POROUS MATERIAL DEFORMABILITY IN FOUR-ROLL PASS ROLLING

2017. Т. 17, No 1. С. 89–92 89 Introduction It is mostly imperative for modern material swaging theory to develop mathematical models and matching methods of calculation of plastic shaping and compacting parameters. This is entirely true for new technique of sintered powder rod rolling in four-roll passes. Presently, such rods made of tungsten and molybdenum powders by their pressing and two-stage sintering are treated basically by swaging. But experimental investigations and commercial production practice evidently show technological and ecological advantages of four-roll pass rolling technique over swaging. Owing to such a situation, there is an urgent need for thorough experimental and theoretical investigations of plastic shaping and sintered powder material compacting at the zone of deformation during the rolling with four-side reduction. Besides, such investigations would be critical for improving existing technologies and developing up-to-date methods of rod rolling with four-side reduction of the rod at the zone of deformation.


Introduction
It is mostly imperative for modern material swaging theory to develop mathematical models and matching methods of calculation of plastic shaping and compacting parameters.This is entirely true for new technique of sintered powder rod rolling in four-roll passes.Presently, such rods made of tungsten and molybdenum powders by their pressing and two-stage sintering are treated basically by swaging.But experimental investigations and commercial production practice evidently show technological and ecological advantages of four-roll pass rolling technique over swaging.Owing to such a situation, there is an urgent need for thorough experimental and theoretical investigations of plastic shaping and sintered powder material compacting at the zone of deformation during the rolling with four-side reduction.Besides, such investigations would be critical for improving existing technologies and developing up-to-date methods of rod rolling with four-side reduction of the rod at the zone of deformation.

