Looking to settle a debate. I am of the understanding that, while the Nazis were successful at recruitment in the early years of the Third Reich, through the duration of the war, the majority of the Wermacht were conscripts. There were 18+ million men that served in the Wermacht, certainly most of these could not be volunteers, no?
As an aside, how "voluntary" was the service of the average grunt who ended up volunteering?
Nazi Germany inherited the Reichwehr, the army of the Weimar Republic, fixed by the treaty of Versailles to 100 000 men. Along with them came the Schwarzer Reichswehr, the illegal and unofficial reserve forces, made up of a mixture of paramlitary police, various freikorps organisations, nazi SA men, the Stahlhelm facist movement and veterans of the first world war. These men were all volunteers.
Hans von Seeckt had shaped the Reischwehr into a very well-trained force, with the intention that every man would be able to take a position a rank above his current one and the ranks below would be filled out with conscripts to rapidly expand the German army.
On the 16th of March 1935, Germany officially announced a re-introduction of conscription (in violation of the Versailles treaty) and an expansion of the army to 36 divisions or 480 000 men.
One needs to understand here that conscription was the world standard at the time and had been since the Napoleonic wars - it was really only Britain and the US that used volunteer forces amalgated by selective drafts (for Britain in 1916-1918 and then again from 1939). While support for conscription was high in Germany, and attempts to get away from it were rare until the last days of the war and the collapse of the German army, service was nevertheless mandatory. You could get deferments if you worked in industries determined as vital to the war effort, such as armaments or munitions factories or with the railroads, but in general that only meant a delay, not a pass. You could either be conscripted to the miltiia, the Volkssturm when the enemy was comning to your region, or be replaced by the various slave labour and other labour schemes introduced by the nazi government to increase production and free up manpower for the army.
Volunteering happened, but was limited and usually with the knowledge that if you did not volunteer, you would soon be conscripted anyway. If you wanted to get into a specific service or unit, attempt to become a career officer or wanted to join up before reaching the normal age of conscription, volunteering could help. However, you were still eligeble for conscription should you flunk out of your cadet school or resign your comission.
So, the absolute majority (I would argue at least 95%) of the Wehrmacht were conscripts.