Technology Description
Technology Description

Technology Description

 R&D of Fabrication of Lotus Metals

1. Fabrication Techniques of Lotus Metals using Gas

Lotus metals (left photo: stainless steel, right photo: magnesium)

Lotus metals

(left photo: stainless steel, right photo: magnesium)

  Lotus metals are fabricated by pore formation of insoluble gas when the molten metals dissolving gas are solidified during unidirectional solidification. Three fabrication techniques were developed by Nakajima group: mold casting technique, continuous zone melting technique and continuous casting technique.

 

 

 

 

 The apparatus of the mold casting technique is shown in Fig.1. In the chamber melting and solidification parts are set up. The metal is melted in the crucible by high-frequency induction heating, and the gas is dissolved into the melt. The melt is poured into the hearth whose bottom is cooled by circulated water. The melt is solidified in the upper direction so that the directional pores are evolved. Although this technique is simple, it is not suitable to solidify long sized metal rod because the solidification rate changes.

 

fig3

 

Fig.1 Mold casting technique                            Fig.2 Continuous zone melting technique

 

 

 The apparatus of the mold casting technique is shown in Fig.1. In the chamber melting and solidification parts are set up. The metal is melted in the crucible by high-frequency induction heating, and the gas is dissolved into the melt. The melt is poured into the hearth whose bottom is cooled by circulated water. The melt is solidified in the upper direction so that the directional pores are evolved. Although this technique is simple, it is not suitable to solidify long sized metal rod because the solidification rate changes.

 

ロータス金属の作製法:連続鋳造法

 

Fig.4 Lotus copper slab fabricated by

continuous  casting technique


 

Fig.3 Continuous casting technique

 

Fig.3 Continuous casting technique

 

 

 

 

  On the other hand, as shown in Fig.2, the metal rod is melted partially by the induction heating coil in the gas and is transferred continuously downward so that lotus metal is fabricated, keeping constant solidification rate. This technique is effective to produce lotus metals with low thermal conductivity such as stainless steel and intermetallic compounds. However, the size of produced ingots is limited; it is not suitable for large sized lotus metals.

 


  Finally, Nakajima group succeeded to invent the continuous casting technique for mass-production of lotus metals. As shown in Fig.3, the melt dissolving gas in the upper crucible heated by induction heating coil is continuously pulled down by dummy bar mechanically through cooled hearth. Thus, long sized lotus metals with uniform pore size and porosity are able to be fabricated (Fig.4). This technique is the most superior to be applied to fabricate any metals and compounds despite of any thermal conductivity.

 

 

2.Fabrication Technique of Lotus Metals by Thermal Decomposition of Gas Compounds

 

ガス化合物熱分解法を用いた鋳型鋳造法によるロータス金属作製装置

 

Fig.5 Fabrication technique by

  


   

 

thermal decomposition method

 

 

 


The fabrication technique as mentioned above must use hydrogen gas. In order to avoid a problem that hydrogen might be explosive when oxygen is mixed, Nakajima group developed to fabricate lotus metals by use of gas compounds instead of gas, which is called as thermal decomposition method. As shown in Fig.5, molten metal is poured into the hearth cooled by circulated water, on the bottom of which a few pellets of hydride are placed. The hydride decomposes into hydrogen in the melt so that lotus metal is fabricated in the similar way as the gas method. This technique has advantages of simple, safe and low cost production.

 

Strength of Lotus Metals

1 Tensile Strength

   The strength of lotus metals is dependent upon the direction of pore growth an

d anisotropic. 

Fig.6. shows the porosity dependence of the tensile strength of lotus copper. 

The specific strength (strength per unit weight) of the parallel direction does not differ from that of non-porous copper. The significant decrease of strength of perpendicular direction is attributed to the stress concentration around pores.The tensile strength parallel to the pore growth direction decreases linearly with increasing porosity, but the strength perpendicular to the growth direction decreases remarkably.

The strength of lotus iron fabricated by using nitrogen gas is almost the same as that of non-porous iron event at the porosity of 40~50%, which is attributed to the solid solution hardening due to nitrogen. This technique was utilized to fabricate the saddles of the machine tools.