Model of powder rod compacting in rolling
Basing on previous experimental data concerning the nature of powder sintered rod compacting at the zone of deformation during the process of four-roll rolling, authors have proposed the following analytic functions describing den-sity changes in a powder strip along the entire length of its deformation zone [1]: where ρ , ρ are rod densities before and after the rolling, respectively, l is the length of deformation zone, k is the parameter of deformation degree, its values being set as follows: k = 0 if ρ = const, k = 1 if the degree of deformation ε is less than 40 %, and k = 2 if ε is greater than 40 %.
Another analytic function [2] is expressed as: where n is the parameter characterizing the rate of powder rod compacting in the direction of rolling; this rate being inversely proportional to rod density at the given profile of deformation zone.However, the latest complex experimental investigations of plastic shaping character and powder rod compacting at the zone of deformation in the process of rolling with four-side reduction have revealed that the proposed functions (1) and ( 2) fail to describe properly the real nature of changes in sintered powder rod density along the entire length of deformation zone.The above functions just set a trend for possible monotonic changing in rod density and leave the question of what degrees of that monotony would be at each separate point of the entire length of deformation zone.
The goal was to obtain a more accurate e pression for density distribution in sintered po der rod along the length of deformation zone.The experimental investigations have been ca ried out to solve the problem using underrolled molybdenum powder rods obtained by rolling in four-roll passes.During the experiment, the cha acter of step-by-step changing in cross area was also observed.
Industrial tests of the following technique were also carried out.There were selected the samples from the batch of sintered pure m lybdenum powder rods with 18×18 mm cross section area and 600 mm length having the smal lest differences in densities of the material far and wide.The selection of samples was carried out in two ways: by hydroweighing a density measuring with the help of special device [3], permitting to measure density characteristics in powder sintered rods so as to gain information about the average density in the rod at a definite cross-section area as well as density over that area or lengthwise.The above device utilizes gamma-radiation source, its principle of operation basing on registration of changes in collimated gamma-ray beam passing through gam ma absorber, e.g.sintered powder rod.The d vice offers ±0.1 g/cm 3 accuracy of density mea surements.
From the selected powder rods underrolled rods were produced by rolling on four industrial mill MK-210×4, at the UZBEK Refra tory Metals Combine.The underrolled rods were subjected to different degrees of deformation, namely, 25 %, 30 %, 33 % and 36 reduction.Special guides held the rods oh their edges during the reduction.The underrolled si tered powder rods were cut into cross 1 and lo gitudinal 2 templates (Fig. 1) to study th macro-and microstructures and to measure the de nsity of rod material along the entire length of deformation zone.would be at each separate point of the entire The goal was to obtain a more accurate expression for density distribution in sintered powder rod along the length of deformation zone.The experimental investigations have been carried out to solve the problem using underrolled m powder rods obtained by rolling in roll passes.During the experiment, the charstep changing in cross-section Industrial tests of the following technique were also carried out.There were selected s from the batch of sintered pure molybdenum powder rods with 18×18 mm crosssection area and 600 mm length having the smaldifferences in densities of the material far and wide.The selection of samples was carried out in two ways: by hydroweighing and radioisotopic density measuring with the help of special device [3], permitting to measure density characteristics in powder sintered rods so as to gain information about the average density in the rod at a definite section area as well as density distribution over that area or lengthwise.The above device radiation source, its principle of operation basing on registration of changes in ray beam passing through gamabsorber, e.g.sintered powder rod.The deaccuracy of density mea-From the selected powder rods underrolled rods were produced by rolling on four-roll pass 210×4, at the UZBEK Refractory Metals Combine.The underrolled rods were degrees of deformation, % and 36 % [1] and edge reduction.Special guides held the rods oh their edges during the reduction.The underrolled sintered powder rods were cut into cross 1 and longitudinal 2 templates (Fig. 1) to study their and microstructures and to measure the deof rod material along the entire length of The changes in material density values along the zone of sintered powder rod deformation were measured by hydroweighing of cross te plates, and the changes in rod cross magnitudes along the length of deformation zone were observed by BMI-1 microscope.The cha racter of density distribution ρ( rial, and rod cross-section areas the length of deformation zone were approx mated by the following analytic functions: where n and m are exactly the parameters whose values are found as a result of data processing by least square method.Setting x equal to zero one can find density values ρ 1 and cross-section area magnitudes the outlet of rod deformation zone: ρ = ρ(0) = ρ (1 + = (0) = (1 − Introducing δ ρ as relative rod compactness after rolling: δ = , and δ S as relative reduction of rod cross area: δ = , one can rewrite functions ( 3) and ( 4) with respect of ( 5), ( 6), ( 7) and ( 8) equations as follows: With regard to the law of constant mass of the material under plastic shaping and compac ting, one can find the relationship rameters of original rod and rolled strip: (1 − δ )λ 1 + δ = where = is the strip elongation in one pass, The changes in material density values along powder rod deformation were measured by hydroweighing of cross templates, and the changes in rod cross-section area magnitudes along the length of deformation zone 1 microscope.The character of density distribution ρ(x), over the matesection areas S(x) along the length of deformation zone were approximated by the following analytic functions: 3) • exp ( ) , ( 4) are exactly the parameters whose values are found as a result of data processing by equal to zero one can find density section area magnitudes S 1 at the outlet of rod deformation zone: ), (5) ).
With respect to the relative compactness δ formula (11) can be expressed as: Formula ( 12) clearly shows that deform tion parameters are interrelated, i.e. knowing the values of any two of them one can easily find the third one from (12).For instance, as geometrical parameters δ S and λ ρ found from the experiment, then the value of parameter δ ρ is obtained from formula (12), thus avoiding labour-consuming experiment for d termination of the character of density distrib tion in powder rod along the length of its d formation zone.
Basing on experimental data obtained the va lues of n and m parameters in equations ( 9) and (10) were calculated according to the standard computer approximation program by method of the least squares.Calculation results and exper mental data are plotted in Fig. 2.
Thorough analysis of curves behavior in Fig. 2, which are plotted according to the above fun tions (12), ( 2) and (9) against experimental data points, reveals function (9) to be the most real description of powder compacting character are rod lengths before and after With respect to the relative compactness δ ρ , Formula ( 12) clearly shows that deformation parameters are interrelated, i.e. knowing lues of any two of them one can easily find the third one from (12).For instance, as can be directly found from the experiment, then the value of ned from formula (12), thus consuming experiment for determination of the character of density distribution in powder rod along the length of its de-Basing on experimental data obtained the vauations ( 9) and (10) were calculated according to the standard computer approximation program by method of the least squares.Calculation results and experi-Thorough analysis of curves behavior in Fig. 2, which are plotted according to the above functions (12), ( 2) and (9) against experimental data points, reveals function (9) to be the most real description of powder compacting character along the length of deformation zone, while fun tions ( 12) and ( 2) show just a qualitatively mon tonic character of compacting.This provides for a further application of function ( 9) to powder material plastic compacting at the zone of its d formation in rolling with four along the length of deformation zone, while funchow just a qualitatively monotonic character of compacting.This provides for a further application of function ( 9) to powder material plastic compacting at the zone of its deformation in rolling with four-side reduction.Прокатка в четырехвалковом калибре предлагает множество технологических и экологических преимуществ по сравнению с обжимом, который в настоящее время используется для обработки спеченных прутков из порошков вольфрама и молибдена.Это объясняет необходимость тщательных экспериментальных и теоретических исследований таких процессов, как пластическая деформация и прессование спеченных порошковых материалов в зоне их деформации при прокатке с четырехсторонним обжатием.Авторы провели многочисленные экспериментальные исследования прокатки порошковых заготовок в четырехвалковых калибрах.Известны эмпирические формулы, описывающие изменение плотности заготовок в процессе их прокатки.В настоящей статье приведены результаты комплексных экспериментальных исследований и дано обоснование предлагаемой аналитической функции, позволяющей дать оценку плотности заготовки путем измерения ее геометрических параметров в процессе прокатки.Эта аналитическая функция основана на законе постоянства массы прутка при пластической деформации и прессовании.

Fig. 2 .
Fig. 2. Powder rod compacting curves.Data taken along the entire length of deformation zone in the process of rod rolling.Straight line 1 is the plot of function (12); Curve 2 is the plot of function (2) when the plot of functi