require careful handling. While some processes offer the ability to recycle excess build material, repeated reuse can result in a reduction in material properties if not replaced regularly. Most additive manufacturing machines do not need to be monitored after the print has begun. The machine will follow an automated process and issues generally only arise when the machine runs out of material or there is an error in the software. A explanation on how each of the different additive manufacturing printers produce parts can be found here.

  • Removal of prints: For some additive manufacturing technologies removal of the print is as simple as separating the printed part from the build platform. For other more industrial 3D printing methods the removal of a print is a highly technical process involving precise extraction of the print while it is still encased in the build material or attached to the build plate. These methods require complicated removal procedures and highly skilled machine operators along with safety equipment and controlled environments.

  • Post processing: Post processing procedures again vary by printer technology. SLA requires a component to cure under UV before handling, metal parts often need to be stress relieved in an oven while FDM parts can be handled right away. For technologies that utilize support, this is also removed at the post-processing stage. Most 3D printing materials are able to be sanded and other post-processing techniques including tumbling, high-pressure air cleaning, polishing, and coloring are implemented to prepare a print for end  use.

2.1.1 Types of Additive Manufacturing Processes suitable for Multi-Material Printing

Different modifications of AM and combination of them with other manufacturing methods aimed to generate multi-material or composite products have been listed below. The subjects are mostly categorized by the process adaptations to implement multi-materials, different materials being used in these processes, and hybrids of AM and other manufacturing methods.

2.1.2 Stereo lithography method

Figure 4 (a) Multiple vat setup for stereo lithography of multi-material parts (b) SLA followed by

Stereo lithography (SLA) [6] has been developed based on photopolymerization phenomena and mostly involves implementing a light source to bond photo-curable resins mixed with other materials to manufacture solid composite parts [6, 7]. Multi material SLA processes have been performed by successively applying and washing off different kinds of photopolymerizable resins in single or multiple vat setups to fabricate each piece of the component with the specific desired materials. The functionality of these approaches has been spread in several fields from fabricating electronic parts to biomedical implants.

2.1.3 Binder jetting methods

Binder jet printing deposits binder materials on powder bed to selectively join powder materials layer by layer to construct three-dimensional parts. In binder jetting, additional extractable powder materials can be engineered in the binding process to reach a desired percentage of materials in different layers of parts. Then by the use of extraction procedures such as solvent materials, additional materials can be removed to obtain a desired porous or functionally graded material (FGM) product [7].

2.1.4 Extrusion-based printing methods

The procedure typically includes depositing the mixture of materials on an ultra-low temperature platform by a sterilized syringe followed by a freezedrying process to remove the solvent material and diminish micropores generated by phase separation. LDM made scaffolds with core-shell composite framework have been created to improve their mechanical and physiochemical properties. An inner and outer feedstock tube and nozzle head were assembled together to extrude the core and the sheath material simultaneously . A multi-nozzle LDM system using disposable syringes has been applied in order to fabricate scaffolds with gradient biomaterials and functions for tissue engineering [8] The use of sacrificial polymeric mixture or UV curable resins combined with ceramic particles is common in many AM methods of ceramic components production. The additive mixtures roles as a binder in the AM process to produce a green product and get removed from the part by a subsequent debinding process. However, every single of these approaches usually suffers from its own certain obstacles such as excessive material consumption, low density after sintering, need for additional steps, lack of functionality and limitations in the produced component dimensions. An addition of photopolymerizable dispersion to the raw material of the extrusionbased AM has been tried to take advantage of the steadiness of the green product with UV-cured resin and economical syringebase AM process. The UV-light irradiation is implemented during the printed process and UV-resin is removed by a typical sintering process. However, deficiencies such as partial polymerization of the layers and cracking of the part due to high shrinkage ratio and during sintering are yet to be dealt with [8,9].

2.1.5 Material jetting printing methods

Multi-jet modelling (MJM), also known as material jetting system or poly-jet printing of materials, uses multiple jet nozzles to deposit photopolymers for the part structures, which are immediately cured after the deposition, and gellike wax materials for the sacrificial support structures. A schematic depiction of the process is shown in Fig 6. This AM technique has the capability to fabricate components with higher resolutions and geometrical complexity [10].

2.2 Modelling and Finite Element Analysis (FEA)

The different CAD models are made and then assembled to form the structure shown below: Geometry

Theoretical Background of FEA:

Joining of the manufactured parts                                                                                                           Joining of Rhenium plate with Aluminium

Effective joining of Rhenium and Aluminum can be achieved by using Diffusion Bonding Technique (DBT). In diffusion bonding technique, temperature and pressure are applied uniformly to a piece to expand it into the other piece . The inter-diffusion of the materials make up the joint in solid state. Since heating is uniform, the entire structure will undergo the same metallurgical changes and there will not be a heat affected zone. The lack of fusion and heat affected zones give unaffected base materials.[20]

 Joining of Titanium and Aluminum Honeycombs:

 Cold metal transfer (CMT) welding–brazing can be used for joining the Titanium and Aluminum honeycombs .[21] Meshing of the CAD Model

Figure of Generated Mesh of the CAD Model


Mesh Statistics: The number of Elements: 141070

The number of Nodes: 68445

High temperature zone formations on the surface of the spacecraft while re-entry The contour shows the variation of high temperatures faced by reentry vehicle heat shield [13]. Based on the the temperature contour the following materials are chosen for our purpose.

Selection of Materials

• The upper-sheet: Rhenium , a 3D printable refractory material [14].

• The upper Honeycomb structure: Titanium, a 3D printable high heat resistance and one of the strongest metals [15].