 

 

 

 

 

 

 

 

                            Fig.6 Porosity dependence of tensile stress

                                                                                     of lotus copper

 

2. Shock Absorbing Property

  When an impact is loaded onto a shock absorber,

 particular part of the shock absorber should be deformed by buckling distortion through compressive deformation. Voids and pores are necessary to absorb such the impact so that porous metals are suitable for shock absorbers. Fig.7  shows the stress-strain curves during compressive loading onto (a) lotus meta


ls and (b) non-porous metals. At the beginning of the compression, the stress suddenly increases with increasing strain, and then the slope gradually decreases and finally keeps constant, showing the plateau region. Non-porous metals do not exhibit any plateau region as shown in Fig. 7(b). The shock absorbing energy is evaluated with the integral of the plateau region. As shown in Fig.8, the absorbing energy of lotus metals is ten to several dozen higher than that of conventional foamed metals and tubes. Thus, lotus  metals exhibit to be the most excellent shock absorber(Fig.9).

 

                            Fig.7 Relation between stress and strain of

                               compression test of shock

 

                               absorbers 

 

 

 

 

 

 

 

   Fig.8 Relation between absorption      Fig.9 Difference of shock absorption behavior

      energy and plateau stress           between foamed metals and lotus                             metals    

 

 

Application of Lotus Metals to Industrial Devices

Lotus metals possess superior functional properties of light-weight, good damping and sufficient heat transfer. Two examples of prospective manufacturing products are introduced, which were developed by Institute for Lotus Materials Research Co., Ltd. and cooperative companies.

 

1 Machine Tools by use of Lotus Carbon Steel

Recently energy saving and high-precision are demanded to the machine tools.It is important to save the weight of the saddle of machine tools and improve the kinematical performance,  which brings about decrease in energy consumption. Cast iron has been usually used as the saddle of the machine tools because of good damping property (Fig.10). However, cast iron is heavy. Instead of cast iron, lotus carbon steel was used as shown in Fig.10. Since lotus carbon steel was fabricated using nitrogen gas, the strength of the saddle made of lotus carbon steel is almost the same as that of non-porous carbon steel because of solid solution hardening.

Use of lotus carbon steel leads to lighten the saddle made of cast iron by 20 to 30%. 

ロータス炭素鋼厚鋼板サドル

Residual vibration during shaving process is significantly suppressed by adopting the saddle made of lotus carbon steel. 

Furthermore, the inertia becomes smaller because of light-weight and performance of acceleration and deceleration is improved more than twice compared with the cast iron saddle. Finally the electricity consumption can be saved by 20%. Thus, light-weight saddle by use of lotus carbon steel remarkably contributed to suppression of the residual vibration and cut-down of the electricity consumption.

 

                       Fig.10 Machine tools and saddle

                           made of lotus carbon steel

 

 

                           

 

2 Heat Sinks fabricated by Lotus Metals

 


Fig.11 setup of heat sink using lotus copper


Since the heat generation from devices drastically increases because of miniaturization and high performance, high-efficient heat sinks for effective cooling are demanded. Foamed metals are considered to be suitable as heat sinks, since the foams have large area contacted on coolant. However, the foamed metals have a demerit; the pressure drop of the heat sinks is large because the flow of the coolant is joining or tributary so that large power is necessary. In order to decrease the pressure drop the pores should be straight and thus, lotus metals are desirable.

  Fig.11 illustrates the setup of the heat sink for water cooling using lotus copper. Lotus copper fins are arranged parallel just below the electric devices and the cooling water flows through the pores of lotus copper. The generating heat at the devices is transferred to the cooling water through the fins. As shown in Fig.12, the heat transfer coefficient per unit the base area of heat sink was measured as a function of the pressure drop. The heat transfer coefficient for lotus heat sink is five times higher than that of the conventional groove-type heat sink and twice higher than that of microchannel groove-type heat sink. Lotus heat sinks exhibit the highest value, 80 kW·m-2·K-1 of the heat transfer coefficients, which is the champion data in the world.  Thus, lotus heat sinks are expected as the best performance heat sinks. AS shown in Fig.13, many heat sinks are mounted in the cars as Power Control Unit. The Power Control Unit can be miniaturized with high performance, if the miniaturization of heat sinks with high heat transfer is realized.

 

 

Fig.12 Relation between heat transfer                   Fig.13 Power control unit mounted

          coefficients and pressure drop                             with many heat sinks

 

Fig.14 shows several examples of applications of lotus metals: golf putter (using damping), dental implant (using bone growth into pores), heat sink, machine tool, airplane engine cooling panel, car body(light-weight, crash box) and machinery parts.

 


                                                   Fig.14 Various applications of lotus metals

 

 

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