• The lower Honeycomb structure: Aluminum, a 3D printable light-weight and effective heat shielding metal.[16]

• The lowers-sheet: Titanium

The Temperature contour of steady state thermal analysis of the Honey-comb structure

The simulation result and its temperature contours shows the effectiveness of the Multi-material Honeycomb structure as a heat shield. In the results section a parametric study has been reported in tabular from 800k to 1800K.

Figure 10 Temperature Contour at 1200 K

The Figure 9 , clearly shows the effectiveness of the structure in heat-shielding. It is observed that the outer surface temperate 1200 K is reduced to 429 K. Referring from previous studies this reduction in temperature is more than sufficient to keep safe the electronic devices and other equipments. Further layers of heat shields will be more useful for reduction of temperature further.. Selective Laser Sintering or SLS printing will be a suitable manufacturing process for the production if this multi-material honeycomb structure. The 3D printable materials are chosen for the analysis, the material are listed down:

fea - Copy.png

The Stress variation Contour

The above Figure (FIG. 10) clearly shows the sustainability of the mechanical structure of the wafer
type honeycomb structure at pressure of 7500 Pa corresponding reentry flight condition at about Mach
6.5. In the Result section the deformation of the structure with increasing pressure faced by a hypersonic
vehicle is tabulated and plotted.


3.1  Parametric study of heat shielding effectiveness with increasing surface temperature

In this section, a parametric study shows the variation of the inner temperature with increasing outer surface temperature. The outer surface temperature is ranged form 800 K to 1800 K which generally corresponds to re-entry flight conditions of a reentry vehicle. It is observed the inner surface temperature and heat shielding effective varies linearly with the increasing outer surface temperature. The variations are tabulated in Table 2 and plotted in figure Fig.11.

3.2  Parametric study of Equivalent Stress and Deformation with varying pressure.

The above plot and table shows the deformation of structure with increasing pressure on the outer surface of the body. The pressure is increased form 1500 to 10500 Pa to visualize the deformation, however, in the pressure varies from 1000 to 8000 Pa while re-entry of a spacecraft. The too rightmost contours show deformation of the body at very extreme condition which is generally not faced, but studied to identify the maximum pressure it can bear. In contrast to a typical existing heat shield which can be used only once or for specific amount of time and this homey-comb multilateral structure can be for a longer time and can be reused also.


4.1 Conclusions 

Different experimental processes including modification of Additive Manufacturing (AM) and their combinations with other manufacturing processes targeting more efficient production of multi-material and composite products have been reviewed in this study. Several investigated potentials of AM in being combined with other manufacturing processes have been mentioned and review. Besides, a review of various multi-material AM processes, this paper describes an application of AM based multi-material wafer type honey-comb structure. The materials are chosen based on 3D printability, heat shielding effectiveness, and high tensile stress. Steady-state thermal analysis and stress analysis has been done using ANSYS simulation software. The structure’s heat shielding effectiveness and its integrity have been simulated with extreme conditions faced by the structure of hypersonic vehicles while reentering the earth’s atmosphere. From the simulation results it is observed, the structure efficiently shields the surface heat and reduces it by 66%. Also, the structural integrity has been tested beyond normal re-entry flight conditions. The structural analysis shows a negligible deformation of the structure, which makes it suitable for long usage and reusability. Also, with the usage of Rapid Manufacturing technology, this type of structural configuration can be produced in sound quality and quickly. Though this study conveys the use of waferbased honeycomb structure in spacecraft for heat shielding, the same concept and design can be used for the auto-motives, high-power electrical devices and other mechanical or electrical devices where there is a need of heat shields with high mechanical strength and virtues of Additive Manufacturing Technologies is applicable.

4.2 Future Work

Multi-Material Additive Manufacturing is an emerging field of the manufacturing process. Various uses of multi-material AM have to be identified and its effectiveness has to be evaluated. This paper described an innovative solution for heat shielding in hypersonic spacecraft using Multi-material based additive manufacturing. Promising results are obtained and this type of structure can only be effectively manufactured using Additive Manufacturing technolgies only. The model is simulated under appropriate conditions, and it shows promising results based on thermal and structural integrity. In the current study two honey combs layers are used, an advance study can be done to validate the used of multiple layers for similar heat shielding purposes. The study will be more effective if we print a multi-material 3D part using an appropriate multi-material additive manufacturing process. A comparison of the simulated results with the experimental works will prove the greatness of this study. In this current study, analysis of application of this concept and design is limited only to hypersonics of aerospace sector,but further studies can be done other aerospace, automotive, high-power electrical machines, and other high energy sectors where there is a need of high heat shielding and mechanical strength is needed.


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“It’s always darkest before dawn”
What a beautiful and motivating quote to start this journey of the once in a lifetime opportunity of attending “The 
Harvard Project for Asian and International Relations (HPAIR) Conference 2021”.

I started my journey with the 2200 delegates representing 62 countries across the globe on January 15th,2021. This year HPAIR saw the largest pool of applicants as well as delegates, not to mention the inspiring world leaders as our keynote speakers.

Aspiring talks from Dr.
Tedros Adhanom GhebreyesusKevin SneaderAndréMilind TambeAmit PradhanBrian A. WongHari NairTodd GolubBandana TewariEric GoosbyLesly GohAnla Cheng, Lan Yang, Joseph LubinAdam Cheyer.
Highly motivating talk with Right Honourable Abhisit Vejjajiva, the 27th Prime Minister of Thailand.

And the conference was ably guided by

Mark WuMartin RollEric LinZeel PatelDavid C. LeeSoyoun ChoiAlexander ChenRupert K.

A great thanks to the HPAIR community for providing this cross-cultural opportunity.